● Stage 01 • Networking & Protocols

In-Memory Key-Value Store

Implement an in-memory database server. Parse the binary Redis wire protocol over raw TCP sockets, handle concurrent connections with non-blocking I/O multiplexing, and support crash recovery.

First steps

Accept one TCP connection on port 6379.
Read a RESP PING. Reply with +PONG\r\n.
Check with redis-cli -p 6379 PING. Add SET and GET before concurrency.

Project Goal

Understand socket programming and raw TCP stream processing.
Parse a binary application-layer protocol (RESP) without third-party frameworks.
Handle thousands of concurrent client connections using an event loop or worker pool.
Implement in-memory data structures with expiration policies and crash recovery logging.

Skills Acquired

POSIX TCP Sockets Wire Protocol Serialization Non-blocking I/O (epoll/kqueue) TCP Buffer Accumulation Active & Passive Expiration Append-Only File (AOF) Durability

Technical Specification

Port: Bind raw TCP socket on 0.0.0.0:6379.
Protocol: Full support for Redis Serialization Protocol (RESP).
Supported Commands: PING, ECHO, SET [key] [val] [EX sec], GET [key], DEL [key], EXPIRE [key] [sec].
Persistence: Append-Only File (appendonly.aof) replayed on startup.

Milestones

01.1

TCP Socket Server & Buffer Accumulator

Bind port 6379 with SO_REUSEADDR. Maintain a read buffer per connection to handle partial packet arrivals split across TCP segments.

01.2

RESP Stream Parser

Implement a state machine parser for RESP frames: Simple Strings (+), Errors (-), Integers (:), Bulk Strings ($), and Arrays (*).

01.3

Non-Blocking I/O Multiplexing

Set sockets to non-blocking mode (O_NONBLOCK). Use an event loop (epoll / kqueue) or worker pool to handle multiple connections without blocking.

01.4

Key Expiration (Active & Passive)

Delete expired keys during GET lookups (passive). Run a periodic timer sampling keys every 100ms to clear expired entries from memory (active).

01.5

Append-Only File (AOF) Durability

Append every write command (SET, DEL) to disk. On server reboot, read and replay the file to restore in-memory state.

Verification Commands

Install the Redis command-line tools, but run these against your own server on port 6379, not a Redis server. The first GET should return "active"; the second should return (nil).

# 1. Test basic operations with official redis-cli
redis-cli -p 6379 PING
redis-cli -p 6379 SET token_user123 "active" EX 2
redis-cli -p 6379 GET token_user123
sleep 2
redis-cli -p 6379 GET token_user123 # Returns (nil)

# 2. Run official redis-benchmark under concurrency
redis-benchmark -p 6379 -t set,get -n 10000 -c 50 -q

Acceptance Criteria

The server must pass all standard redis-cli commands. redis-benchmark must complete 10,000 operations across 50 concurrent connections with zero dropped sockets. Terminating the process with kill -9 and restarting must restore all unexpired keys.

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