🚀 Java Redis Server

A high-performance, lightweight, in-memory Redis-compatible server built from the ground up using Java 25 and Netty. This project implements core Redis functionality with a focus on low latency, efficient memory usage, clean architecture, and production-grade replication.

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✨ Features

Core Capabilities

• ⚡ High Performance: Built on Netty's asynchronous event-driven framework for massive concurrency
• 💾 In-Memory Storage: Optimized data structures using ConcurrentHashMap for thread-safe, lock-free reads
• 🔌 Redis Protocol (RESP): Full RESP implementation, compatible with any standard Redis client (redis-cli, jedis, lettuce, redis-py, etc.)
• ⏳ Advanced Expiration: Dual-strategy expiration (Lazy + Active background cleanup via DelayQueue)
• 🔄 Transaction Support: Full MULTI/EXEC/DISCARD support with optimized batch execution
• 📊 Pipelining: Full support for command pipelining with efficient batch processing

Replication (Master-Replica)

• 🔗 Master-Replica Architecture: Production-grade single-writer, log-based state machine
• 📡 Command Propagation: Automatic propagation with backpressure handling
• 🔄 Deterministic Replication: Commands are canonicalized before replication to ensure consistent state:
  • XADD stream * → XADD stream <actual-id> (auto-generated IDs resolved)
  • SET key value EX 60 → SET key value PXAT <timestamp> (relative → absolute time)
  • EXPIRE key 60 → PEXPIREAT key <timestamp> (relative → absolute time)
• 📦 Partial Resync (PSYNC2): Efficient reconnection with backlog-based partial resync
• 💾 RDB Snapshots: Point-in-time snapshots for full resync
• 🚫 Write Protection: Replicas automatically reject write commands (READONLY error)
• 📊 Replication Log: Lock-free ring buffer with configurable size (64KB - 512MB)

Advanced Features

• 🌊 Redis Streams: Full XADD/XRANGE/XREAD support with blocking reads
• 🚫 Blocking Operations: BLPOP with efficient async handling (no thread blocking)
• 🏗️ Extensible Command Registry: Easy to add new commands via simple interface

🚀 Key Differentiators from Redis

Feature	Redis	This Implementation
Offset Tracking	Mutex-protected long	Lock-free AtomicLong
Statistics	Atomic increments	LongAdder (10x faster under contention)
Backlog	Single producer lock	Lock-free ring buffer with minimal contention
Propagation	Synchronous per-replica	Async with circuit breaker pattern
WAIT Polling	Fixed interval	Adaptive exponential backoff
Memory	Unbounded backlog growth	Bounded with automatic eviction
Failure Handling	Simple disconnect	Circuit breaker + self-healing recovery
Health Monitoring	Basic lag tracking	Per-replica health scores & backpressure detection

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🛠️ Supported Commands

Detailed documentation for each command can be found in the docs/commands directory.

🔑 Connection & Utility

Command	Usage	Description
PING	PING [message]	Test connection, returns PONG or echoes message
ECHO	ECHO message	Echo the given message
EXPIRE	EXPIRE key seconds	Set key expiration in seconds
TTL	TTL key	Get remaining time to live in seconds
TYPE	TYPE key	Get the type of value stored at key
DEL	DEL key [key ...]	Delete one or more keys

📝 String Operations

Command	Usage	Description
SET	SET key value [EX s] [PX ms] [PXAT ms] [NX|XX]	Set string value with optional expiry
GET	GET key	Get the value of a key
INCR	INCR key	Increment integer value by 1

🔄 Transactions

Command	Usage	Description
MULTI	MULTI	Start a transaction block
EXEC	EXEC	Execute all commands in transaction
DISCARD	DISCARD	Discard all commands in transaction

📋 List Operations

Command	Usage	Description
LPUSH	LPUSH key element [element ...]	Push elements to head of list
RPUSH	RPUSH key element [element ...]	Push elements to tail of list
LPOP	LPOP key [count]	Pop elements from head of list
LLEN	LLEN key	Get list length
LRANGE	LRANGE key start stop	Get range of elements
BLPOP	BLPOP key [key ...] timeout	Blocking pop from head

🌊 Stream Operations

Command	Usage	Description
XADD	XADD key ID field value [field value ...]	Append entry to stream
XRANGE	XRANGE key start end [COUNT count]	Get range of entries
XREAD	XREAD [COUNT c] [BLOCK ms] STREAMS key [key ...] id [id ...]	Read from streams

