# Language Guide (proto 2)

Covers how to use the proto2 revision of Protocol Buffers language in your project.

This guide describes how to use the protocol buffer language to structure your protocol buffer data, including `.proto` file syntax and how to generate data access classes from your `.proto` files. It covers the **proto2** revision of the protocol buffers language.

For information on **editions** syntax, see the [Protobuf Editions Language Guide](https://protobuf.dev/programming-guides/editions).

For information on **proto3** syntax, see the [Proto3 Language Guide](https://protobuf.dev/programming-guides/proto3).

This is a reference guide – for a step by step example that uses many of the features described in this document, see the [tutorial](https://protobuf.dev/getting-started) for your chosen language.

## Defining A Message Type

First let’s look at a very simple example. Let’s say you want to define a search request message format, where each search request has a query string, the particular page of results you are interested in, and a number of results per page. Here’s the `.proto` file you use to define the message type.

    syntax = "proto2";

    message SearchRequest {
      optional string query = 1;
      optional int32 page_number = 2;
      optional int32 results_per_page = 3;

  * The first line of the file specifies that you’re using the proto2 revision of the protobuf language spec.

    * The `syntax` must be the first non-empty, non-comment line of the file.
    * If no `syntax` is specified, the protocol buffer compiler will assume you are using proto2.
  * The `SearchRequest` message definition specifies three fields (name/value pairs), one for each piece of data that you want to include in this type of message. Each field has a name and a type.

### Specifying Field Types

In the earlier example, all the fields are scalar types: two integers (`page_number` and `results_per_page`) and a string (`query`). You can also specify enumerations and composite types like other message types for your field.

### Assigning Field Numbers

You must give each field in your message definition a number between `1` and `536,870,911` with the following restrictions:

  * The given number **must be unique** among all fields for that message.
  * Field numbers `19,000` to `19,999` are reserved for the Protocol Buffers implementation. The protocol buffer compiler will complain if you use one of these reserved field numbers in your message.
  * You cannot use any previously reserved field numbers or any field numbers that have been allocated to extensions.

This number **cannot be changed once your message type is in use** because it identifies the field in the [message wire format](https://protobuf.dev/programming-guides/encoding). “Changing” a field number is equivalent to deleting that field and creating a new field with the same type but a new number. See Deleting Fields for how to do this properly.

Field numbers **should never be reused**. Never take a field number out of the reserved list for reuse with a new field definition. See Consequences of Reusing Field Numbers.

You should use the field numbers 1 through 15 for the most-frequently-set fields. Lower field number values take less space in the wire format. For example, field numbers in the range 1 through 15 take one byte to encode. Field numbers in the range 16 through 2047 take two bytes. You can find out more about this in [Protocol Buffer Encoding](https://protobuf.dev/programming-guides/encoding#structure).

#### Consequences of Reusing Field Numbers

Reusing a field number makes decoding wire-format messages ambiguous.

The protobuf wire format is lean and doesn’t provide a way to detect fields encoded using one definition and decoded using another.

Encoding a field using one definition and then decoding that same field with a different definition can lead to:

  * Developer time lost to debugging
  * A parse/merge error (best case scenario)
  * Leaked PII/SPII
  * Data corruption

Common causes of field number reuse:

  * renumbering fields (sometimes done to achieve a more aesthetically pleasing number order for fields). Renumbering effectively deletes and re-adds all the fields involved in the renumbering, resulting in incompatible wire-format changes.

  * deleting a field and not reserving the number to prevent future reuse.

    * This has been a very easy mistake to make with extension fields for several reasons. [Extension Declarations](https://protobuf.dev/programming-guides/extension_declarations) provide a mechanism for reserving extension fields.

The field number is limited to 29 bits rather than 32 bits because three bits are used to specify the field’s wire format. For more on this, see the [Encoding topic](https://protobuf.dev/programming-guides/encoding#structure).

### Specifying Field Cardinality

Message fields can be one of the following:

  * _Singular_ :

In proto2, there are two types of singular fields:

    * `optional`: (recommended) An `optional` field is in one of two possible states:

      * the field is set, and contains a value that was explicitly set or parsed from the wire. It will be serialized to the wire.
      * the field is unset, and will return the default value. It will not be serialized to the wire.

You can check to see if the value was explicitly set.

    * `required`: **Do not use.** Required fields are so problematic they were removed from proto3 and editions. Semantics for required field should be implemented at the application layer. When it _is_ used, a well-formed message must have exactly one of this field.

  * `repeated`: this field type can be repeated zero or more times in a well-formed message. The order of the repeated values will be preserved.

  * `map`: this is a paired key/value field type. See [Maps](https://protobuf.dev/programming-guides/encoding#maps) for more on this field type.

#### Use Packed Encoding for New Repeated Fields

For historical reasons, `repeated` fields of scalar numeric types (for example, `int32`, `int64`, `enum`) aren’t encoded as efficiently as they could be. New code should use the special option `[packed = true]` to get a more efficient encoding. For example:

    repeated int32 samples = 4 [packed = true];
    repeated ProtoEnum results = 5 [packed = true];

You can find out more about `packed` encoding in [Protocol Buffer Encoding](https://protobuf.dev/programming-guides/encoding#packed).

#### Required is Strongly Deprecated

#### Important

**Required Is Forever** As mentioned earlier **`required` must not be used for new fields**. Semantics for required fields should be implemented at the application layer instead. Existing `required` fields should be treated as permanent, immutable elements of the message definition. It is nearly impossible to safely change a field from `required` to `optional`. If there is any chance that a stale reader exists, it will consider messages without this field to be incomplete and may reject or drop them.

A second issue with required fields appears when someone adds a value to an enum. In this case, the unrecognized enum value is treated as if it were missing, which also causes the required value check to fail.

#### Well-formed Messages

The term “well-formed,” when applied to protobuf messages, refers to the bytes serialized/deserialized. The protoc parser validates that a given proto definition file is parseable.

Singular fields can appear more than once in wire-format bytes. The parser will accept the input, but only the last instance of that field will be accessible through the generated bindings. See [Last One Wins](https://protobuf.dev/programming-guides/encoding#last-one-wins) for more on this topic.

### Adding More Message Types

Multiple message types can be defined in a single `.proto` file. This is useful if you are defining multiple related messages – so, for example, if you wanted to define the reply message format that corresponds to your `SearchResponse` message type, you could add it to the same `.proto`:

    message SearchRequest {
      optional string query = 1;
      optional int32 page_number = 2;
      optional int32 results_per_page = 3;

    message SearchResponse {
     ...

**Combining Messages leads to bloat** While multiple message types (such as message, enum, and service) can be defined in a single `.proto` file, it can also lead to dependency bloat when large numbers of messages with varying dependencies are defined in a single file. It’s recommended to include as few message types per `.proto` file as possible.

### Adding Comments

To add comments to your `.proto` files:

  * Prefer C/C++/Java line-end-style comments ‘//’ on the line before the .proto code element

  * C-style inline/multi-line comments `/* ... */` are also accepted.

    * When using multi-line comments, a margin line of ‘*’ is preferred.

     * SearchRequest represents a search query, with pagination options to
     * indicate which results to include in the response.
    message SearchRequest {
      optional string query = 1;

      // Which page number do we want?
      optional int32 page_number = 2;

      // Number of results to return per page.
      optional int32 results_per_page = 3;

### Deleting Fields

Deleting fields can cause serious problems if not done properly.

**Do not delete** `required` fields. This is almost impossible to do safely. If you must delete a `required` field, you should first mark the field `optional` and `deprecated` and ensure that all systems which in any way observe the message have been deployed with the new schema. Then you can consider removing the field (but note that this is still an error-prone process).

When you no longer need a field that is not `required`, first delete all references to the field from client code, and then delete the field definition from the message. However, you **must** reserve the deleted field number. If you do not reserve the field number, it is possible for a developer to reuse that number in the future and cause a breakage.

You should also reserve the field name to allow JSON and TextFormat encodings of your message to continue to parse.

#### Reserved Field Numbers

If you update a message type by entirely deleting a field, or commenting it out, future developers can reuse the field number when making their own updates to the type. This can cause severe issues, as described in Consequences of Reusing Field Numbers. To make sure this doesn’t happen, add your deleted field number to the `reserved` list.

The protoc compiler will generate error messages if any future developers try to use these reserved field numbers.

    message Foo {
      reserved 2, 15, 9 to 11;

Reserved field number ranges are inclusive (`9 to 11` is the same as `9, 10, 11`).

#### Reserved Field Names

Reusing an old field name later is generally safe, except when using TextProto or JSON encodings where the field name is serialized. To avoid this risk, you can add the deleted field name to the `reserved` list.

Reserved names affect only the protoc compiler behavior and not runtime behavior, with one exception: TextProto implementations may discard unknown fields (without raising an error like with other unknown fields) with reserved names at parse time (only the C++ and Go implementations do so today). Runtime JSON parsing is not affected by reserved names.

    message Foo {
      reserved 2, 15, 9 to 11;
      reserved "foo", "bar";

Note that you can’t mix field names and field numbers in the same `reserved` statement.

### What’s Generated from Your `.proto`?

When you run the protocol buffer compiler on a `.proto`, the compiler generates the code in your chosen language you’ll need to work with the message types you’ve described in the file, including getting and setting field values, serializing your messages to an output stream, and parsing your messages from an input stream.

