Idempotent Consumer Microservice Pattern #2683

idempotent-consumer/README.md

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+---
+title: "Idempotent Consumer Pattern in Java: Ensuring Reliable Message Processing"
+shortTitle: Idempotent Consumer
+description: "Learn about the Idempotent Consumer pattern in Java. Discover how it ensures reliable and consistent message processing, even in cases of duplicate messages."
+category: Messaging
+language: en
+tag:
+    - Messaging
+    - Fault tolerance
+    - Event-driven
+    - Reliability
+---
+
+## Also known as
+
+* Idempotency Pattern
+
+## Intent of Idempotent Consumer Pattern
+
+The Idempotent Consumer pattern is used to handle duplicate messages in distributed systems, ensuring that multiple processing of the same message does not cause undesired side effects. This pattern guarantees that the same message can be processed repeatedly with the same outcome, which is critical in ensuring reliable communication and data consistency in systems where message duplicates are possible.
+
+## Detailed Explanation of Idempotent Consumer Pattern with Real-World Examples
+
+### Real-world Example
+
+> In a payment processing system, ensuring that payment messages are idempotent prevents duplicate transactions. For example, if a user’s payment message is accidentally processed twice, the system should recognize the second message as a duplicate and prevent it from executing a second time. By storing unique identifiers for each processed message, such as a transaction ID, the system can skip any duplicate messages. This ensures that a user is not charged twice for the same transaction, maintaining system integrity and customer satisfaction.
+
+### In Plain Words
+
+> The Idempotent Consumer pattern prevents duplicate messages from causing unintended side effects by ensuring that processing the same message multiple times results in the same outcome. This makes message processing safe in distributed systems where duplicates may occur.
+
+### Wikipedia says
+
+> In computing, idempotence is the property of certain operations in mathematics and computer science whereby they can be applied multiple times without changing the result beyond the initial application.
+
+## When to Use the Idempotent Consumer Pattern
+
+The Idempotent Consumer pattern is particularly useful in scenarios:
+
+* When messages can be duplicated due to network retries or communication issues.
+* In distributed systems where message ordering is not guaranteed, making deduplication necessary to avoid repeated processing.
+* In financial or critical systems, where duplicate processing would have significant side effects.
+
+## Real-World Applications of Idempotent Consumer Pattern
+
+* Payment processing systems that avoid duplicate transactions.
+* E-commerce systems to prevent multiple entries of the same order.
+* Inventory management systems to prevent multiple entries when updating stock levels.
+
+## Benefits and Trade-offs of the Idempotent Consumer Pattern
+
+### Benefits
+
+* **Reliability**: Ensures that messages can be processed without unwanted side effects from duplicates.
+* **Consistency**: Maintains data integrity by ensuring that duplicate messages do not cause redundant updates or actions.
+* **Fault Tolerance**: Handles message retries gracefully, preventing them from causing errors.
+
+### Trade-offs
+
+* **State Management**: Requires storing processed message IDs, which can add memory overhead.
+* **Complexity**: Implementing deduplication mechanisms can increase the complexity of the system.
+* **Scalability**: In high-throughput systems, maintaining a large set of processed messages can impact performance and resource usage.
+
+## Related Patterns in Java
+
+* [Retry Pattern](https://java-design-patterns.com/patterns/retry/): Works well with the Idempotent Consumer pattern to handle failed messages.
+* [Circuit Breaker Pattern](https://java-design-patterns.com/patterns/circuitbreaker/): Often used alongside idempotent consumers to prevent repeated failures from causing overload.
+
+## References and Credits
+
+* [Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions](https://amzn.to/4dznP2Y)
+* [Designing Data-Intensive Applications](https://amzn.to/3UADv7Q)

