Java Append-Only Log (WAL)

Junior implementation of a Write-Ahead Log (WAL) in Java.
Append-only logs are the foundation of modern databases and distributed systems, ensuring durability, recovery, and sequential I/O efficiency.

This project started as a simple LogFile class (append + read), then evolved into a more production-like design inspired by LevelDB, RocksDB, and Kafka.

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✨ Features

• Append-only writes (append(record)) and sequential reads.
• Binary record format: [len:4B][data:len][crc:4B].
• CRC32 checksums for corruption detection.
• Page buffering with padding (configurable size).
• Configurable fsync policy (always / never / every N / time-based).
• Record fragmentation across pages (FULL/FIRST/MIDDLE/LAST).
• Recovery scan: detects incomplete/corrupt pages and truncates safely.
• Segment rotation with wal-000001.log, wal-000002.log, etc.
• Iterator API for log replay.

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📖 Why?

Write-Ahead Logs are used in:

• Databases (Postgres, MySQL/InnoDB, FoundationDB, LevelDB, RocksDB)
• Distributed consensus (Raft, Paxos)
• Event streaming systems (Kafka, Pulsar)

Understanding WAL internals builds strong foundations for storage engines, transaction systems, and distributed databases.

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🚀 Quick Start

// Create log
LogFile log = new LogFile("mylog.bin");
log.append(new Record("hello".getBytes()));
log.append(new Record("bye".getBytes()));
log.close();

// Replay log
LogFile reader = new LogFile("mylog.bin");
Record rec;
while ((rec = reader.readNext()) != null) {
    System.out.println(rec);
}
reader.close();

📊 Diagram