🔗 Replication Commands

Command	Usage	Description
REPLCONF	REPLCONF <option> <value>	Configure replication
PSYNC	PSYNC replicationid offset	Initiate replication sync
INFO	INFO [section]	Get server information

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🚀 Getting Started

Prerequisites

• JDK 25 or higher (with --enable-preview)
• Maven 3.9+

Build & Run

# Clone the repository
git clone <repository-url>
cd redis-java

# Build the project
mvn clean package

# Start as master (default)
java --enable-preview -jar target/redis-server.jar

# Start as replica
java --enable-preview -jar target/redis-server.jar --port 6380 --replicaof localhost 6379

The server listens on port 6379 by default.

Connect with redis-cli

redis-cli -p 6379
127.0.0.1:6379> PING
PONG
127.0.0.1:6379> SET mykey "Hello, Redis!"
OK
127.0.0.1:6379> GET mykey
"Hello, Redis!"
127.0.0.1:6379> SET counter 100
OK
127.0.0.1:6379> INCR counter
(integer) 101

Replication Example

# Terminal 1: Start master
java --enable-preview -jar target/redis-server.jar --port 6379

# Terminal 2: Start replica
java --enable-preview -jar target/redis-server.jar --port 6380 --replicaof localhost 6379

# Terminal 3: Write to master
redis-cli -p 6379 SET foo bar
OK

# Terminal 4: Read from replica (automatically replicated!)
redis-cli -p 6380 GET foo
"bar"

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🏗️ Architecture

┌─────────────────────────────────────────────────────────────────────────────┐
│                              Client Connections                              │
└─────────────────────────────────────────────────────────────────────────────┘
                                       │
                                       ▼
┌─────────────────────────────────────────────────────────────────────────────┐
│                         Netty EventLoop (Boss + Worker)                      │
│  • Async I/O multiplexing       • Zero-copy buffer management               │
│  • Connection handling          • Backpressure support                      │
└─────────────────────────────────────────────────────────────────────────────┘
                                       │
                                       ▼
┌─────────────────────────────────────────────────────────────────────────────┐
│                           RedisCommandHandler                                │
│  • RESP protocol parsing        • Pipelining support                        │
│  • Transaction management       • Command routing                           │
└─────────────────────────────────────────────────────────────────────────────┘
                                       │
                        ┌──────────────┴──────────────┐
                        ▼                              ▼
┌──────────────────────────────────┐  ┌──────────────────────────────────────┐
│        CommandRegistry           │  │        ReplicationManager             │
│  • O(1) command lookup           │  │  • Master/Replica role management    │
│  • Extensible registration       │  │  • Command propagation               │
└──────────────────────────────────┘  │  • Backlog for partial resync        │
                        │              │  • Lock-free offset tracking         │
                        ▼              └──────────────────────────────────────┘
┌──────────────────────────────────┐                    │
│      Command Implementations     │                    │
│  • String: SET, GET, INCR        │                    ▼
│  • List: LPUSH, RPUSH, BLPOP     │  ┌──────────────────────────────────────┐
│  • Stream: XADD, XRANGE, XREAD   │  │        CommandPropagator             │
│  • Transaction: MULTI, EXEC      │  │  • Deterministic command rewriting   │
└──────────────────────────────────┘  │  • XADD * → XADD <actual-id>         │
                        │              │  • SET EX → SET PXAT                 │
                        ▼              │  • EXPIRE → PEXPIREAT                │
┌──────────────────────────────────┐  └──────────────────────────────────────┘
│          RedisDatabase           │                    │
│  • ConcurrentHashMap storage     │                    ▼
│  • Atomic compute operations     │  ┌──────────────────────────────────────┐
│  • Type-safe RedisValue wrapper  │  │         Replica Connections          │
└──────────────────────────────────┘  │  • Async command propagation         │
                        │              │  • Backpressure handling             │
                        ▼              │  • Lag monitoring                    │
┌──────────────────────────────────┐  └──────────────────────────────────────┘
│          ExpiryManager           │
│  • DelayQueue-based scheduling   │
│  • Background cleanup thread     │
│  • O(log n) insertion            │
└──────────────────────────────────┘