  * For **C++**, the compiler generates a `.h` and `.cc` file from each `.proto`, with a class for each message type described in your file.
  * For **Java**, the compiler generates a `.java` file with a class for each message type, as well as a special `Builder` class for creating message class instances.
  * For **Kotlin**, in addition to the Java generated code, the compiler generates a `.kt` file for each message type with an improved Kotlin API. This includes a DSL that simplifies creating message instances, a nullable field accessor, and a copy function.
  * **Python** is a little different — the Python compiler generates a module with a static descriptor of each message type in your `.proto`, which is then used with a _metaclass_ to create the necessary Python data access class at runtime.
  * For **Go**, the compiler generates a `.pb.go` file with a type for each message type in your file.
  * For **Ruby**, the compiler generates a `.rb` file with a Ruby module containing your message types.
  * For **Objective-C**, the compiler generates a `pbobjc.h` and `pbobjc.m` file from each `.proto`, with a class for each message type described in your file.
  * For **C#**, the compiler generates a `.cs` file from each `.proto`, with a class for each message type described in your file.
  * For **PHP**, the compiler generates a `.php` message file for each message type described in your file, and a `.php` metadata file for each `.proto` file you compile. The metadata file is used to load the valid message types into the descriptor pool.
  * For **Dart**, the compiler generates a `.pb.dart` file with a class for each message type in your file.

You can find out more about using the APIs for each language by following the tutorial for your chosen language. For even more API details, see the relevant [API reference](https://protobuf.dev/reference/).

## Scalar Value Types

A scalar message field can have one of the following types – the table shows the type specified in the `.proto` file, and the corresponding type in the automatically generated class:

Proto Type| Notes
double|
float|
int32| Uses variable-length encoding. Inefficient for encoding negative numbers – if your field is likely to have negative values, use sint32 instead.
int64| Uses variable-length encoding. Inefficient for encoding negative numbers – if your field is likely to have negative values, use sint64 instead.
uint32| Uses variable-length encoding.
uint64| Uses variable-length encoding.
sint32| Uses variable-length encoding. Signed int value. These more efficiently encode negative numbers than regular int32s.
sint64| Uses variable-length encoding. Signed int value. These more efficiently encode negative numbers than regular int64s.
fixed32| Always four bytes. More efficient than uint32 if values are often greater than 228.
fixed64| Always eight bytes. More efficient than uint64 if values are often greater than 256.
sfixed32| Always four bytes.
sfixed64| Always eight bytes.
bool|
string| A string must always contain UTF-8 encoded or 7-bit ASCII text, and cannot be longer than 232.
bytes| May contain any arbitrary sequence of bytes no longer than 232.

Proto Type| C++ Type| Java/Kotlin Type[1]| Python Type[3]| Go Type| Ruby Type| C# Type| PHP Type| Dart Type| Rust Type
---|---|---|---|---|---|---|---|---|---
double| double| double| float| *float64| Float| double| float| double| f64
float| float| float| float| *float32| Float| float| float| double| f32
int32| int32_t| int| int| int32| Fixnum or Bignum (as required)| int| integer| *int32| i32
int64| int64_t| long| int/long[4]| *int64| Bignum| long| integer/string[6]| Int64| i64
uint32| uint32_t| int[2]| int/long[4]| *uint32| Fixnum or Bignum (as required)| uint| integer| int| u32
uint64| uint64_t| long[2]| int/long[4]| *uint64| Bignum| ulong| integer/string[6]| Int64| u64
sint32| int32_t| int| int| int32| Fixnum or Bignum (as required)| int| integer| *int32| i32
sint64| int64_t| long| int/long[4]| *int64| Bignum| long| integer/string[6]| Int64| i64
fixed32| uint32_t| int[2]| int/long[4]| *uint32| Fixnum or Bignum (as required)| uint| integer| int| u32
fixed64| uint64_t| long[2]| int/long[4]| *uint64| Bignum| ulong| integer/string[6]| Int64| u64
sfixed32| int32_t| int| int| *int32| Fixnum or Bignum (as required)| int| integer| int| i32
sfixed64| int64_t| long| int/long[4]| *int64| Bignum| long| integer/string[6]| Int64| i64
bool| bool| boolean| bool| *bool| TrueClass/FalseClass| bool| boolean| bool| bool
string| string| String| unicode (Python 2), str (Python 3)| *string| String (UTF-8)| string| string| String| ProtoString
bytes| string| ByteString| bytes| []byte| String (ASCII-8BIT)| ByteString| string| List| ProtoBytes

[1] Kotlin uses the corresponding types from Java, even for unsigned types, to ensure compatibility in mixed Java/Kotlin codebases.

[2] In Java, unsigned 32-bit and 64-bit integers are represented using their signed counterparts, with the top bit simply being stored in the sign bit.

[3] In all cases, setting values to a field will perform type checking to make sure it is valid.

[4] 64-bit or unsigned 32-bit integers are always represented as long when decoded, but can be an int if an int is given when setting the field. In all cases, the value must fit in the type represented when set. See [2].

[5] Proto2 typically doesn’t ever check the UTF-8 validity of string fields. Behavior varies between languages though, and invalid UTF-8 data should not be stored in string fields.

[6] Integer is used on 64-bit machines and string is used on 32-bit machines.

You can find out more about how these types are encoded when you serialize your message in [Protocol Buffer Encoding](https://protobuf.dev/programming-guides/encoding).

## Default Field Values

When a message is parsed, if the encoded message bytes do not contain a particular field, accessing that field in the parsed object returns the default value for that field. The default values are type-specific:

  * For strings, the default value is the empty string.
  * For bytes, the default value is empty bytes.
  * For bools, the default value is false.
  * For numeric types, the default value is zero.
  * For message fields, the field is not set. Its exact value is language-dependent. See the [generated code guide](https://protobuf.dev/reference/) for your language for details.
  * For enums, the default value is the **first defined enum value**, which should be 0 (recommended for compatibility with open enums). See Enum Default Value.

The default value for repeated fields is empty (generally an empty list in the appropriate language).

The default value for map fields is empty (generally an empty map in the appropriate language).

### Overriding Default Scalar Values

In proto2, you can specify explicit default values for singular non-message fields. For example, let’s say you want to provide a default value of 10 for the `SearchRequest.results_per_page` field:

    optional int32 results_per_page = 3 [default = 10];

If the sender does not specify `results_per_page`, the receiver will observe the following state:

  * The `results_per_page` field is not present. That is, the `has_results_per_page()` (hazzer method) method would return `false`.
  * The value of `results_per_page` (returned from the “getter”) is `10`.

If the sender does send a value for `results_per_page` the default value of 10 is ignored and the sender’s value is returned from the “getter”.

See the [generated code guide](https://protobuf.dev/reference/) for your chosen language for more details about how defaults work in generated code.

Because the default value for enums is the first defined enum value, take care when adding a value to the beginning of an enum value list. See the Updating a Message Type section for guidelines on how to safely change definitions.

## Enumerations

When you’re defining a message type, you might want one of its fields to only have one of a predefined list of values. For example, let’s say you want to add a `corpus` field for each `SearchRequest`, where the corpus can be `UNIVERSAL`, `WEB`, `IMAGES`, `LOCAL`, `NEWS`, `PRODUCTS` or `VIDEO`. You can do this very simply by adding an `enum` to your message definition with a constant for each possible value.

In the following example we’ve added an `enum` called `Corpus` with all the possible values, and a field of type `Corpus`:

    enum Corpus {
      CORPUS_UNSPECIFIED = 0;
      CORPUS_UNIVERSAL = 1;
      CORPUS_WEB = 2;
      CORPUS_IMAGES = 3;
      CORPUS_LOCAL = 4;
      CORPUS_NEWS = 5;
      CORPUS_PRODUCTS = 6;
      CORPUS_VIDEO = 7;

    message SearchRequest {
      optional string query = 1;
      optional int32 page_number = 2;
      optional int32 results_per_page = 3;
      optional Corpus corpus = 4;

### Prefixing Enum Values

When prefixing enum values, the remainder of the name with the prefix stripped should still be a legal and style-conformant enum name. For example, avoid the following:

    enum DeviceTier {
      DEVICE_TIER_UNKNOWN = 0;
      DEVICE_TIER_1 = 1;
      DEVICE_TIER_2 = 2;

Instead, use a value name like `DEVICE_TIER_TIER1`, where the `DEVICE_TIER_` portion is viewed as scoping the enum value rather than as part of the individual enum value name. Some Protobuf implementations automatically strip the prefix that matches the containing enum name where it is safe to do so, but could not in this example since a bare `1` is not a legal enum value name.

We plan for a future Edition to add support for scoped enums, which will eliminate the need to manually prefix each enum value and enable this to be written succinctly as `TIER1 = 1`.

### Enum Default Value

The default value for the `SearchRequest.corpus` field is `CORPUS_UNSPECIFIED` because that is the first value defined in the enum.

It is strongly recommended to define the first value of every enum as `ENUM_TYPE_NAME_UNSPECIFIED = 0;` or `ENUM_TYPE_NAME_UNKNOWN = 0;`. This is because of the way proto2 handles unknown values for enum fields.

It is also recommended that this first, default value have no semantic meaning other than “this value was unspecified”.

The default value for an enum field like `SearchRequest.corpus` field can be explicitly overridden like this:

      optional Corpus corpus = 4 [default = CORPUS_UNIVERSAL];

### Enum Value Aliases

You can define aliases by assigning the same value to different enum constants. To do this you need to set the `allow_alias` option to `true`. Otherwise, the protocol buffer compiler generates a warning message when aliases are found. Though all alias values are valid for serialization, only the first value is used when deserializing.

    enum EnumAllowingAlias {
      option allow_alias = true;
      EAA_UNSPECIFIED = 0;
      EAA_STARTED = 1;
      EAA_RUNNING = 1;
      EAA_FINISHED = 2;

    enum EnumNotAllowingAlias {
      ENAA_UNSPECIFIED = 0;
      ENAA_STARTED = 1;
      // ENAA_RUNNING = 1;  // Uncommenting this line will cause a warning message.
      ENAA_FINISHED = 2;

Enumerator constants must be in the range of a 32-bit integer. Since `enum` values use [varint encoding](https://protobuf.dev/programming-guides/encoding) on the wire, negative values are inefficient and thus not recommended. You can define `enum`s within a message definition, as in the earlier example, or outside – these `enum`s can be reused in any message definition in your `.proto` file. You can also use an `enum` type declared in one message as the type of a field in a different message, using the syntax `_MessageType_._EnumType_`.

