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Hello dear students, in this lesson we are going to learn what reactive programming is and how we can

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apply reactive programming practices using Java via flow API.

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We are going to start the lesson from learning general concepts.

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We'll learn what reactive programming approach is.

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We'll discuss what the most popular libraries for reactive programming in Java are.

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But in scope of this lesson, we will focus our attention on the flow API that is available in JDK for

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reactive programming.

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As we will go.

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We'll learn different terms like for example Backpressure aware concept.

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We'll learn key interfaces from flow API.

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Then we'll talk about use cases.

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And I will try to answer the question when it is recommended to use reactive programming approach.

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And at the end of the lesson we are going to review code examples in order to understand how reactive

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streams work on practice.

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Let's start our lesson.

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Let's start from learning of what reactive programming is.

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Reactive programming is a programming paradigm focused on handling asynchronous data streams and the

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propagation of changes.

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It provides a set of principles, patterns, and libraries that enable developers to build responsive

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and scalable systems, especially in the context of handling events, input output operations, and

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real time data.

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Let's review key characteristics of reactive programming.

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They include asynchronous and non-blocking.

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Reactive programming emphasizes asynchronous and non-blocking operations.

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It allows components to continue processing other tasks while waiting for asynchronous operations to

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complete, leading to more efficient resource utilization.

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Data streams.

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At the core of reactive programming is the concept of data streams.

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Sequences of events or values that occur over time.

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These streams can represent various types of data such as user inputs, sensor readings, or messages

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from a server.

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Event-driven reactive systems are often event driven, meaning they respond to events or changes in

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the system.

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Events can trigger reactions and the system adapts dynamically to these changes.

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Back pressure.

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Back pressure is a key concept in reactive programming.

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It refers to the ability of downstream component to signal its capacity to handle data to an upstream

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component.

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This helps prevent overwhelming the system with more data than it can process efficiently.

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Responsive and scalable.

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Reactive programming aims to create responsive applications that can handle varied workloads.

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By embracing asynchrony and scalability, reactive systems can efficiently adapt to changes in demand.

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Functional programming.

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Reactive programming often aligns with functional programming principles.

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It emphasizes immutability, pure functions, and the use of higher order functions, enabling developers

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to write more concise and maintainable code.

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Reactive extensions.

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Reactive extensions, often referred to as RCS, is a set of libraries that provide a uniform way to

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work with reactive programming concepts across multiple programming languages.

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RCS libraries exist for languages like Java, JavaScript, C, sharp, and more.

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Reactive programming has become popular in building modern applications, especially those that involve

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real time data, user interfaces, and distributed systems.

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Frameworks and libraries like Project Reactor, Erik's Java provide tools and abstractions to implement

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reactive programming in various programming languages.

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These tools simplify the handling of asynchronous and streaming data, making it easier for developers

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to build responsive and scalable applications.

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In Java, reactive programming approach may be implemented with different libraries, for example a

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Java reactor core.

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Reactive streams are socket martini reactor, but starting from Java version nine, you can use JDK

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to implement reactive programming approach.

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Flow API is available starting from Java nine.

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In this lesson we will focus on flow API.

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So let's learn what a flow API is.

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The flow API, also known as Reactive Streams, was introduced in Java nine to address challenges associated

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with asynchronous programming and handling streams of data in a more efficient and scalable way.

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Reactive streams provide a standard for asynchronous stream processing with non-blocking back pressure,

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enabling better control over the flow of data between components.

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Reactive streams define a set of interfaces, classes, and masses that standardize the exchange of

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asynchronous data streams between components in a non-blocking, back pressure aware manner.

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Now let me share with you key interfaces in the flow API.

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So the main interfaces include publisher, subscriber, subscription and processor.

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I will make just a brief overview of them and we will learn more during the review of real life example.

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Publisher amidst a potentially unbounded sequence of elements to one or more subscribers.

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Subscriber consumes elements from a publisher and signals when it can handle more elements.

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Subscription represents the link between a subscriber and a publisher, allowing the subscriber to request

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and cancel elements.

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Processor.

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The processor interface in the Reactive Streams API serves as a dual role component, acting both as

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a subscriber and a publisher, enabling custom data processing within a reactive stream.

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So let's try to answer the question why flow API is needed?

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Traditional stream processing in Java lacked a standardized way to handle asynchronous data streams,

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which could lead to issues like buffer overflow or excessive resource consumption.