Key Design Decisions

Component	Design Choice	Rationale
Storage	ConcurrentHashMap	Lock-free reads, segment-level locking for writes
Lists	LinkedList	O(1) head/tail operations for LPUSH/RPUSH/LPOP
Streams	ConcurrentSkipListMap	O(log n) range queries, thread-safe
Expiry	DelayQueue + lazy	Hybrid approach balances memory and CPU
Replication	Lock-free AtomicLong	High-throughput offset tracking
Statistics	LongAdder	10x better than AtomicLong under contention

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🔄 Replication Deep Dive

Architecture Overview

This implementation follows a single-writer, log-based state machine architecture with snapshot checkpoints and asynchronous followers:

┌─────────────────────────────────────────────────────────────────────────────┐
│                            REPLICATION ARCHITECTURE                          │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│   ┌─────────────────────────────────────────────────────────────────────┐   │
│   │                           MASTER                                     │   │
│   │  ┌──────────┐    ┌──────────────┐    ┌─────────────────────────┐   │   │
│   │  │ Accepts  │───▶│ Execute &    │───▶│ Append to               │   │   │
│   │  │ Writes   │    │ Canonicalize │    │ Replication Log         │   │   │
│   │  └──────────┘    └──────────────┘    └───────────┬─────────────┘   │   │
│   │                                                   │                  │   │
│   │                              ┌────────────────────┴──────────────┐  │   │
│   │                              ▼                                    ▼  │   │
│   │                   ┌──────────────────┐              ┌───────────────┐│   │
│   │                   │ Propagate to     │              │ Snapshot      ││   │
│   │                   │ Replicas         │              │ Producer      ││   │
│   │                   └──────────────────┘              └───────────────┘│   │
│   └─────────────────────────────────────────────────────────────────────┘   │
│                                    │                                         │
│            ┌───────────────────────┼───────────────────────┐                │
│            ▼                       ▼                       ▼                │
│   ┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐        │
│   │    REPLICA 1    │    │    REPLICA 2    │    │    REPLICA N    │        │
│   │  ┌───────────┐  │    │  ┌───────────┐  │    │  ┌───────────┐  │        │
│   │  │ Rejects   │  │    │  │ Rejects   │  │    │  │ Rejects   │  │        │
│   │  │ Writes    │  │    │  │ Writes    │  │    │  │ Writes    │  │        │
│   │  └───────────┘  │    │  └───────────┘  │    │  └───────────┘  │        │
│   │  ┌───────────┐  │    │  ┌───────────┐  │    │  ┌───────────┐  │        │
│   │  │ Replays   │  │    │  │ Replays   │  │    │  │ Replays   │  │        │
│   │  │ Commands  │  │    │  │ Commands  │  │    │  │ Commands  │  │        │
│   │  └───────────┘  │    │  └───────────┘  │    │  └───────────┘  │        │
│   └─────────────────┘    └─────────────────┘    └─────────────────┘        │
└─────────────────────────────────────────────────────────────────────────────┘

Core Roles

Role	Responsibilities	Behavior
Master	Accepts writes, executes commands, owns replication log	Propagates canonicalized commands to replicas
Replica	Rejects writes (READONLY), replays commands, tracks offset	Requests PSYNC for synchronization
Snapshot Producer	Creates point-in-time state snapshots	Independent of live mutations

Command Pipeline

The exact command execution flow ensures consistency:

parse → validate → canonicalize → execute → append to log → send to replicas

Key principle: Replication is always downstream, never upstream.

Command Canonicalization

To ensure consistent state across master and replicas, commands are rewritten to their canonical (deterministic) form before propagation:

Original Command	Propagated Command	Reason
XADD stream * field value	XADD stream 1706745600000-0 field value	Auto-generated IDs depend on timestamp
SET key value EX 60	SET key value PXAT 1706745660000	Relative time → absolute timestamp
SET key value PX 5000	SET key value PXAT 1706745605000	Relative time → absolute timestamp
EXPIRE key 60	PEXPIREAT key 1706745660000	Relative seconds → absolute milliseconds

Full Sync vs Partial Sync Decision

When a replica connects, the master uses this decision logic:

Replica connects → sends replicationId + offset
                          │
                          ▼
              ┌─────────────────────┐
              │ ID matches master?  │─── No ──▶ FULL SYNC
              └─────────────────────┘
                          │ Yes
                          ▼
              ┌─────────────────────┐
              │ Offset in backlog?  │─── No ──▶ FULL SYNC  
              └─────────────────────┘
                          │ Yes
                          ▼
                    PARTIAL SYNC

No heuristics. No guessing. The decision is deterministic.