When you run the protocol buffer compiler on a `.proto` that uses an `enum`, the generated code will have a corresponding `enum` for Java, Kotlin, or C++, or a special `EnumDescriptor` class for Python that’s used to create a set of symbolic constants with integer values in the runtime-generated class.

#### Important

The generated code may be subject to language-specific limitations on the number of enumerators (low thousands for one language). Review the limitations for the languages you plan to use.

#### Important

For information on how enums should work contrasted with how they currently work in different languages, see [Enum Behavior](https://protobuf.dev/programming-guides/enum).

Removing enum values is a breaking change for persisted protos. Instead of removing a value, mark the value with the `reserved` keyword to prevent the enum value from being code-generated, or keep the value but indicate that it will be removed later by using the `deprecated` field option:

    enum PhoneType {
      PHONE_TYPE_UNSPECIFIED = 0;
      PHONE_TYPE_MOBILE = 1;
      PHONE_TYPE_HOME = 2;
      PHONE_TYPE_WORK = 3 [deprecated = true];
      reserved 4,5;

For more information about how to work with message `enum`s in your applications, see the [generated code guide](https://protobuf.dev/reference/) for your chosen language.

### Reserved Values

If you update an enum type by entirely removing an enum entry, or commenting it out, future users can reuse the numeric value when making their own updates to the type. This can cause severe issues if they later load old instances of the same `.proto`, including data corruption, privacy bugs, and so on. One way to make sure this doesn’t happen is to specify that the numeric values (and/or names, which can also cause issues for JSON serialization) of your deleted entries are `reserved`. The protocol buffer compiler will complain if any future users try to use these identifiers. You can specify that your reserved numeric value range goes up to the maximum possible value using the `max` keyword.

    enum Foo {
      reserved 2, 15, 9 to 11, 40 to max;
      reserved "FOO", "BAR";

Note that you can’t mix field names and numeric values in the same `reserved` statement.

## Using Other Message Types

You can use other message types as field types. For example, let’s say you wanted to include `Result` messages in each `SearchResponse` message – to do this, you can define a `Result` message type in the same `.proto` and then specify a field of type `Result` in `SearchResponse`:

    message SearchResponse {
      repeated Result results = 1;

    message Result {
      optional string url = 1;
      optional string title = 2;
      repeated string snippets = 3;

### Importing Definitions

In the earlier example, the `Result` message type is defined in the same file as `SearchResponse` – what if the message type you want to use as a field type is already defined in another `.proto` file?

You can use definitions from other `.proto` files by _importing_ them. To import another `.proto`’s definitions, you add an import statement to the top of your file:

    import "myproject/other_protos.proto";

By default, you can use definitions only from directly imported `.proto` files. However, sometimes you may need to move a `.proto` file to a new location. Instead of moving the `.proto` file directly and updating all the call sites in a single change, you can put a placeholder `.proto` file in the old location to forward all the imports to the new location using the `import public` notion.

**Note that the public import functionality is not available in Java, Kotlin, TypeScript, JavaScript or GCL.**

`import public` dependencies can be transitively relied upon by any code importing the proto containing the `import public` statement. For example:

    // new.proto
    // All definitions are moved here

    // old.proto
    // This is the proto that all clients are importing.
    import public "new.proto";
    import "other.proto";

    // client.proto
    import "old.proto";
    // You use definitions from old.proto and new.proto, but not other.proto

The protocol compiler searches for imported files in a set of directories specified on the protocol compiler command line using the `-I`/`--proto_path` flag. If no flag was given, it looks in the directory in which the compiler was invoked. In general you should set the `--proto_path` flag to the root of your project and use fully qualified names for all imports.

### Using proto3 Message Types

It’s possible to import [proto3](https://protobuf.dev/programming-guides/proto3) and [edition 2023](https://protobuf.dev/programming-guides/editions) message types and use them in your proto2 messages, and vice versa. However, proto2 enums cannot be used directly in proto3 syntax (it’s okay if an imported proto2 message uses them).

## Nested Types

You can define and use message types inside other message types, as in the following example – here the `Result` message is defined inside the `SearchResponse` message:

    message SearchResponse {
      message Result {
        optional string url = 1;
        optional string title = 2;
        repeated string snippets = 3;
      repeated Result results = 1;

If you want to reuse this message type outside its parent message type, you refer to it as `_Parent_._Type_`:

    message SomeOtherMessage {
      optional SearchResponse.Result result = 1;

You can nest messages as deeply as you like. In the example below, note that the two nested types named `Inner` are entirely independent, since they are defined within different messages:

    message Outer {       // Level 0
      message MiddleAA {  // Level 1
        message Inner {   // Level 2
          optional int64 ival = 1;
          optional bool  booly = 2;
      message MiddleBB {  // Level 1
        message Inner {   // Level 2
          optional int32  ival = 1;
          optional bool   booly = 2;

### Groups

**Note that the groups feature is deprecated and should not be used when creating new message types. Use nested message types instead.**

Groups are another way to nest information in your message definitions. For example, another way to specify a `SearchResponse` containing a number of `Result`s is as follows:

    message SearchResponse {
      repeated group Result = 1 {
        optional string url = 1;
        optional string title = 2;
        repeated string snippets = 3;

A group simply combines a nested message type and a field into a single declaration. In your code, you can treat this message just as if it had a `Result` type field called `result` (the latter name is converted to lower-case so that it does not conflict with the former). Therefore, this example is exactly equivalent to the `SearchResponse` earlier, except that the message has a different [wire format](https://protobuf.dev/programming-guides/encoding).

## Updating a Message Type

If an existing message type no longer meets all your needs – for example, you’d like the message format to have an extra field – but you’d still like to use code created with the old format, don’t worry! It’s very simple to update message types without breaking any of your existing code when you use the binary wire format.

#### Note

If you use ProtoJSON or [proto text format](https://protobuf.dev/reference/protobuf/textformat-spec) to store your protocol buffer messages, the changes that you can make in your proto definition are different. The ProtoJSON wire format safe changes are described [here](https://protobuf.dev/programming-guides/json#json-wire-safety).

Check [Proto Best Practices](https://protobuf.dev/best-practices/dos-donts) and the following rules:

### Binary Wire-unsafe Changes

Wire-unsafe changes are schema changes that will break if you use parse data that was serialized using the old schema with a parser that is using the new schema (or vice versa). Only make wire-unsafe changes if you know that all serializers and deserializers of the data are using the new schema.

  * Changing field numbers for any existing field is not safe.
    * Changing the field number is equivalent to deleting the field and adding a new field with the same type. If you want to renumber a field, see the instructions for deleting a field.
  * Moving fields into an existing `oneof` is not safe.

### Binary Wire-safe Changes

Wire-safe changes are ones where it is fully safe to evolve the schema in this way without risk of data loss or new parse failures.

Note that any wire-safe changes may be a breaking change to application code in a given language. For example, adding a value to a preexisting enum would be a compilation break for any code with an exhaustive switch on that enum. For that reason, Google may avoid making some of these types of changes on public messages: the AIPs contain guidance for which of these changes are safe to make there.

  * Adding new fields is safe.
    * If you add new fields, any messages serialized by code using your “old” message format can still be parsed by your new generated code. You should keep in mind the default values for these elements so that new code can properly interact with messages generated by old code. Similarly, messages created by your new code can be parsed by your old code: old binaries simply ignore the new field when parsing. See the Unknown Fields section for details.
  * Removing fields is safe.
    * The same field number must not used again in your updated message type. You may want to rename the field instead, perhaps adding the prefix “OBSOLETE_”, or make the field number reserved, so that future users of your `.proto` can’t accidentally reuse the number.
  * Adding additional values to an enum is safe.
  * Changing a single explicit presence field or extension into a member of a **new** `oneof` is safe.
  * Changing a `oneof` which contains only one field to an explicit presence field is safe.
  * Changing a field into an extension of same number and type is safe.

### Binary Wire-compatible Changes (Conditionally Safe)

Unlike Wire-safe changes, wire-compatible means that the same data can be parsed both before and after a given change. However, a parse of the data may be lossy under this shape of change. For example, changing an int32 to an int64 is a compatible change, but if a value larger than INT32_MAX is written, a client that reads it as an int32 will discard the high order bits of the number.

You can make compatible changes to your schema only if you manage the roll out to your system carefully. For example, you may change an int32 to an int64 but ensure you continue to only write legal int32 values until the new schema is deployed to all endpoints, and then subsequently start writing larger values after that.

If your schema is published outside of your organization, you should generally not make wire-compatible changes, as you cannot manage the deployment of the new schema to know when the different range of values may be safe to use.

