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docs: add complete README for Actor Model pattern also implemented changes #3251 
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Diff for: ‎actor-model/README.md

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# Actor Model
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---
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title: "Actor Model Pattern in Java: Building Concurrent Systems with Elegance"
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shortTitle: Actor Model
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description: "Explore the Actor Model pattern in Java with real-world examples and practical implementation. Learn how to build scalable, message-driven systems using actors, messages, and asynchronous communication."
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category: Concurrency
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language: en
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tag:
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- Concurrency
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- Messaging
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- Isolation
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- Asynchronous
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- Distributed Systems
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- Actor Model
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---
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## Intent
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## Also Known As
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The Actor Model is a concurrency pattern that treats "actors" as the fundamental units of computation. Each actor has its own state and behavior and interacts with other actors exclusively through message passing.
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- Message-passing concurrency
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- Actor-based concurrency
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## Explanation
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---
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### Real-world Example
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## Intent of Actor Model Pattern
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Imagine a team of people (actors) working in an office. They don’t share the same brain (memory), but instead communicate by passing notes (messages). Each person reads one note at a time and responds accordingly.
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The Actor Model pattern enables the construction of highly concurrent, distributed, and fault-tolerant systems by using isolated components (actors) that interact exclusively through asynchronous message passing.
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### Problem
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---
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Managing concurrent behavior in a safe and scalable way is difficult, especially with shared memory and race conditions.
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## Detailed Explanation of Actor Model Pattern with Real-World Examples
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### Solution
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### 📦 Real-world Example
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Encapsulate state and behavior within individual actors that communicate through asynchronous messages.
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Imagine a customer service system:
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- Each **customer support agent** is an **actor**.
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- Customers **send questions (messages)** to agents.
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- Each agent handles one request at a time and can **respond asynchronously** without interfering with other agents.
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## Class Diagram
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---
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![UML Diagram](etc/actor-model.png)
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### 🧠 In Plain Words
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## Participants
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> "Actors are like independent workers that never share memory and only communicate through messages."
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- **Actor**: Base class that defines a mailbox and handles incoming messages sequentially.
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- **Message**: Represents communication between actors.
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- **ActorSystem**: Creates and manages actors.
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- **PrinterActor**: Sample actor that prints incoming messages.
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---
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## Applicability
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### 📖 Wikipedia Says
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Use the Actor Model pattern when:
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> [Actor model](https://en.wikipedia.org/wiki/Actor_model) is a mathematical model of concurrent computation that treats "actors" as the universal primitives of concurrent computation.
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- You need a simple and safe way to handle concurrency.
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- You want to avoid thread synchronization issues like race conditions and deadlocks.
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- You want each object to process messages independently.
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---
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## Example
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### 🧹 Architecture Diagram
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![UML Class Diagram](./etc/Actor_Model_UML_Class_Diagram.png)
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---
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## Programmatic Example of Actor Model Pattern in Java
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### Actor.java
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```java
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public abstract class Actor implements Runnable {
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@Setter @Getter private String actorId;
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private final BlockingQueue<Message> mailbox = new LinkedBlockingQueue<>();
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private volatile boolean active = true;
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public void send(Message message) {
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mailbox.add(message);
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}
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public void stop() {
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active = false;
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}
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@Override
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public void run() {
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}
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protected abstract void onReceive(Message message);
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}
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```
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### Message.java
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```java
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@AllArgsConstructor
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@Getter
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@Setter
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public class Message {
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private final String content;
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private final String senderId;
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}
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```
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### ActorSystem.java
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```java
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public class ActorSystem {
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public void startActor(Actor actor) {
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String actorId = "actor-" + idCounter.incrementAndGet(); // Generate a new and unique ID
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actor.setActorId(actorId); // assign the actor it's ID
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actorRegister.put(actorId, actor); // Register and save the actor with it's ID
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executor.submit(actor); // Run the actor in a thread
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}
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public Actor getActorById(String actorId) {
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return actorRegister.get(actorId); // Find by Id
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}
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public void shutdown() {
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executor.shutdownNow(); // Stop all threads
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}
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}
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```
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### App.java
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```java
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ActorSystem system = new ActorSystem();
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Actor printer = system.actorOf(new PrinterActor());
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public class App {
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public static void main(String[] args) {
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ActorSystem system = new ActorSystem();
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Actor srijan = new ExampleActor(system);
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Actor ansh = new ExampleActor2(system);
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system.startActor(srijan);
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system.startActor(ansh);
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ansh.send(new Message("Hello ansh", srijan.getActorId()));
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srijan.send(new Message("Hello srijan!", ansh.getActorId()));
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Thread.sleep(1000); // Give time for messages to process
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srijan.stop(); // Stop the actor gracefully
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ansh.stop();
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system.shutdown(); // Stop the actor system
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}
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}
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```
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---
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## When to Use the Actor Model Pattern in Java
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- When building **concurrent or distributed systems**
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- When you want **no shared mutable state**
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- When you need **asynchronous, message-driven communication**
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- When components should be **isolated and loosely coupled**
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---
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## Actor Model Pattern Java Tutorials
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- [Baeldung – Akka with Java](https://www.baeldung.com/java-akka)
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- [Vaughn Vernon – Reactive Messaging Patterns](https://vaughnvernon.co/?p=1143)
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---
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## Real-World Applications of Actor Model Pattern in Java
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- [Akka Framework](https://akka.io/)
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- [Erlang and Elixir concurrency](https://www.erlang.org/)
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- [Microsoft Orleans](https://learn.microsoft.com/en-us/dotnet/orleans/)
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- JVM-based game engines and simulators
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---
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## Benefits and Trade-offs of Actor Model Pattern
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### ✅ Benefits
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- High concurrency support
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- Easy scaling across threads or machines
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- Fault isolation and recovery
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- Message ordering within actors
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### ⚠️ Trade-offs
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- Harder to debug due to asynchronous behavior
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- Slight performance overhead due to message queues
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- More complex to design than simple method calls
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---
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## Related Java Design Patterns
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- [Command Pattern](../command)
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- [Mediator Pattern](../mediator)
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- [Event-Driven Architecture](../event-driven-architecture)
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- [Observer Pattern](../observer)
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---
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printer.send(new Message("Hello Actor!", null));
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printer.send(new Message("Another message", null));
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## References and Credits
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Thread.sleep(1000);
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- *Programming Erlang*, Joe Armstrong
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- *Reactive Design Patterns*, Roland Kuhn
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- *The Actor Model in 10 Minutes*, [InfoQ Article](https://www.infoq.com/articles/actor-model/)
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- [Akka Documentation](https://doc.akka.io/docs/akka/current/index.html)
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((PrinterActor) printer).stop();
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system.shutdown();

