Implement Microservice pattern: Transactional outbox

[tanya-johari]

What does this PR do?

This pull request implements the Transactional Outbox design pattern. It demonstrates how to ensure reliable, at-least-once message delivery from a service by atomically saving business data and its corresponding event in the same database transaction. This PR includes a fully documented code implementation, comprehensive unit tests covering success and failure scenarios, a detailed README and a PlantUML class diagram.

Key features

Customer: The main business entity model that is being created and persisted.

OutboxEvent: The entity representing the message to be sent, which is stored transactionally in the outbox table.

CustomerService: The service that handles the core business logic, responsible for creating the Customer and the OutboxEvent in a single, atomic transaction.

OutboxRepository: The data access layer for reading from and writing OutboxEvent entities to the database.

EventPoller: A background process that periodically scans the outbox table for unprocessed events and publishes them to the message broker.

MessageBroker: A mock implementation that simulates sending the event to an external messaging system.

This pull request is submitted as part of my participation in Hacktoberfest 2025

Closes #2671

[github-actions]

⏳ Analyzing changes in this PR... ⏳

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📥 Commits

Analyzing changes from base (ede37bd) to latest commit (d840eea):

• d840eea: fix: fix test

Resolves #2671

• a121f93: fix: Apply spotless

Resolves #2671

• fddc220: feat: Add docs

Resolves #2671

• 199067f: feat: Add test cases

Resolves #2671

• 3e1e516: feat: Implement Transactional Outbox pattern

Resolves #2671

📁 Files being considered (20)

➕ microservices-transactional-outbox/README.md (1 hunk)
➕ microservices-transactional-outbox/etc/microservices-transactional-outbox-flowchart.png (0 hunks)
➕ microservices-transactional-outbox/etc/microservices-transactional-outbox.puml (1 hunk)
➕ microservices-transactional-outbox/pom.xml (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/App.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/Customer.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/CustomerService.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/EventPoller.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/MessageBroker.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/OutboxEvent.java (1 hunk)
➕ microservices-transactional-outbox/src/main/java/com/iluwatar/transactionaloutbox/OutboxRepository.java (1 hunk)
➕ microservices-transactional-outbox/src/main/resources/META-INF/persistence.xml (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/AppTest.java (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/CustomerServiceTests.java (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/EventPollerTests.java (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/MessageBrokerTests.java (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/OutboxEventTests.java (1 hunk)
➕ microservices-transactional-outbox/src/test/java/com/iluwatar/transactionaloutbox/OutboxRepositoryTests.java (1 hunk)
➕ microservices-transactional-outbox/src/test/resources/META-INF/persistence.xml (1 hunk)
🔄 pom.xml (1 hunk)

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autogenerated by presubmit.ai

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
99.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[tanya-johari]

Hello @iluwatar the implementation is complete and it's ready for review whenever you have a moment. Please let me know if there are any changes or additional information you need from my side.