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Reactive streams address these problems by introducing Backpressure, allowing subscribers to signal

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when they are ready to receive more data, preventing overwhelming downstream components.

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And in which cases it is recommended to use flow API.

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Asynchronous processing.

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Reactive streams are beneficial in scenarios where asynchronous processing is required, such as handling

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events, input output operations, or real time data.

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Backpressure handling when dealing with data streams, where the rate of data production is different

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from the rate of data consumption.

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Reactive streams help manage backpressure, preventing overwhelming downstream components.

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Scalable and responsive applications.

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Reactive programming was.

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Reactive Streams is well suited for building responsive and scalable applications, especially in scenarios

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like web services, where responsiveness is crucial.

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Use reactive streams when you need to.

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Handle asynchronous data streams.

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Implement Backpressure to control the flow of data and prevent resource exhaustion.

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Build scalable and responsive applications with non-blocking input output operations.

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Reactive streams have gained popularity in modern Java development and are widely used in frameworks

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like Project Reactor to build reactive and resilient systems.

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I believe we learned enough theory, and now we are ready to review examples that I prepared especially

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for this lesson.

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You can find the source code of examples that I am going to share with you in attachments to the lesson.

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Remember, Tim, that you are always welcome to ask questions in case something is not clear.

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Just post your question below the video and I will be happy to answer.

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As you can see, examples related to reactive streams are in flow API package.

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We're going to start review from the flow API example class.

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This is a class where I put client code and main method.

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So we will go together with the flow of execution.

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And I will explain you what is happening here line by line.

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Let me also run this file in order to have all console output printed.

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And you will be able to see the sequence of events that is happening here.

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Pay attention that today we will review types that are stored on the Java.util.concurrent package inside

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final class flow.

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There are such important interfaces as publisher, subscriber, subscription and processor.

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These are types that we are going to use today.

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I start example from creation of the submission publisher.

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The submission publisher is a built in implementation of the publisher interface in Java Flow API.

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It emits a potentially unbounded sequence of elements, in this case integers to one or more subscribers.

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The next thing I create instance of my subscriber.

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It is parametrized by integer type because in this example I plan to receive integer elements.

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To implement subscriber interface, you need to give implementation to all following methods.

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Let's review each of them one by one.

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Unsubscribe method.

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This method signals about the subscription sets the subscription object, and requests the first item

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to initialize the data flow with the provided subscription.

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Basically, with the request method, you create demand for elements that subscriber can process depending

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on the amount of requested elements, the same amount of times on next method will be invoked.

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On next method.

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This method processes in common integer items.

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In our example, I just double all in common integers and transform them to string just for the sake

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of example, because theoretically we can implement processor interface that I am going to explain a

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little bit later, and you can supply process data downstream in such way you can build processing chain

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and data flow.

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That's why I created submit method just for the sake of example.

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And just to show you that in submit method, you can put the logic of processed item submission downstream

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in simple words.

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Send processed item further down the processing chain depending on your business logic.

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Onerror method handles errors during processing by printing an error message, allowing the processor

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to respond appropriately to error conditions.

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On complete signals, the completion of processing.

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Printing a message to indicate the end of the data stream.

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If you build a longer processing chain, this is the method where you can notify downstream components

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that processing is completed, and in this method I cancel subscription.

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Let's get back to flow API example class.

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The next thing that I do here I subscribe my subscriber to publisher in the context of the Reactive

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Streams API.

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The relationship between a publisher and a subscriber is established through the subscribe method.

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However, it is important to note that it is the publisher who initiates the subscription process by

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calling the subscribe method on the publisher object.

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Now there will be a connection between them.

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Publisher will be aware about subscriber and when this will happen, then unsubscribe method will be

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triggered and I will initialize my subscription property inside.

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But wait, something should be wrong here.

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Where missing subscription object because we have publisher, we have subscriber.

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But where is subscription object?

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You can implement it by yourself by implementing subscription interface.

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But what is happening in this case?

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You can see that I decided to print subscription object to console in order to understand the type of

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this object.

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And we can see that we received here.

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Object of type buffered subscription.

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This is nested class inside submission publisher class.

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But who created this object?

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And when a buffered subscription is created when a subscriber subscribes to a submission publisher,

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this subscription object manages the communication between the publisher and the subscriber, facilitating

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the flow of data.