idempotent-consumer/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>idempotent-consumer</artifactId>
+    <dependencyManagement>
+        <dependencies>
+            <dependency>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-dependencies</artifactId>
+                <type>pom</type>
+                <version>3.2.3</version>
+                <scope>import</scope>
+            </dependency>
+            <dependency>
+                <groupId>org.hibernate</groupId>
+                <artifactId>hibernate-core</artifactId>
+                <version>6.4.4.Final</version>
+            </dependency>
+        </dependencies>
+    </dependencyManagement>
+    <dependencies>
+        <!-- Spring Boot Dependencies -->
+
+        <!-- Spring Boot Data JPA for database access -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
+        </dependency>
+
+        <!-- Testing Dependencies -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- JUnit Jupiter Engine for testing -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- Mockito for mocking in tests -->
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- H2 for mocking database -->
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>runtime</scope>
+        </dependency>
+
+    </dependencies>
+
+
+    <build>
+        <plugins>
+            <!-- Maven Assembly Plugin for creating a single distributable JAR -->
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-assembly-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <phase>package</phase>
+                        <goals>
+                            <goal>single</goal>
+                        </goals>
+                        <configuration>
+                            <descriptorRefs>
+                                <descriptorRef>jar-with-dependencies</descriptorRef>
+                            </descriptorRefs>
+                            <archive>
+                                <manifest>
+                                    <!-- Define the main class for the executable JAR -->
+                                    <mainClass>com.iluwatar.idempotentconsumer.App</mainClass>
+                                </manifest>
+                            </archive>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+
+            <!-- Other plugins as needed -->
+
+        </plugins>
+    </build>
+</project>

idempotent-consumer/src/main/java/com/iluwatar/idempotentconsumer/App.java

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+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.idempotentconsumer;
+
+import java.util.UUID;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.boot.CommandLineRunner;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.context.annotation.Bean;
+
+/**
+ * The main entry point for the idempotent-consumer application.
+ * This application demonstrates the use of the Idempotent Consumer
+ * pattern which ensures that a message is processed exactly once
+ * in scenarios where the same message can be delivered multiple times.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Idempotence">Idempotence (Wikipedia)</a>
+ * @see <a href="https://camel.apache.org/components/latest/eips/idempotentConsumer-eip.html">Idempotent Consumer Pattern (Apache Camel)</a>
+ */
+@SpringBootApplication
+@Slf4j
+public class App {
+  public static void main(String[] args) {
+    SpringApplication.run(App.class, args);
+  }
+  /**
+   * The starting point of the CommandLineRunner
+   * where the main program is run.
+   *
+   * @param requestService idempotent request service
+   * @param requestRepository request jpa repository
+   */
+  @Bean
+  public CommandLineRunner run(RequestService requestService, RequestRepository requestRepository) {
+    return args -> {
+      Request req = requestService.create(UUID.randomUUID());
+      requestService.create(req.getUuid());
+      requestService.create(req.getUuid());
+      LOGGER.info("Nb of requests : {}", requestRepository.count()); // 1, processRequest is idempotent
+      req = requestService.start(req.getUuid());
+      try {
+        req = requestService.start(req.getUuid());
+      } catch (InvalidNextStateException ex) {
+        LOGGER.error("Cannot start request twice!");
+      }
+      req = requestService.complete(req.getUuid());
+      LOGGER.info("Request: {}", req);
+    };
+  }
+}
+

idempotent-consumer/src/main/java/com/iluwatar/idempotentconsumer/InvalidNextStateException.java

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+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.idempotentconsumer;
+
+/**
+ * This exception is thrown when an invalid transition is attempted in the Statemachine
+ * for the request status. This can occur when attempting to move to a state that is not valid
+ * from the current state.
+ */
+public class InvalidNextStateException extends RuntimeException {
+  public InvalidNextStateException(String s) {
+    super(s);
+  }
+}

idempotent-consumer/src/main/java/com/iluwatar/idempotentconsumer/Request.java

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+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.idempotentconsumer;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import java.util.UUID;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+/**
+ * The {@code Request} class represents a request with a unique UUID and a status.
+ * The status of a request can be one of four values: PENDING, STARTED, COMPLETED, or INERROR.
+ */
+@Entity
+@NoArgsConstructor
+@Data
+public class Request {
+  enum Status {
+    PENDING,
+    STARTED,
+    COMPLETED
+  }
+
+  @Id
+  private UUID uuid;
+  private Status status;
+
+  public Request(UUID uuid) {
+    this(uuid, Status.PENDING);
+  }
+
+  public Request(UUID uuid, Status status) {
+    this.uuid = uuid;
+    this.status = status;
+  }
+}