  * `int32`, `uint32`, `int64`, `uint64`, and `bool` are all compatible.
    * If a number is parsed from the wire which doesn’t fit in the corresponding type, you will get the same effect as if you had cast the number to that type in C++ (for example, if a 64-bit number is read as an int32, it will be truncated to 32 bits).
  * `sint32` and `sint64` are compatible with each other but are _not_ compatible with the other integer types.
    * If the value written was between INT_MIN and INT_MAX inclusive it will parse as the same value with either type. If an sint64 value was written outside of that range and parsed as an sint32, the varint is truncated to 32 bits and then zigzag decoding occurs (which will cause a different value to be observed).
  * `string` and `bytes` are compatible as long as the bytes are valid UTF-8.
  * Embedded messages are compatible with `bytes` if the bytes contain an encoded instance of the message.
  * `fixed32` is compatible with `sfixed32`, and `fixed64` with `sfixed64`.
  * For `string`, `bytes`, and message fields, singular is compatible with `repeated`.
    * Given serialized data of a repeated field as input, clients that expect this field to be singular will take the last input value if it’s a primitive type field or merge all input elements if it’s a message type field. Note that this is **not** generally safe for numeric types, including bools and enums. Repeated fields of numeric types are serialized in the [packed](https://protobuf.dev/programming-guides/encoding#packed) format by default, which will not be parsed correctly when a singular field is expected.
  * `enum` is compatible with `int32`, `uint32`, `int64`, and `uint64`
    * Be aware that client code may treat them differently when the message is deserialized: for example, unrecognized proto3 `enum` values will be preserved in the message, but how this is represented when the message is deserialized is language-dependent.
  * Changing a field between a `map<K, V>` and the corresponding `repeated` message field is binary compatible (see Maps, below, for the message layout and other restrictions).
    * However, the safety of the change is application-dependent: when deserializing and reserializing a message, clients using the `repeated` field definition will produce a semantically identical result; however, clients using the `map` field definition may reorder entries and drop entries with duplicate keys.

## Unknown Fields

Unknown fields are well-formed protocol buffer serialized data representing fields that the parser does not recognize. For example, when an old binary parses data sent by a new binary with new fields, those new fields become unknown fields in the old binary.

Proto2 messages preserve unknown fields and include them during parsing and in the serialized output.

### Retaining Unknown Fields

Some actions can cause unknown fields to be lost. For example, if you do one of the following, unknown fields are lost:

  * Serialize a proto to JSON.
  * Iterate over all of the fields in a message to populate a new message.

To avoid losing unknown fields, do the following:

  * Use binary; avoid using text formats for data exchange.
  * Use message-oriented APIs, such as `CopyFrom()` and `MergeFrom()`, to copy data rather than copying field-by-field

TextFormat is a bit of a special case. Serializing to TextFormat prints unknown fields using their field numbers. But parsing TextFormat data back into a binary proto fails if there are entries that use field numbers.

## Extensions

An extension is a field defined outside of its container message; usually in a `.proto` file separate from the container message’s `.proto` file.

### Why Use Extensions?

There are two main reasons to use extensions:

  * The container message’s `.proto` file will have fewer imports/dependencies. This can improve build times, break circular dependencies, and otherwise promote loose coupling. Extensions are very good for this.
  * Allow systems to attach data to a container message with minimal dependency and coordination. Extensions are not a great solution for this because of the limited field number space and the Consequences of Reusing Field Numbers. If your use case requires very low coordination for a large number of extensions, consider using the [`Any` message type](https://protobuf.dev/reference/protobuf/google.protobuf#any) instead.

### Example Extension

Let’s look at an example extension:

    // file kittens/video_ext.proto

    import "kittens/video.proto";
    import "media/user_content.proto";

    package kittens;

    // This extension allows kitten videos in a media.UserContent message.
    extend media.UserContent {
      // Video is a message imported from kittens/video.proto
      repeated Video kitten_videos = 126;

Note that the file defining the extension (`kittens/video_ext.proto`) imports the container message’s file (`media/user_content.proto`).

The container message must reserve a subset of its field numbers for extensions.

    // file media/user_content.proto

    package media;

    // A container message to hold stuff that a user has created.
    message UserContent {
      // Set verification to `DECLARATION` to enforce extension declarations for all
      // extensions in this range.
      extensions 100 to 199 [verification = DECLARATION];

The container message’s file (`media/user_content.proto`) defines the message `UserContent`, which reserves field numbers [100 to 199] for extensions. It is recommended to set `verification = DECLARATION` for the range to require declarations for all its extensions.

When the new extension (`kittens/video_ext.proto`) is added, a corresponding declaration should be added to `UserContent` and `verification` should be removed.

    // A container message to hold stuff that a user has created.
    message UserContent {
      extensions 100 to 199 [
        declaration = {
          number: 126,
          full_name: ".kittens.kitten_videos",
          type: ".kittens.Video",
          repeated: true
      ];

`UserContent` declares that field number `126` will be used by a `repeated` extension field with the fully-qualified name `.kittens.kitten_videos` and the fully-qualified type `.kittens.Video`. To learn more about extension declarations see [Extension Declarations](https://protobuf.dev/programming-guides/extension_declarations).

Note that the container message’s file (`media/user_content.proto`) **does not** import the kitten_video extension definition (`kittens/video_ext.proto`)

There is no difference in the wire-format encoding of extension fields as compared to a standard field with the same field number, type, and cardinality. Therefore, it is safe to move a standard field out of its container to be an extension or to move an extension field into its container message as a standard field so long as the field number, type, and cardinality remain constant.

However, because extensions are defined outside of the container message, no specialized accessors are generated to get and set specific extension fields. For our example, the protobuf compiler **will not generate** `AddKittenVideos()` or `GetKittenVideos()` accessors. Instead, extensions are accessed through parameterized functions like: `HasExtension()`, `ClearExtension()`, `GetExtension()`, `MutableExtension()`, and `AddExtension()`.

In C++, it would look something like:

    UserContent user_content;
    user_content.AddExtension(kittens::kitten_videos, new kittens::Video());
    assert(1 == user_content.GetExtensionCount(kittens::kitten_videos));
    user_content.GetExtension(kittens::kitten_videos, 0);

### Defining Extension Ranges

If you are the owner of a container message, you will need to define an extension range for the extensions to your message.

Field numbers allocated to extension fields cannot be reused for standard fields.

It is safe to expand an extension range after it is defined. A good default is to allocate 1000 relatively small numbers, and densely populate that space using extension declarations:

    message ModernExtendableMessage {
      // All extensions in this range should use extension declarations.
      extensions 1000 to 2000 [verification = DECLARATION];

When adding a range for extension declarations before the actual extensions, you should add `verification = DECLARATION` to enforce that declarations are used for this new range. This placeholder can be removed once an actual declaration is added.

It is safe to split an existing extension range into separate ranges that cover the same total range. This might be necessary for migrating a legacy message type to [Extension Declarations](https://protobuf.dev/programming-guides/extension_declarations). For example, before migration, the range might be defined as:

    message LegacyMessage {
      extensions 1000 to max;

And after migration (splitting the range) it can be:

    message LegacyMessage {
      // Legacy range that was using an unverified allocation scheme.
      extensions 1000 to 524999999 [verification = UNVERIFIED];
      // Current range that uses extension declarations.
      extensions 525000000 to max  [verification = DECLARATION];

It is not safe to increase the start field number nor decrease the end field number to move or shrink an extension range. These changes can invalidate an existing extension.

Prefer using field numbers 1 to 15 for standard fields that are populated in most instances of your proto. It is not recommended to use these numbers for extensions.

If your numbering convention might involve extensions having very large field numbers, you can specify that your extension range goes up to the maximum possible field number using the `max` keyword:

    message Foo {
      extensions 1000 to max;

`max` is 229 \- 1, or 536,870,911.

### Choosing Extension Numbers

Extensions are just fields that can be specified outside of their container messages. All the same rules for Assigning Field Numbers apply to extension field numbers. The same Consequences of Reusing Field Numbers also apply to reusing extension field numbers.

Choosing unique extension field numbers is simple if the container message uses [extension declarations](https://protobuf.dev/programming-guides/extension_declarations). When defining a new extension, choose the lowest field number above all other declarations from the highest extension range defined in the container message. For example, if a container message is defined like this:

    message Container {
      // Legacy range that was using an unverified allocation scheme
      extensions 1000 to 524999999;
      // Current range that uses extension declarations. (highest extension range)
      extensions 525000000 to max  [
        declaration = {
          number: 525000001,
          full_name: ".bar.baz_ext",
          type: ".bar.Baz"
        // 525,000,002 is the lowest field number above all other declarations
      ];

The next extension of `Container` should add a new declaration with the number `525000002`.

#### Unverified Extension Number Allocation (not recommended)

The owner of a container message may choose to forgo extension declarations in favor of their own unverified extension number allocation strategy.

An unverified allocation scheme uses a mechanism external to the protobuf ecosystem to allocate extension field numbers within the selected extension range. One example could be using a monorepo’s commit number. This system is “unverified” from the protobuf compiler’s point of view since there is no way to check that an extension is using a properly acquired extension field number.

The benefit of an unverified system over a verified system like extension declarations is the ability to define an extension without coordinating with the container message owner.

The downside of an unverified system is that the protobuf compiler cannot protect participants from reusing extension field numbers.

**Unverified extension field number allocation strategies are not recommended** because the Consequences of Reusing Field Numbers fall on all extenders of a message (not just the developer that didn’t follow the recommendations). If your use case requires very low coordination, consider using the [`Any` message](https://protobuf.dev/reference/protobuf/google.protobuf#any) instead.

Unverified extension field number allocation strategies are limited to the range 1 to 524,999,999. Field numbers 525,000,000 and above can only be used with extension declarations.

### Specifying Extension Types

Extensions can be of any field type except `oneof`s and `map`s.

### Nested Extensions (not recommended)

You can declare extensions in the scope of another message:

    import "common/user_profile.proto";

    package puppies;

    message Photo {
      extend common.UserProfile {
        optional int32 likes_count = 111;
      ...

