Tiered Queue:

TieredQueue is a high-performance, embedded hybrid (memory + file backed) message queue built for systems where:

• Low-latency reads are essential
• Durability is required
• Scalability beyond memory limits is needed

It combines an in-memory FIFO queue for fast access and a disk-backed write-ahead log (WAL) using memory-mapped segmented files for durability and capacity. It behaves like a sliding pipe where the head lives in memory (hot path), and the tail is written to disk (cold path).

Features:

• Sliding memory window for fast, FIFO in-memory reads
• Disk-backed persistence using memory-mapped segment files
• Automatic backfill of memory from disk
• Zero-copy IO via MappedByteBuffer
• Strict FIFO ordering preserved across memory and disk
• Efficient resource usage (memory + disk)
• Embeddable: no external dependencies or brokers

Architecture:

        [ Memory Queue (Head) ] →→→→→ Readers
                ↑
Writers ---→    |  
                ↓
         [ Disk Queue (Tail) ]   

1. Writers:

• Prefer memory queue if disk has no backlog.
• Otherwise write to Disk Queue i.e disk log segments.

2. Memory Queue:

• Fixed-size FIFO buffer for low-latency reads.

3. Disk Queue:

• Segmented, memory-mapped files for durable storage.
• Loader reads from disk to refill memory when it runs low.

Future Improvements:

• Standardized API for easier integration with file based queues like Chronicle Queue.
• Concurrent access optimizations.
• Kryo serialization support.
• Metrics and monitoring.