tasks/CROSS3_protocol_bridge/spec.md

CROSS3: Protocol Bridge — JSON to Message Translation

Goal

Fix 4 translation bugs and 2 error mapping bugs in the bridge service that translates Service A's JSON REST API responses into structured messages for Service B's consumer.

Requirements

1. int64 fields must be translated as Python int (no truncation to 32-bit)
2. bytes fields must be base64-decoded from JSON strings
3. oneof fields must have exactly one variant set (not multiple)
4. enum fields must be mapped from string names to integer values
5. HTTP 404 from Service A must map to NOT_FOUND error code (5) for Service B
6. HTTP 429 from Service A must map to RESOURCE_EXHAUSTED error code (8) for Service B
7. All tests must pass: pytest tests/

Supporting Documents

• service_a/models.py — JSON data models from Service A
• service_b/schema.py — Message schema for Service B (proto3-style)
• bridge/translator.py — JSON→Message translation (4 bugs)
• bridge/error_mapper.py — HTTP status → error code (2 bugs)

Background

The bridge service sits between Service A (a REST API returning JSON) and Service B (a queue consumer expecting structured proto3-style messages). Because JSON and proto3 have different type semantics, every field crossing this boundary needs a careful type conversion.

Type Semantic Differences

JSON Type	Proto3 Type	Issue
number	int64	JSON numbers lose precision for values > 2^53; must not be masked to 32-bit
string	bytes	Binary data is base64-encoded in JSON; must be decoded to bytes
object	oneof	JSON may include multiple keys; proto3 oneof allows exactly one
string	enum	Enum names in JSON must be converted to integer codes

Error Code Mapping

Service A returns HTTP status codes. Service B uses gRPC-style integer error codes:

HTTP Status	Expected Error Code	Code Number
200	OK	0
400	INVALID_ARGUMENT	3
401/403	INVALID_ARGUMENT	3
404	NOT_FOUND	5
429	RESOURCE_EXHAUSTED	8
5xx	INTERNAL	13

Real-World Context

JSON-to-gRPC transcoding bridges are a common integration layer in microservice architectures, and the 6 bugs in this task reflect bugs documented in real transcoding implementations:

• int64 truncation (Bug 1): JavaScript's JSON.parse() silently truncates 64-bit integers beyond Number.MAX_SAFE_INTEGER (2^53-1). This caused data loss in the Twitter Snowflake ID migration (2010) and is documented in the gRPC-Gateway project FAQ. Fix: serialize int64 as strings in JSON.
• bytes base64 handling (Bug 2): Proto3 bytes fields must be base64-encoded in JSON (RFC 4648). The gRPC-Gateway library had exactly this bug in v1.x — binary fields were passed through raw, corrupting payloads. CVE-class issue in several open-source gRPC-JSON bridges.
• oneof variant mapping (Bug 3): Proto3 oneof fields in JSON use the field name as the key, not the oneof name. Incorrect mapping silently drops the value.
• enum integer vs string (Bug 4): Proto3 JSON canonical form uses enum names (strings), not integers. Several gRPC web clients (grpc-web, grpc-gateway) have shipped this mismatch causing 0-value enums for all inputs.
• Error code mapping (Bugs 5–6): gRPC status codes (google.rpc.Code) are a strict mapping from HTTP status; 404→NOT_FOUND(5) and 429→RESOURCE_EXHAUSTED(8) are specified in google.rpc.Code and commonly misimplemented.