In this case, the C++ code to access this extension is:

    UserProfile user_profile;
    user_profile.SetExtension(puppies::Photo::likes_count, 42);

In other words, the only effect is that `likes_count` is defined within the scope of `puppies.Photo`.

This is a common source of confusion: Declaring an `extend` block nested inside a message type _does not_ imply any relationship between the outer type and the extended type. In particular, the earlier example _does not_ mean that `Photo` is any sort of subclass of `UserProfile`. All it means is that the symbol `likes_count` is declared inside the scope of `Photo`; it’s simply a static member.

A common pattern is to define extensions inside the scope of the extension’s field type - for example, here’s an extension to `media.UserContent` of type `puppies.Photo`, where the extension is defined as part of `Photo`:

    import "media/user_content.proto";

    package puppies;

    message Photo {
      extend media.UserContent {
        optional Photo puppy_photo = 127;
      ...

However, there is no requirement that an extension with a message type be defined inside that type. You can also use the standard definition pattern:

    import "media/user_content.proto";

    package puppies;

    message Photo {
      ...

    // This can even be in a different file.
    extend media.UserContent {
      optional Photo puppy_photo = 127;

This **standard (file-level) syntax is preferred** to avoid confusion. The nested syntax is often mistaken for subclassing by users who are not already familiar with extensions.

## Any

The `Any` message type lets you use messages as embedded types without having their .proto definition. An `Any` contains an arbitrary serialized message as `bytes`, along with a URL that acts as a globally unique identifier for and resolves to that message’s type. To use the `Any` type, you need to import `google/protobuf/any.proto`.

    import "google/protobuf/any.proto";

    message ErrorStatus {
      string message = 1;
      repeated google.protobuf.Any details = 2;

The default type URL for a given message type is `type.googleapis.com/_packagename_._messagename_`.

Different language implementations will support runtime library helpers to pack and unpack `Any` values in a typesafe manner – for example, in Java, the `Any` type will have special `pack()` and `unpack()` accessors, while in C++ there are `PackFrom()` and `UnpackTo()` methods:

    // Storing an arbitrary message type in Any.
    NetworkErrorDetails details = ...;
    ErrorStatus status;
    status.add_details()->PackFrom(details);

    // Reading an arbitrary message from Any.
    ErrorStatus status = ...;
    for (const google::protobuf::Any& detail : status.details()) {
      if (detail.Is<NetworkErrorDetails>()) {
        NetworkErrorDetails network_error;
        detail.UnpackTo(&network_error);
        ... processing network_error ...

If you want to limit contained messages to a small number of types and to require permission before adding new types to the list, consider using extensions with [extension declarations](https://protobuf.dev/programming-guides/extension_declarations) instead of `Any` message types.

## Oneof

If you have a message with many optional fields and where at most one field will be set at the same time, you can enforce this behavior and save memory by using the oneof feature.

Oneof fields are like optional fields except all the fields in a oneof share memory, and at most one field can be set at the same time. Setting any member of the oneof automatically clears all the other members. You can check which value in a oneof is set (if any) using a special `case()` or `WhichOneof()` method, depending on your chosen language.

Note that if _multiple values are set, the last set value as determined by the order in the proto will overwrite all previous ones_.

Field numbers for oneof fields must be unique within the enclosing message.

### Using Oneof

To define a oneof in your `.proto` you use the `oneof` keyword followed by your oneof name, in this case `test_oneof`:

    message SampleMessage {
      oneof test_oneof {
         string name = 4;
         SubMessage sub_message = 9;

You then add your oneof fields to the oneof definition. You can add fields of any type except `map` fields, but you cannot use the `required`, `optional`, or `repeated` keywords. If you need to add a repeated field to a oneof, you can use a message containing the repeated field.

In your generated code, oneof fields have the same getters and setters as regular `optional` fields. You also get a special method for checking which value (if any) in the oneof is set. You can find out more about the oneof API for your chosen language in the relevant [API reference](https://protobuf.dev/reference/).

### Oneof Features

  * Setting a oneof field will automatically clear all other members of the oneof. So if you set several oneof fields, only the _last_ field you set will still have a value.

        SampleMessage message;
        message.set_name("name");
        CHECK(message.has_name());
        // Calling mutable_sub_message() will clear the name field and will set
        // sub_message to a new instance of SubMessage with none of its fields set.
        message.mutable_sub_message();
        CHECK(!message.has_name());

  * If the parser encounters multiple members of the same oneof on the wire, only the last member seen is used in the parsed message. When parsing data on the wire, starting at the beginning of the bytes, evaluate the next value, and apply the following parsing rules:

    * First, check if a _different_ field in the same oneof is currently set, and if so clear it.

    * Then apply the contents as though the field was not in a oneof:

      * A primitive will overwrite any value already set
      * A message will merge into any value already set
  * Extensions are not supported for oneof.

  * A oneof cannot be `repeated`.

  * Reflection APIs work for oneof fields.

  * If you set a oneof field to the default value (such as setting an int32 oneof field to 0), the “case” of that oneof field will be set, and the value will be serialized on the wire.

  * If you’re using C++, make sure your code doesn’t cause memory crashes. The following sample code will crash because `sub_message` was already deleted by calling the `set_name()` method.

        SampleMessage message;
        SubMessage* sub_message = message.mutable_sub_message();
        message.set_name("name");      // Will delete sub_message
        sub_message->set_...            // Crashes here

  * Again in C++, if you `Swap()` two messages with oneofs, each message will end up with the other’s oneof case: in the example below, `msg1` will have a `sub_message` and `msg2` will have a `name`.

        SampleMessage msg1;
        msg1.set_name("name");
        SampleMessage msg2;
        msg2.mutable_sub_message();
        msg1.swap(&msg2);
        CHECK(msg1.has_sub_message());
        CHECK(msg2.has_name());

### Backwards-compatibility issues

Be careful when adding or removing oneof fields. If checking the value of a oneof returns `None`/`NOT_SET`, it could mean that the oneof has not been set or it has been set to a field in a different version of the oneof. There is no way to tell the difference, since there’s no way to know if an unknown field on the wire is a member of the oneof.

#### Tag Reuse Issues

  * **Move optional fields into or out of a oneof**: You may lose some of your information (some fields will be cleared) after the message is serialized and parsed. However, you can safely move a single field into a **new** oneof and may be able to move multiple fields if it is known that only one is ever set. See Updating A Message Type for further details.
  * **Delete a oneof field and add it back**: This may clear your currently set oneof field after the message is serialized and parsed.
  * **Split or merge oneof**: This has similar issues to moving `optional` fields.

## Maps

If you want to create an associative map as part of your data definition, protocol buffers provides a handy shortcut syntax:

    map<key_type, value_type> map_field = N;

…where the `key_type` can be any integral or string type (so, any scalar type except for floating point types and `bytes`). Note that neither enum nor proto messages are valid for `key_type`. The `value_type` can be any type except another map.

So, for example, if you wanted to create a map of projects where each `Project` message is associated with a string key, you could define it like this:

    map<string, Project> projects = 3;

### Maps Features

  * Extensions are not supported for maps.
  * Maps cannot be `repeated`, `optional`, or `required`.
  * Wire format ordering and map iteration ordering of map values is undefined, so you cannot rely on your map items being in a particular order.
  * When generating text format for a `.proto`, maps are sorted by key. Numeric keys are sorted numerically.
  * When parsing from the wire or when merging, if there are duplicate map keys the last key seen is used. When parsing a map from text format, parsing may fail if there are duplicate keys.
  * If you provide a key but no value for a map field, the behavior when the field is serialized is language-dependent. In C++, Java, Kotlin, and Python the default value for the type is serialized, while in other languages nothing is serialized.
  * No symbol `FooEntry` can exist in the same scope as a map `foo`, because `FooEntry` is already used by the implementation of the map.

The generated map API is currently available for all supported languages. You can find out more about the map API for your chosen language in the relevant [API reference](https://protobuf.dev/reference/).

### Backwards Compatibility

The map syntax is equivalent to the following on the wire, so protocol buffers implementations that do not support maps can still handle your data:

    message MapFieldEntry {
      optional key_type key = 1;
      optional value_type value = 2;

    repeated MapFieldEntry map_field = N;

Any protocol buffers implementation that supports maps must both produce and accept data that can be accepted by the earlier definition.

## Packages

You can add an optional `package` specifier to a `.proto` file to prevent name clashes between protocol message types.

    package foo.bar;
    message Open { ... }

You can then use the package specifier when defining fields of your message type:

    message Foo {
      ...
      optional foo.bar.Open open = 1;
      ...

The way a package specifier affects the generated code depends on your chosen language:

  * In **C++** the generated classes are wrapped inside a C++ namespace. For example, `Open` would be in the namespace `foo::bar`.
  * In **Java** and **Kotlin**, the package is used as the Java package, unless you explicitly provide an `option java_package` in your `.proto` file.
  * In **Python**, the `package` directive is ignored, since Python modules are organized according to their location in the file system.
  * In **Go**, the `package` directive is ignored, and the generated `.pb.go` file is in the package named after the corresponding `go_proto_library` Bazel rule. For open source projects, you **must** provide either a `go_package` option or set the Bazel `-M` flag.
  * In **Ruby**, the generated classes are wrapped inside nested Ruby namespaces, converted to the required Ruby capitalization style (first letter capitalized; if the first character is not a letter, `PB_` is prepended). For example, `Open` would be in the namespace `Foo::Bar`.
  * In **PHP** the package is used as the namespace after converting to PascalCase, unless you explicitly provide an `option php_namespace` in your `.proto` file. For example, `Open` would be in the namespace `Foo\Bar`.
  * In **C#** the package is used as the namespace after converting to PascalCase, unless you explicitly provide an `option csharp_namespace` in your `.proto` file. For example, `Open` would be in the namespace `Foo.Bar`.