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The charm buffer.

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It suggests that this subscription might have some internal buffer to store items when the subscriber

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can't keep up with the rate of data production.

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Here is a breakdown of what happens during the subscription process.

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Publisher calls subscribe method and subscribe subscriber object.

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Submission publisher creates a buffered subscription object internally.

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This is happening inside subscribe method.

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If we would open the source code of subscribe method, we will see how instance of buffered subscription

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is created.

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Unsubscribe method is called on the subscriber with created buffered subscription.

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The unsubscribe method is then invoked on the subscriber, passing the buffered subscription as an argument.

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This allows the subscriber to request items and manage backpressure.

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The buffered aspect of the subscription typically comes into play when the subscriber is unable to keep

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up with the rate of data production.

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In such cases, the buffered subscription may buffer items until the subscriber signals that it can

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process more data by calling the request method.

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So after subscription is happened, then I set the timeout.

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We need this time out to make sure that subscription is created.

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But using timeout is very primitive way that I decided to use for the sake of example.

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In reality, ensuring that everyone is successfully subscribed in a reactive system can be a complicated

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task, and the approach might depend on the specific requirements of your application.

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Some points to consider using of countdown Lodge.

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Using of completablefuture.

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Using of timeout mechanism, including using it with completablefuture.

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In my course Java from zero to first job.

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I have multi-threading section where I explained in details API of countdown, Lodge, Completablefuture

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and other things.

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That's why I wouldn't stop too much on this right now.

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In console you can see subscriber subscribed.

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After the timeout, I declare a for loop with counter and I submit five integers using publisher object.

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I submit them one by one, and in console you can see that publisher submits one element and subscriber

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receives it.

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Taking into account that println method is synchronized, text and console not necessarily represents

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the exact sequence of events, but at least it can give us an understanding of what is going on.

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Subscriber after receiving integers, doubles, received elements and prints them to console two and

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when publisher is closed, subscriber got notified two and Oncomplete method is triggered.

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And that's it.

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Knowing this, you can build more complex data flows with multiple subscribers, different publishers.

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You can connect them between each other as your business logic would require.

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And the last but not the least thing that I would like to show you for today is processor interface.

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You can create processor class that would implement processor interface if you want to make it acting

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as both a publisher and a subscriber.

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This can be helpful when you want to build a processing chain.

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For example, you can receive some data from publisher, process it, and pass it further to the next

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subscriber.

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And like this.

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You can build complex data flows.

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So the processor interface in the Reactive Streams API is used when you need to create a component that

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acts as a both a publisher and a subscriber.

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It is particularly useful in scenarios where you want to transform or process the data flowing through

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a reactive stream before it reaches the downstream subscribers.

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Here are some situations where you might consider using the processor interface.

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Transforming or processing data.

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If you need to apply transformations, filtering or any other processing logic to the data before it

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is delivered to subscribers, the processor interface allows you to implement this logic.

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Connecting different parts of a reactive pipeline.

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When you have a reactive pipeline with multiple stages, and you want to connect these stages with a

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component that can both consume data from the upstream and emit processed data to the downstream.

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Implementing a custom flow control mechanism.

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If you need to implement a custom flow control mechanism between the publisher and subscribers, the

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processor allows you to manage backpressure and control the rate at which data is consumed.

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Aggregating or combining data streams.

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When you want to combine or aggregate data from multiple publishers before delivering it to downstream

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subscribers.

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A processor can be used to implement the necessary logic.

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If you check the source code of processor interface, then you will find that processor is a static

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interface that extends subscriber and publisher at the same time.

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In Java, an interface can extend multiple other interfaces.

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This feature allows an interface to inherit abstract masses from multiple parent interfaces, providing

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a way to express multiple types of behavior in a single interface.

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So feel free to use processor interface implementations when it will be needed in your business cases.

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That's all what I wanted to share with you today in this lesson.

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Let's recap what we have learned in the video.

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We learned reactive programming approach.

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Now you know the most popular libraries for reactive programming in Java.

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I explained you what flow API in JDK is, and we learned key interfaces and types in flow API.

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I explained when and in which cases it is recommended to use Reactive Programming, Reactive Streams

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flow API, and at the end of the lesson we reviewed code example in order to understand how it works

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in practice.

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That's all what I wanted to share with you today.

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Have a great day and see you in the next lesson.