Diff for: ‎actor-model/etc/actor_model-Actor_Model___UML_Class_Diagram.png renamed to ‎actor-model/etc/Actor_Model_UML_Class_Diagram.png

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Diff for: ‎actor-model/pom.xml

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<artifactId>actor-model</artifactId>
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<name>Actor Model</name>
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<!-- Force unified JUnit version to avoid classpath mismatches -->
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<dependencyManagement>
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<dependencies>
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<dependency>
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<groupId>org.junit</groupId>
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<artifactId>junit-bom</artifactId>
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<version>5.11.0</version>
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<type>pom</type>
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<scope>import</scope>
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</dependency>
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</dependencies>
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</dependencyManagement>
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<dependencies>
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<!-- JUnit 5 Jupiter API -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-api</artifactId>
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<version>5.10.0</version>
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<scope>test</scope>
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</dependency>
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<!-- JUnit 5 Jupiter Engine -->
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-engine</artifactId>
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<version>5.10.0</version>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.junit.platform</groupId>
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<artifactId>junit-platform-launcher</artifactId>
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<scope>test</scope>
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</dependency>
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<dependency>
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<groupId>org.slf4j</groupId>
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<artifactId>slf4j-api</artifactId>
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</dependency>
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<dependency>
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<groupId>ch.qos.logback</groupId>
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<artifactId>logback-classic</artifactId>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<!-- Assembly plugin for creating fat JARs -->
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-assembly-plugin</artifactId>
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<version>3.3.0</version>
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<configuration>
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<descriptorRefs>
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<descriptorRef>jar-with-dependencies</descriptorRef>
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</descriptorRefs>
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<archive>
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<manifest>
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<mainClass>com.iluwatar.actormodel.App</mainClass>
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</manifest>
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</archive>
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</configuration>
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<executions>
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<execution>
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<id>make-assembly</id>
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<phase>package</phase>
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<goals>
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<goal>single</goal>
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</goals>
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</execution>
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</executions>
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</plugin>
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</plugins>
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</build>
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</project>