Note that even when the `package` directive does not directly affect the generated code, for example in Python, it is still strongly recommended to specify the package for the `.proto` file, as otherwise it may lead to naming conflicts in descriptors and make the proto not portable for other languages.

### Packages and Name Resolution

Type name resolution in the protocol buffer language works like C++: first the innermost scope is searched, then the next-innermost, and so on, with each package considered to be “inner” to its parent package. A leading ‘.’ (for example, `.foo.bar.Baz`) means to start from the outermost scope instead.

The protocol buffer compiler resolves all type names by parsing the imported `.proto` files. The code generator for each language knows how to refer to each type in that language, even if it has different scoping rules.

## Defining Services

If you want to use your message types with an RPC (Remote Procedure Call) system, you can define an RPC service interface in a `.proto` file and the protocol buffer compiler will generate service interface code and stubs in your chosen language. So, for example, if you want to define an RPC service with a method that takes your `SearchRequest` and returns a `SearchResponse`, you can define it in your `.proto` file as follows:

    service SearchService {
      rpc Search(SearchRequest) returns (SearchResponse);

By default, the protocol compiler will then generate an abstract interface called `SearchService` and a corresponding “stub” implementation. The stub forwards all calls to an `RpcChannel`, which in turn is an abstract interface that you must define yourself in terms of your own RPC system. For example, you might implement an `RpcChannel` which serializes the message and sends it to a server via HTTP. In other words, the generated stub provides a type-safe interface for making protocol-buffer-based RPC calls, without locking you into any particular RPC implementation. So, in C++, you might end up with code like this:

    using google::protobuf;

    protobuf::RpcChannel* channel;
    protobuf::RpcController* controller;
    SearchService* service;
    SearchRequest request;
    SearchResponse response;

    void DoSearch() {
      // You provide classes MyRpcChannel and MyRpcController, which implement
      // the abstract interfaces protobuf::RpcChannel and protobuf::RpcController.
      channel = new MyRpcChannel("somehost.example.com:1234");
      controller = new MyRpcController;

      // The protocol compiler generates the SearchService class based on the
      // definition given earlier.
      service = new SearchService::Stub(channel);

      // Set up the request.
      request.set_query("protocol buffers");

      // Execute the RPC.
      service->Search(controller, &request, &response,
                      protobuf::NewCallback(&Done));

    void Done() {
      delete service;
      delete channel;
      delete controller;

All service classes also implement the `Service` interface, which provides a way to call specific methods without knowing the method name or its input and output types at compile time. On the server side, this can be used to implement an RPC server with which you could register services.

    using google::protobuf;

    class ExampleSearchService : public SearchService {
     public:
      void Search(protobuf::RpcController* controller,
                  const SearchRequest* request,
                  SearchResponse* response,
                  protobuf::Closure* done) {
        if (request->query() == "google") {
          response->add_result()->set_url("http://www.google.com");
        } else if (request->query() == "protocol buffers") {
          response->add_result()->set_url("http://protobuf.googlecode.com");
        done->Run();
    };

    int main() {
      // You provide class MyRpcServer.  It does not have to implement any
      // particular interface; this is just an example.
      MyRpcServer server;

      protobuf::Service* service = new ExampleSearchService;
      server.ExportOnPort(1234, service);
      server.Run();

      delete service;
      return 0;

If you don’t want to plug in your own existing RPC system, you can use [gRPC](https://github.com/grpc/grpc-common): a language- and platform-neutral open source RPC system developed at Google. gRPC works particularly well with protocol buffers and lets you generate the relevant RPC code directly from your `.proto` files using a special protocol buffer compiler plugin. However, as there are potential compatibility issues between clients and servers generated with proto2 and proto3, we recommend that you use proto3 or edition 2023 for defining gRPC services. You can find out more about proto3 syntax in the [Proto3 Language Guide](https://protobuf.dev/programming-guides/proto3) and about edition 2023 in [Edition 2023 Language Guide](https://protobuf.dev/programming-guides/editions).

In addition to gRPC, there are also a number of ongoing third-party projects to develop RPC implementations for Protocol Buffers. For a list of links to projects we know about, see the [third-party add-ons wiki page](https://github.com/protocolbuffers/protobuf/blob/master/docs/third_party.md).

## JSON Mapping

The standard protobuf binary wire format is the preferred serialization format for communication between two systems that use protobufs. For communicating with systems that use JSON rather than protobuf wire format, Protobuf supports a canonical encoding in [JSON](https://protobuf.dev/programming-guides/json).

## Options

Individual declarations in a `.proto` file can be annotated with a number of _options_. Options do not change the overall meaning of a declaration, but may affect the way it is handled in a particular context. The complete list of available options is defined in [`/google/protobuf/descriptor.proto`](https://github.com/protocolbuffers/protobuf/blob/main/src/google/protobuf/descriptor.proto).

Some options are file-level options, meaning they should be written at the top-level scope, not inside any message, enum, or service definition. Some options are message-level options, meaning they should be written inside message definitions. Some options are field-level options, meaning they should be written inside field definitions. Options can also be written on enum types, enum values, oneof fields, service types, and service methods; however, no useful options currently exist for any of these.

Here are a few of the most commonly used options:

  * `java_package` (file option): The package you want to use for your generated Java/Kotlin classes. If no explicit `java_package` option is given in the `.proto` file, then by default the proto package (specified using the “package” keyword in the `.proto` file) will be used. However, proto packages generally do not make good Java packages since proto packages are not expected to start with reverse domain names. If not generating Java or Kotlin code, this option has no effect.

        option java_package = "com.example.foo";

  * `java_outer_classname` (file option): The class name (and hence the file name) for the wrapper Java class you want to generate. If no explicit `java_outer_classname` is specified in the `.proto` file, the class name will be constructed by converting the `.proto` file name to camel-case (so `foo_bar.proto` becomes `FooBar.java`). If the `java_multiple_files` option is disabled, then all other classes/enums/etc. generated for the `.proto` file will be generated _within_ this outer wrapper Java class as nested classes/enums/etc. If not generating Java code, this option has no effect.

        option java_outer_classname = "Ponycopter";

  * `java_multiple_files` (file option): If false, only a single `.java` file will be generated for this `.proto` file, and all the Java classes/enums/etc. generated for the top-level messages, services, and enumerations will be nested inside of an outer class (see `java_outer_classname`). If true, separate `.java` files will be generated for each of the Java classes/enums/etc. generated for the top-level messages, services, and enumerations, and the wrapper Java class generated for this `.proto` file won’t contain any nested classes/enums/etc. This is a Boolean option which defaults to `false`. If not generating Java code, this option has no effect.

        option java_multiple_files = true;

  * `optimize_for` (file option): Can be set to `SPEED`, `CODE_SIZE`, or `LITE_RUNTIME`. This affects the C++ and Java code generators (and possibly third-party generators) in the following ways:

    * `SPEED` (default): The protocol buffer compiler will generate code for serializing, parsing, and performing other common operations on your message types. This code is highly optimized.
    * `CODE_SIZE`: The protocol buffer compiler will generate minimal classes and will rely on shared, reflection-based code to implement serialization, parsing, and various other operations. The generated code will thus be much smaller than with `SPEED`, but operations will be slower. Classes will still implement exactly the same public API as they do in `SPEED` mode. This mode is most useful in apps that contain a very large number of `.proto` files and do not need all of them to be blindingly fast.
    * `LITE_RUNTIME`: The protocol buffer compiler will generate classes that depend only on the “lite” runtime library (`libprotobuf-lite` instead of `libprotobuf`). The lite runtime is much smaller than the full library (around an order of magnitude smaller) but omits certain features like descriptors and reflection. This is particularly useful for apps running on constrained platforms like mobile phones. The compiler will still generate fast implementations of all methods as it does in `SPEED` mode. Generated classes will only implement the `MessageLite` interface in each language, which provides only a subset of the methods of the full `Message` interface.

    option optimize_for = CODE_SIZE;

  * `cc_generic_services`, `java_generic_services`, `py_generic_services` (file options): **Generic services are deprecated.** Whether or not the protocol buffer compiler should generate abstract service code based on services definitions in C++, Java, and Python, respectively. For legacy reasons, these default to `true`. However, as of version 2.3.0 (January 2010), it is considered preferable for RPC implementations to provide [code generator plugins](https://protobuf.dev/reference/cpp/api-docs/google.protobuf.compiler.plugin.pb) to generate code more specific to each system, rather than rely on the “abstract” services.

        // This file relies on plugins to generate service code.
        option cc_generic_services = false;
        option java_generic_services = false;
        option py_generic_services = false;

  * `cc_enable_arenas` (file option): Enables [arena allocation](https://protobuf.dev/reference/cpp/arenas) for C++ generated code.

  * `objc_class_prefix` (file option): Sets the Objective-C class prefix which is prepended to all Objective-C generated classes and enums from this .proto. There is no default. You should use prefixes that are between 3-5 uppercase characters as [recommended by Apple](https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html#//apple_ref/doc/uid/TP40011210-CH10-SW4). Note that all 2 letter prefixes are reserved by Apple.