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🧪 Testing

We maintain high confidence through comprehensive unit and integration tests.

# Run all tests (Unit + Integration)
./run_all_tests.sh

# Run only unit tests
mvn test

# Run only integration tests  
mvn verify -DskipUnitTests

# Run specific test class
mvn test -Dtest=CommandPropagatorTest

# Run with verbose output
mvn test -X

Test Coverage

Category	Tests	Description
Unit Tests	400+	Individual command and component testing
Integration Tests	100+	End-to-end RESP protocol testing
Replication Tests	100+	Master-replica sync, backlog, snapshots, scale tests
Total	599	Comprehensive coverage verified by CI

Replication Testing

# Run replication-specific tests
mvn test -Dtest=MasterReplicaIntegrationTest

# Run scale/stress tests (benchmarks)
mvn test -Dtest=ReplicationScaleTest

# Run end-to-end replication test with actual servers
./test_replication.sh

# Run with stress test
./test_replication.sh --stress

# Run with multiple replicas
./test_replication.sh --scale

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⚙️ Configuration

Command Line Arguments

java --enable-preview -jar target/redis-server.jar [options]

Options:
  --port <port>              Server port (default: 6379)
  --replicaof <host> <port>  Start as replica of specified master

Configuration File

Edit src/main/resources/application.properties:

Property	Default	Description
redis.port	6379	Port to listen on
redis.boss.threads	1	Number of acceptor threads
redis.worker.threads	1	Number of I/O threads
redis.cleanup.interval.ms	5000	Expiry cleanup interval
redis.expiry.enabled	true	Enable/disable key expiration
redis.repl.backlog.size	1048576	Replication backlog size (1MB)

Environment Variables

Variable	Description
REDIS_PORT	Override server port
REDIS_EXPIRY_ENABLED	Override expiry setting

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🔒 Production Considerations

Memory Management

• Keys are stored in a ConcurrentHashMap with lazy expiration
• Background cleanup prevents memory leaks from expired keys
• Replication backlog is bounded (1MB default) with automatic eviction
• Lists use LinkedList for O(1) push/pop operations
• Streams use ConcurrentSkipListMap for efficient range queries

Thread Safety

• All storage operations are atomic via compute() operations
• Lock-free statistics using LongAdder (10x better than AtomicLong under contention)
• Per-connection state isolation (no shared mutable state between connections)
• Replication offsets tracked with AtomicLong for lock-free updates

Monitoring

• INFO command provides comprehensive server statistics
• INFO replication shows per-replica lag, offset, and health
• Circuit breaker status and backpressure events tracked
• Enhanced metrics: repl_commands_propagated, repl_bytes_propagated, repl_backpressure_events

High Availability

• Replicas automatically reject writes with -READONLY error
• Circuit breaker pattern protects against slow/failing replicas
• Automatic circuit breaker recovery after 5 seconds
• PSYNC2 enables efficient partial resync after brief disconnections

Capacity Planning

Workload	Backlog Size	Notes
Low (< 100 ops/s)	64KB	Minimum for basic partial resync
Medium (< 10K ops/s)	1MB (default)	Handles most workloads
High (> 10K ops/s)	16-64MB	For write-heavy applications
Enterprise (> 100K ops/s)	256-512MB	Maximum supported

Recommended JVM Settings

java --enable-preview \
  -Xms256m -Xmx1g \
  -XX:+UseZGC \
  -XX:+ZGenerational \
  -XX:+AlwaysPreTouch \
  -Djava.lang.Integer.IntegerCache.high=10000 \
  -jar target/redis-server.jar

Production Checklist

• Set appropriate JVM heap size (-Xmx)
• Configure backlog size based on write volume
• Enable ZGC for low-latency GC pauses
• Monitor INFO replication for lag metrics
• Set up replica health alerts for circuit breaker trips

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🤝 Contributing

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

Code Style

• Follow existing code patterns
• Add comprehensive JavaDoc for public APIs
• Include unit tests for new functionality
• Ensure all tests pass before submitting PR

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📄 License

Distributed under the MIT License. See LICENSE for more information.

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🙏 Acknowledgments

• Redis - The original inspiration
• Netty - High-performance networking framework
• JUnit 5 - Testing framework