Diff for: ‎actor-model/src/main/java/com/iluwatar/actormodel/App.java

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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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/**
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* The Actor Model is a design pattern used to handle concurrency in a safe, scalable, and
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* message-driven way.
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*
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* <p>In the Actor Model: - An **Actor** is an independent unit that has its own state and behavior.
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* - Actors **communicate only through messages** — they do not share memory. - An **ActorSystem**
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* is responsible for creating, starting, and managing the lifecycle of actors. - Messages are
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* delivered asynchronously, and each actor processes them one at a time.
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*
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* <p>💡 Key benefits: - No shared memory = no need for complex thread-safety - Easy to scale with
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* many actors - Suitable for highly concurrent or distributed systems
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*
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* <p>🔍 This example demonstrates the Actor Model: - `ActorSystem` starts two actors: `srijan` and
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* `ansh`. - `ExampleActor` and `ExampleActor2` extend the `Actor` class and override the
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* `onReceive()` method to handle messages. - Actors communicate using `send()` to pass `Message`
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* objects that include the message content and sender's ID. - The actors process messages
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* **asynchronously in separate threads**, and we allow a short delay (`Thread.sleep`) to let them
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* run. - The system is shut down gracefully at the end.
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*/
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package com.iluwatar.actormodel;
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public class App {
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public static void main(String[] args) throws InterruptedException {
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ActorSystem system = new ActorSystem();
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Actor srijan = new ExampleActor(system);
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system.startActor(srijan);
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Actor ansh = new ExampleActor2(system);
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system.startActor(srijan);
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system.startActor(ansh);
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ansh.send(new Message("Hello Srijan", srijan.getActorId()));
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srijan.send(new Message("Hello ansh!", srijan.getActorId()));
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ansh.send(new Message("Hello ansh", srijan.getActorId()));
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srijan.send(new Message("Hello srijan!", ansh.getActorId()));
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Thread.sleep(1000); // Give time for messages to process
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Diff for: ‎actor-model/src/main/java/com/iluwatar/actormodel/ExampleActor.java

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*/
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package com.iluwatar.actormodel;
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import java.util.logging.Logger;
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import java.util.ArrayList;
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import java.util.List;
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import lombok.Getter;
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import lombok.extern.slf4j.Slf4j;
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@Slf4j
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public class ExampleActor extends Actor {
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private final ActorSystem actorSystem;
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@Getter private final List<String> receivedMessages = new ArrayList<>();
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public ExampleActor(ActorSystem actorSystem) {
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this.actorSystem = actorSystem;
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}
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Logger logger = Logger.getLogger(getClass().getName());
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// Logger log = Logger.getLogger(getClass().getName());
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@Override
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protected void onReceive(Message message) {
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logger.info("[" + getActorId() + "]" + "Received : " + message.getContent());
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LOGGER.info(
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"[{}]Received : {} from : [{}]", getActorId(), message.getContent(), message.getSenderId());
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Actor sender = actorSystem.getActorById(message.getSenderId()); // sender actor id
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// Reply of the message
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if (sender != null && !message.getSenderId().equals(getActorId())) {
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sender.send(new Message("I got your message ", getActorId()));
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}

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