  * `message_set_wire_format` (message option): If set to `true`, the message uses a different binary format intended to be compatible with an old format used inside Google called `MessageSet`. Users outside Google will probably never need to use this option. The message must be declared exactly as follows:

        message Foo {
          option message_set_wire_format = true;
          extensions 4 to max;

  * `packed` (field option): If set to `true` on a repeated field of a basic numeric type, it causes a more compact [encoding](https://protobuf.dev/programming-guides/encoding#packed) to be used. The only reason to not use this option is if you need compatibility with parsers prior to version 2.3.0. These older parsers would ignore packed data when it was not expected. Therefore, it was not possible to change an existing field to packed format without breaking wire compatibility. In 2.3.0 and later, this change is safe, as parsers for packable fields will always accept both formats, but be careful if you have to deal with old programs using old protobuf versions.

        repeated int32 samples = 4 [packed = true];

  * `deprecated` (field option): If set to `true`, indicates that the field is deprecated and should not be used by new code. In most languages this has no actual effect. In Java, this becomes a `@Deprecated` annotation. For C++, clang-tidy will generate warnings whenever deprecated fields are used. In the future, other language-specific code generators may generate deprecation annotations on the field’s accessors, which will in turn cause a warning to be emitted when compiling code which attempts to use the field. If the field is not used by anyone and you want to prevent new users from using it, consider replacing the field declaration with a reserved statement.

        optional int32 old_field = 6 [deprecated = true];

### Enum Value Options

Enum value options are supported. You can use the `deprecated` option to indicate that a value shouldn’t be used anymore. You can also create custom options using extensions.

The following example shows the syntax for adding these options:

    import "google/protobuf/descriptor.proto";

    extend google.protobuf.EnumValueOptions {
      optional string string_name = 123456789;

    enum Data {
      DATA_UNSPECIFIED = 0;
      DATA_SEARCH = 1 [deprecated = true];
      DATA_DISPLAY = 2 [
        (string_name) = "display_value"
      ];

The C++ code to read the `string_name` option might look something like this:

    const absl::string_view foo = proto2::GetEnumDescriptor<Data>()
        ->FindValueByName("DATA_DISPLAY")->options().GetExtension(string_name);

See Custom Options to see how to apply custom options to enum values and to fields.

### Custom Options

Protocol Buffers also allows you to define and use your own options. Note that this is an **advanced feature** which most people don’t need. Since options are defined by the messages defined in `google/protobuf/descriptor.proto` (like `FileOptions` or `FieldOptions`), defining your own options is simply a matter of extending those messages. For example:

    import "google/protobuf/descriptor.proto";

    extend google.protobuf.MessageOptions {
      optional string my_option = 51234;

    message MyMessage {
      option (my_option) = "Hello world!";

Here we have defined a new message-level option by extending `MessageOptions`. When we then use the option, the option name must be enclosed in parentheses to indicate that it is an extension. We can now read the value of `my_option` in C++ like so:

    string value = MyMessage::descriptor()->options().GetExtension(my_option);

Here, `MyMessage::descriptor()->options()` returns the `MessageOptions` protocol message for `MyMessage`. Reading custom options from it is just like reading any other extension.

Similarly, in Java we would write:

    String value = MyProtoFile.MyMessage.getDescriptor().getOptions()
      .getExtension(MyProtoFile.myOption);

In Python it would be:

    value = my_proto_file_pb2.MyMessage.DESCRIPTOR.GetOptions()
      .Extensions[my_proto_file_pb2.my_option]

Custom options can be defined for every kind of construct in the Protocol Buffers language. Here is an example that uses every kind of option:

    import "google/protobuf/descriptor.proto";

    extend google.protobuf.FileOptions {
      optional string my_file_option = 50000;
    extend google.protobuf.MessageOptions {
      optional int32 my_message_option = 50001;
    extend google.protobuf.FieldOptions {
      optional float my_field_option = 50002;
    extend google.protobuf.OneofOptions {
      optional int64 my_oneof_option = 50003;
    extend google.protobuf.EnumOptions {
      optional bool my_enum_option = 50004;
    extend google.protobuf.EnumValueOptions {
      optional uint32 my_enum_value_option = 50005;
    extend google.protobuf.ServiceOptions {
      optional MyEnum my_service_option = 50006;
    extend google.protobuf.MethodOptions {
      optional MyMessage my_method_option = 50007;

    option (my_file_option) = "Hello world!";

    message MyMessage {
      option (my_message_option) = 1234;

      optional int32 foo = 1 [(my_field_option) = 4.5];
      optional string bar = 2;
      oneof qux {
        option (my_oneof_option) = 42;

        string quux = 3;

    enum MyEnum {
      option (my_enum_option) = true;

      FOO = 1 [(my_enum_value_option) = 321];
      BAR = 2;

    message RequestType {}
    message ResponseType {}

    service MyService {
      option (my_service_option) = FOO;

      rpc MyMethod(RequestType) returns(ResponseType) {
        // Note:  my_method_option has type MyMessage.  We can set each field
        //   within it using a separate "option" line.
        option (my_method_option).foo = 567;
        option (my_method_option).bar = "Some string";

Note that if you want to use a custom option in a package other than the one in which it was defined, you must prefix the option name with the package name, just as you would for type names. For example:

    // foo.proto
    import "google/protobuf/descriptor.proto";
    package foo;
    extend google.protobuf.MessageOptions {
      optional string my_option = 51234;

    // bar.proto
    import "foo.proto";
    package bar;
    message MyMessage {
      option (foo.my_option) = "Hello world!";

One last thing: Since custom options are extensions, they must be assigned field numbers like any other field or extension. In the examples earlier, we have used field numbers in the range 50000-99999. This range is reserved for internal use within individual organizations, so you can use numbers in this range freely for in-house applications. If you intend to use custom options in public applications, however, then it is important that you make sure that your field numbers are globally unique. To obtain globally unique field numbers, send a request to add an entry to [protobuf global extension registry](https://github.com/protocolbuffers/protobuf/blob/master/docs/options.md). Usually you only need one extension number. You can declare multiple options with only one extension number by putting them in a sub-message:

    message FooOptions {
      optional int32 opt1 = 1;
      optional string opt2 = 2;

    extend google.protobuf.FieldOptions {
      optional FooOptions foo_options = 1234;

    // usage:
    message Bar {
      optional int32 a = 1 [(foo_options).opt1 = 123, (foo_options).opt2 = "baz"];
      // alternative aggregate syntax (uses TextFormat):
      optional int32 b = 2 [(foo_options) = { opt1: 123 opt2: "baz" }];

Also, note that each option type (file-level, message-level, field-level, etc.) has its own number space, so, for example, you could declare extensions of FieldOptions and MessageOptions with the same number.

### Option Retention

Options have a notion of _retention_ , which controls whether an option is retained in the generated code. Options have _runtime retention_ by default, meaning that they are retained in the generated code and are thus visible at runtime in the generated descriptor pool. However, you can set `retention = RETENTION_SOURCE` to specify that an option (or field within an option) must not be retained at runtime. This is called _source retention_.

Option retention is an advanced feature that most users should not need to worry about, but it can be useful if you would like to use certain options without paying the code size cost of retaining them in your binaries. Options with source retention are still visible to `protoc` and `protoc` plugins, so code generators can use them to customize their behavior.

Retention can be set directly on an option, like this:

    extend google.protobuf.FileOptions {
      optional int32 source_retention_option = 1234
          [retention = RETENTION_SOURCE];

It can also be set on a plain field, in which case it takes effect only when that field appears inside an option:

    message OptionsMessage {
      optional int32 source_retention_field = 1 [retention = RETENTION_SOURCE];

You can set `retention = RETENTION_RUNTIME` if you like, but this has no effect since it is the default behavior. When a message field is marked `RETENTION_SOURCE`, its entire contents are dropped; fields inside it cannot override that by trying to set `RETENTION_RUNTIME`.

#### Note

As of Protocol Buffers 22.0, support for option retention is still in progress and only C++ and Java are supported. Go has support starting from 1.29.0. Python support is complete but has not made it into a release yet.

### Option Targets

Fields have a `targets` option which controls the types of entities that the field may apply to when used as an option. For example, if a field has `targets = TARGET_TYPE_MESSAGE` then that field cannot be set in a custom option on an enum (or any other non-message entity). Protoc enforces this and will raise an error if there is a violation of the target constraints.

At first glance, this feature may seem unnecessary given that every custom option is an extension of the options message for a specific entity, which already constrains the option to that one entity. However, option targets are useful in the case where you have a shared options message applied to multiple entity types and you want to control the usage of individual fields in that message. For example:

    message MyOptions {
      optional string file_only_option = 1 [targets = TARGET_TYPE_FILE];
      optional int32 message_and_enum_option = 2 [targets = TARGET_TYPE_MESSAGE,
                                                  targets = TARGET_TYPE_ENUM];

    extend google.protobuf.FileOptions {
      optional MyOptions file_options = 50000;

    extend google.protobuf.MessageOptions {
      optional MyOptions message_options = 50000;

    extend google.protobuf.EnumOptions {
      optional MyOptions enum_options = 50000;

    // OK: this field is allowed on file options
    option (file_options).file_only_option = "abc";

    message MyMessage {
      // OK: this field is allowed on both message and enum options
      option (message_options).message_and_enum_option = 42;

    enum MyEnum {
      MY_ENUM_UNSPECIFIED = 0;
      // Error: file_only_option cannot be set on an enum.
      option (enum_options).file_only_option = "xyz";

## Generating Your Classes

To generate the Java, Kotlin, Python, C++, Go, Ruby, Objective-C, or C# code that you need to work with the message types defined in a `.proto` file, you need to run the protocol buffer compiler `protoc` on the `.proto` file. If you haven’t installed the compiler, [download the package](https://protobuf.dev/downloads) and follow the instructions in the README. For Go, you also need to install a special code generator plugin for the compiler; you can find this and installation instructions in the [golang/protobuf](https://github.com/golang/protobuf/) repository on GitHub.

