feat: Implement Write-Ahead Log (WAL) pattern (#3576)

[devikae]

Fixes #3576

Pattern Name

Write-Ahead Log (WAL) Pattern

Intent

The Write-Ahead Log (WAL) pattern ensures data durability and system recoverability in database engines, distributed consensus protocols, and transactional systems by writing all state mutations sequentially to an append-only log file on stable storage before modifying in-memory storage structures.

Summary of Changes

• Added new standalone module write-ahead-log to parent pom.xml.
• Implemented core classes: WriteAheadLog, DatabaseStore, LogEntry, OperationType, and App.
• Added unit tests (LogEntryTest, WriteAheadLogTest, DatabaseStoreTest, AppTest) with 100% pass rate.
• Provided complete README.md with intent, Mermaid UML class diagram, code snippets, and real-world use cases.
• Ensured formatting compliance via Spotless (spotless:check).

[github-actions]

PR Summary

Introduced a new standalone module write-ahead-log implementing the Write-Ahead Log (WAL) pattern. Adds core classes (WriteAheadLog, DatabaseStore, LogEntry, OperationType, App), along with unit tests and a comprehensive README. Updated root Maven POM to declare the new module. All tests pass successfully.

Changes

File	Summary
pom.xml	Modified root POM to declare new write-ahead-log module, enabling Maven to build the WAL pattern alongside existing modules.
write-ahead-log/README.md	Added comprehensive README with pattern intent, class diagram, practical examples, and usage guidance.
write-ahead-log/pom.xml	Introduced write-ahead-log module with dependencies (slf4j, logback, lombok, JUnit 5 engine, Mockito) and build configuration including main-class for App.
write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/App.java	Implemented App demonstrating WAL usage: initialization, data operations, crash simulation, and recovery.
write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/DatabaseStore.java	Implemented DatabaseStore coordinating WriteAheadLog with an in-memory memTable and supporting put, delete, get, checkpoint, crash simulation, and recovery.
write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/LogEntry.java	Implemented LogEntry with fields sequenceNumber, type, key, value, and serialization/deserialization to/from pipe-delimited strings.
write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/OperationType.java	Defined OperationType enum with SET, DELETE, and CHECKPOINT values.
write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/WriteAheadLog.java	Implemented WriteAheadLog to append, read all, and clear WAL entries with sequential numbering and persistence.
write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/AppTest.java	Added test ensuring App.main executes without exceptions.
write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/DatabaseStoreTest.java	Tests for put/get, delete, crash simulation, checkpoint, and recovery behavior.
write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/LogEntryTest.java	Tests for LogEntry serialization, deserialization, validation, and equality.
write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java	Tests for append/readAll, sequence resumption on reopen, clear and IO handling.

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[github-actions]

🚨 Pull request needs attention.

Review Summary

Commits Considered (1)

• 058fc40: feat: Implement Write-Ahead Log (WAL) pattern (#3576)

Files Processed (12)

• pom.xml (1 hunk)
• write-ahead-log/README.md (1 hunk)
• write-ahead-log/pom.xml (1 hunk)
• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/App.java (1 hunk)
• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/DatabaseStore.java (1 hunk)
• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/LogEntry.java (1 hunk)
• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/OperationType.java (1 hunk)
• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/WriteAheadLog.java (1 hunk)
• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/AppTest.java (1 hunk)
• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/DatabaseStoreTest.java (1 hunk)
• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/LogEntryTest.java (1 hunk)
• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java (1 hunk)

Actionable Comments (5)

• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/App.java [52-55]

  bug: "Incorrect logger usage (undefined LOGGER)"

• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/WriteAheadLog.java [91-91]

  bug: "Logger usage mismatch in WAL append"

• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/WriteAheadLog.java [114-114]

  bug: "Logger usage mismatch in WAL read error"

• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/DatabaseStore.java [65-66]

  bug: "Logger usage in put method"

• write-ahead-log/src/main/java/com/iluwatar/writeaheadlog/DatabaseStore.java [78-78]

  bug: "Logger usage in delete method"

Skipped Comments (0)

[codecov]

Codecov Report

❌ Patch coverage is 93.96552% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.69%. Comparing base (22a3412) to head (e604256).

Files with missing lines	Patch %	Lines
.../src/main/java/com/iluwatar/writeaheadlog/App.java	85.71%	3 Missing and 1 partial ⚠️
...java/com/iluwatar/writeaheadlog/WriteAheadLog.java	92.30%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3582      +/-   ##
============================================
+ Coverage     83.54%   83.69%   +0.14%     
- Complexity     4212     4248      +36     
============================================
  Files          1107     1112       +5     
  Lines         14907    15023     +116     
  Branches        704      717      +13     
============================================
+ Hits          12454    12573     +119     
+ Misses         2160     2158       -2     
+ Partials        293      292       -1     

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[github-actions]

✅ LGTM!

Review Summary

Commits Considered (1)

• e604256: test: Add exception handling tests and verify coverage

Files Processed (1)

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java (1 hunk)

Actionable Comments (0)

Skipped Comments (5)

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java [67-82]

  enhancement: "Verify durability semantics across restarts."

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java [85-93]

  possible issue: "Assume durability on append for sequence numbers after reopen."

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java [109-118]

  enhancement: "Stabilize IO-error test with temp directory facilities."

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java [121-133]

  enhancement: "IO error coverage for clear() edge cases."

• write-ahead-log/src/test/java/com/iluwatar/writeaheadlog/WriteAheadLogTest.java [67-93]

  possible issue: "Restart resume relies on flush semantics."