feat: Implement Fallback design pattern #2846

[priyanshuvishwakarma273403]

What does this PR do?

This PR implements the Fallback design pattern as a standalone, self-contained Maven module (fallback). The implementation demonstrates a robust resiliency mechanism commonly used in microservices architectures.

Key Components Implemented:

• RemoteService: A simple interface representing a service/dependency call that might fail.
• PrimaryService: Implements RemoteService to simulate the main dependency with configurable latency, timeout, and exceptions.
• FallbackService: Implements RemoteService and represents the fallback strategy, returning cached or degraded responses.
• SimpleCircuitBreaker: A thread-safe state machine (CLOSED, OPEN, HALF_OPEN) that monitors service health to fast-fail subsequent requests when the primary service is down.
• FallbackExecutor: Leverages Java 21's Virtual Threads to orchestrate the service call execution, applying timeouts and delegating to the fallback handler when failures, timeouts, or OPEN circuit states are encountered.
• App: Main application illustrating different runtime scenarios and state transitions (healthy call, exception, timeout, fast-failing during OPEN, and recovery back to CLOSED).
• Unit & Integration Tests: Fully verified using JUnit 5 (covering all state transitions, timeouts, short-circuits, and recovery). Passed spotless checks.

Fixes #2846

[github-actions]

PR Summary

Adds a standalone Maven module fallback implementing the Fallback design pattern with a circuit breaker. Includes interfaces, concrete services, a simple circuit breaker, an executor that uses virtual threads, a demo App, and comprehensive tests. Also ships a README and updates the root pom.xml to include the new module. Demonstrates healthy, failing, timed, OPEN-state fast-fail, and recovery scenarios.

Changes

File	Summary
fallback/README.md	Introduces the Fallback pattern and its circuit-breaker–driven resilience in a standalone Maven module. Documents components (RemoteService, PrimaryService, FallbackService, SimpleCircuitBreaker, FallbackExecutor), a demo App, and tests, with scenario explanations and usage guidance.
fallback/pom.xml	Defines the fallback Maven module, its dependencies (slf4j-api, logback-classic, JUnit Jupiter Engine), and assembly configuration for a runnable App. Establishes parent-child relation and marks the module as test-enabled.
fallback/src/main/java/com/iluwatar/fallback/App.java	Demo application orchestrating healthy, failing, timeout, OPEN circuit, and recovery scenarios using FallbackExecutor, PrimaryService, FallbackService, and SimpleCircuitBreaker.
fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java	Executes primary service calls with timeout handling, observes circuit breaker state, and delegates to the fallback when primary fails, times out, or circuit is OPEN. Uses a virtual-thread executor and implements AutoCloseable.
fallback/src/main/java/com/iluwatar/fallback/FallbackService.java	Concrete FallbackService returning a predefined degraded response when the primary fails or times out.
fallback/src/main/java/com/iluwatar/fallback/PrimaryService.java	Simulates the main dependency with configurable latency and exception throwing to test resilience features.
fallback/src/main/java/com/iluwatar/fallback/RemoteService.java	Interface representing a potentially failing external service with an execute() method.
fallback/src/main/java/com/iluwatar/fallback/SimpleCircuitBreaker.java	Thread-safe state machine with CLOSED, OPEN, and HALF_OPEN states, tracking failures and retry windows.
fallback/src/test/java/com/iluwatar/fallback/AppTest.java	Ensures the App main method runs without throwing exceptions.
fallback/src/test/java/com/iluwatar/fallback/FallbackPatternTest.java	JUnit tests cover healthy calls, failures, timeouts, OPEN fast-fail, and recovery to CLOSED, validating circuit-breaker behavior.
pom.xml	Adds the fallback module to the root Maven build, enabling building and testing of the new pattern module.

---

autogenerated by presubmit.ai

[github-actions]

✅ LGTM!

Review Summary

Commits Considered (2)

• ab58c76: docs: add README for Fallback design pattern implementation
• 488c4d7: feat: implement Fallback design pattern with circuit breaker and service monitoring

Files Processed (11)

• fallback/README.md (1 hunk)
• fallback/pom.xml (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/App.java (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/FallbackService.java (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/PrimaryService.java (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/RemoteService.java (1 hunk)
• fallback/src/main/java/com/iluwatar/fallback/SimpleCircuitBreaker.java (1 hunk)
• fallback/src/test/java/com/iluwatar/fallback/AppTest.java (1 hunk)
• fallback/src/test/java/com/iluwatar/fallback/FallbackPatternTest.java (1 hunk)
• pom.xml (1 hunk)

Actionable Comments (0)

Skipped Comments (7)

• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java [46-46]

  possible issue: "Handle potential null responses from primary service."

• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java [66-66]

  behavior: "Circuit breaker fast-fail path."

• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java [81-81]

  best-practice: "Timeout handling."

• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java [86-86]

  best-practice: "Cancel task on timeout to free resources."

• fallback/src/main/java/com/iluwatar/fallback/FallbackExecutor.java [94-94]

  maintainability: "Narrow exception handling in executor."

• fallback/src/main/java/com/iluwatar/fallback/SimpleCircuitBreaker.java [69-69]

  design: "Circuit breaker state transitions."

• fallback/pom.xml [47-47]

  best-practice: "Test dependencies: add junit-jupiter-api for API usage in tests."

[codecov]

Codecov Report

❌ Patch coverage is 85.96491% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.34%. Comparing base (f1dcddf) to head (ea12c2b).

Files with missing lines	Patch %	Lines
...n/java/com/iluwatar/fallback/FallbackExecutor.java	71.87%	9 Missing ⚠️
...va/com/iluwatar/fallback/SimpleCircuitBreaker.java	87.50%	2 Missing and 2 partials ⚠️
...lback/src/main/java/com/iluwatar/fallback/App.java	91.66%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3538      +/-   ##
============================================
- Coverage     83.35%   83.34%   -0.01%     
- Complexity     4087     4107      +20     
============================================
  Files          1073     1078       +5     
  Lines         14414    14528     +114     
  Branches        687      691       +4     
============================================
+ Hits          12015    12109      +94     
- Misses         2113     2128      +15     
- Partials        286      291       +5     

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[github-actions]

🚨 Pull request needs attention.

Review Summary

Commits Considered (1)

• ea12c2b: Merge branch 'master' into feat/fallback

Files Processed (2)

• fallback/README.md (1 hunk)
• pom.xml (1 hunk)

Actionable Comments (1)

• pom.xml [255-255]

  maintainability: "Add new Maven module to aggregator"

Skipped Comments (2)

• fallback/README.md [2-11]

  maintainability: "Validate YAML front matter formatting"

• fallback/README.md [50-57]

  readability: "Use explicit types or Java 10+ 'var' safety in code samples"

[github-actions]

<module>fallback</module>

[iluwatar]

@all-contributors please add @priyanshuvishwakarma273403 for code

[allcontributors]

@iluwatar

I've put up a pull request to add @priyanshuvishwakarma273403! 🎉