The Protocol Compiler is invoked as follows:

    protoc --proto_path=IMPORT_PATH --cpp_out=DST_DIR --java_out=DST_DIR --python_out=DST_DIR --go_out=DST_DIR --ruby_out=DST_DIR --objc_out=DST_DIR --csharp_out=DST_DIR path/to/file.proto

  * `IMPORT_PATH` specifies a directory in which to look for `.proto` files when resolving `import` directives. If omitted, the current directory is used. Multiple import directories can be specified by passing the `--proto_path` option multiple times; they will be searched in order. `-I=_IMPORT_PATH_` can be used as a short form of `--proto_path`.

**Note:** File paths relative to their `proto_path` must be globally unique in a given binary. For example, if you have `proto/lib1/data.proto` and `proto/lib2/data.proto`, those two files cannot be used together with `-I=proto/lib1 -I=proto/lib2` because it would be ambiguous which file `import "data.proto"` will mean. Instead `-Iproto/` should be used and the global names will be `lib1/data.proto` and `lib2/data.proto`.

If you are publishing a library and other users may use your messages directly, you should include a unique library name in the path that they are expected to be used under to avoid file name collisions. If you have multiple directories in one project, it is best practice to prefer setting one `-I` to a top level directory of the project.

  * You can provide one or more _output directives_ :

    * `--cpp_out` generates C++ code in `DST_DIR`. See the [C++ generated code reference](https://protobuf.dev/reference/cpp/cpp-generated) for more.
    * `--java_out` generates Java code in `DST_DIR`. See the [Java generated code reference](https://protobuf.dev/reference/java/java-generated) for more.
    * `--kotlin_out` generates additional Kotlin code in `DST_DIR`. See the [Kotlin generated code reference](https://protobuf.dev/reference/kotlin/kotlin-generated) for more.
    * `--python_out` generates Python code in `DST_DIR`. See the [Python generated code reference](https://protobuf.dev/reference/python/python-generated) for more.
    * `--go_out` generates Go code in `DST_DIR`. See the [Go generated code reference](https://protobuf.dev/reference/go/go-generated-opaque) for more.
    * `--ruby_out` generates Ruby code in `DST_DIR`. See the [Ruby generated code reference](https://protobuf.dev/reference/ruby/ruby-generated) for more.
    * `--objc_out` generates Objective-C code in `DST_DIR`. See the [Objective-C generated code reference](https://protobuf.dev/reference/objective-c/objective-c-generated) for more.
    * `--csharp_out` generates C# code in `DST_DIR`. See the [C# generated code reference](https://protobuf.dev/reference/csharp/csharp-generated) for more.
    * `--php_out` generates PHP code in `DST_DIR`. See the [PHP generated code reference](https://protobuf.dev/reference/php/php-generated) for more.

As an extra convenience, if the `DST_DIR` ends in `.zip` or `.jar`, the compiler will write the output to a single ZIP-format archive file with the given name. `.jar` outputs will also be given a manifest file as required by the Java JAR specification. Note that if the output archive already exists, it will be overwritten.

  * You must provide one or more `.proto` files as input. Multiple `.proto` files can be specified at once. Although the files are named relative to the current directory, each file must reside in one of the `IMPORT_PATH`s so that the compiler can determine its canonical name.

## File location

Prefer not to put `.proto` files in the same directory as other language sources. Consider creating a subpackage `proto` for `.proto` files, under the root package for your project.

### Location Should be Language-agnostic

When working with Java code, it’s handy to put related `.proto` files in the same directory as the Java source. However, if any non-Java code ever uses the same protos, the path prefix will no longer make sense. So in general, put the protos in a related language-agnostic directory such as `//myteam/mypackage`.

The exception to this rule is when it’s clear that the protos will be used only in a Java context, such as for testing.

## Supported Platforms

For information about:

  * the operating systems, compilers, build systems, and C++ versions that are supported, see [Foundational C++ Support Policy](https://opensource.google/documentation/policies/cplusplus-support).
  * the PHP versions that are supported, see [Supported PHP versions](https://cloud.google.com/php/getting-started/supported-php-versions).


# Go Opaque API Migration

Describes the automated migration to the Opaque API.

The Opaque API is the latest version of the Protocol Buffers implementation for the Go programming language. The old version is now called Open Struct API. See the [Go Protobuf: Releasing the Opaque API](https://go.dev/blog/protobuf-opaque) blog post for an introduction.

The migration to the Opaque API happens incrementally, on a per-proto-message or per-`.proto`-file basis, by setting the `api_level` feature to one of its possible values:

  * `API_OPEN` selects the Open Struct API. This was backported into edition 2023, so older versions of the Go plugin may not honor it.
  * `API_HYBRID` is a step between Open and Opaque: The Hybrid API also includes accessor methods (so you can update your code), but still exports the struct fields as before. There is no performance difference; this API level only helps with the migration.
  * `API_OPAQUE` selects the Opaque API; this is the default for Edition 2024 and newer.

To override the default for a specific `.proto` file, set the `api_level` feature:

    edition = "2024";

    package log;

    import "google/protobuf/go_features.proto";
    option features.(pb.go).api_level = API_OPEN;

    message LogEntry { … }

Before you can change the `api_level` to `API_OPAQUE` for existing files, all existing usages of the generated proto code need to be updated. The `open2opaque` tool helps with this.

For your convenience, you can also override the default API level with a `protoc` command-line flag:

    protoc […] --go_opt=default_api_level=API_OPEN

To override the default API level for a specific file (instead of all files), use the `apilevelM` mapping flag (similar to [the `M` flag for import paths](https://protobuf.dev/reference/go/go-generated/#package)):

    protoc […] --go_opt=apilevelMhello.proto=API_OPEN

## Automated Migration

We try to make migrating existing projects to the Opaque API as easy as possible for you: our open2opaque tool does most of the work!

To install the migration tool, use:

    go install google.golang.org/open2opaque@latest
    go install golang.org/x/tools/cmd/goimports@latest

#### Note

If you encounter any issues with the automated migration approach, refer to the [Opaque API: Manual Migration](https://protobuf.dev/reference/go/opaque-migration-manual) guide.

### Project Preparation

Ensure your build environment and project are using recent-enough versions of Protocol Buffers and Go Protobuf:

  1. Update the protobuf compiler (protoc) from [the protobuf release page](https://github.com/protocolbuffers/protobuf/releases/latest) to version 29.0 or newer.

  2. Update the protobuf compiler Go plugin (protoc-gen-go) to version 1.36.0 or newer:

         go install google.golang.org/protobuf/cmd/protoc-gen-go@latest

  3. In each project, update the `go.mod` file to use the protobuf module in version 1.36.0 or newer:

         go get google.golang.org/protobuf@latest

#### Note

If you are not yet importing `google.golang.org/protobuf`, you might still be on an older module. See [the `google.golang.org/protobuf` announcement (from 2020)](https://go.dev/blog/protobuf-apiv2) and migrate your code before returning to this page.

### Step 1. Switch to the Hybrid API

Use the `open2opaque` tool to switch your `.proto` files to the Hybrid API:

    open2opaque setapi -api HYBRID $(find . -name "*.proto")

Then, [re-compile](https://protobuf.dev/getting-started/gotutorial#compiling-protocol-buffers) your protocol buffers.

Your existing code will continue to build. The Hybrid API is a step between the Open and Opaque API which adds the new accessor methods but keeps struct fields visible.

### Step 2. `open2opaque rewrite`

To rewrite your Go code to use the Opaque API, run the `open2opaque rewrite` command:

    open2opaque rewrite -levels=red github.com/robustirc/robustirc/...

You can specify one or more [packages or patterns](https://pkg.go.dev/cmd/go#hdr-Package_lists_and_patterns).

As an example, if you had code like this:

    logEntry := &logpb.LogEntry{}
    if req.IPAddress != nil {
        logEntry.IPAddress = redactIP(req.IPAddress)
    logEntry.BackendServer = proto.String(host)

The tool would rewrite it to use accessors:

    logEntry := &logpb.LogEntry{}
    if req.HasIPAddress() {
        logEntry.SetIPAddress(redactIP(req.GetIPAddress()))
    logEntry.SetBackendServer(host)

Another common example is to initialize a protobuf message with a struct literal:

    return &logpb.LogEntry{
        BackendServer: proto.String(host),

In the Opaque API, the equivalent is to use a Builder:

    return logpb.LogEntry_builder{
        BackendServer: proto.String(host),
    }.Build()

The tool classifies its available rewrites into different levels. The `-levels=red` argument enables all rewrites, including those that require human review. The following levels are available:

  * green: Safe rewrites (high confidence). Includes most changes the tool makes. These changes do not require a close look and could even be submitted by automation, without any human oversight.
  * yellow: (reasonable confidence) These rewrites require human review. They should be correct, but please review them.
  * red: Potentially dangerous rewrites, changing rare and complicated patterns. These require careful human review. For example, when an existing function takes a `*string` parameter, the typical fix of using `proto.String(msg.GetFoo())` does not work if the function meant to change the field value by writing to the pointer (`*foo = "value"`).

Many programs can be fully migrated with only green changes. Before you can migrate a proto message or file to the Opaque API, you need to complete all rewrites of all levels, at which point no direct struct access remains in your code.

### Step 3. Migrate and Verify

To complete the migration, use the `open2opaque` tool to switch your `.proto` files to the Opaque API:

    open2opaque setapi -api OPAQUE $(find . -name "*.proto")

Now, any remaining code that was not rewritten to the Opaque API yet will no longer compile.

Run your unit tests, integration tests and other verification steps, if any.

## Questions? Issues?

First, check out the [Opaque API FAQ](https://protobuf.dev/reference/go/opaque-faq). If that doesn’t answer your question or resolve your issue, see [Where can I ask questions or report issues?](https://protobuf.dev/reference/go/opaque-faq#questions)