Redis-Clone-Cpp

A from-scratch Redis implementation in C++ — raw sockets, real threads, zero dependencies on Redis itself.

Built over TCP/IPv4 using POSIX sockets and std::thread, this project reimplements the core of Redis from the ground up. It speaks the RESP2 wire protocol, meaning any standard redis-cli or Redis client library connects to it out of the box.

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Features

• RESP2 protocol — fully compatible with redis-cli and standard Redis clients
• Multi-client — each connection is handled in a detached std::thread
• Concurrent list operations — BLPOP uses std::mutex + std::condition_variable for safe blocking across threads
• Stream support — append-only streams with monotonic ID validation
• TTL expiry — millisecond-precision key expiration via std::chrono

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Getting Started

# Build
g++ -std=c++17 -pthread -o redis-clone main.cpp
 
# Run
./redis-clone
 
# Connect (in another terminal)
redis-cli -p 6379

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Supported Commands

General

Command	Syntax	Description
PING	PING	Returns PONG. Used to test connectivity.
ECHO	ECHO <message>	Returns the given message as a bulk string.
TYPE	TYPE <key>	Returns the type of the value stored at a key: string, list, stream, or none.

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Strings

Command	Syntax	Description
SET	SET <key> <value> [EX seconds | PX milliseconds]	Stores a string value. Optionally set expiry in seconds (EX) or milliseconds (PX).
GET	GET <key>	Returns the value at key, or nil if expired / not found.

Examples

SET name "alice"
SET session "abc123" EX 60
GET name       → "alice"
GET session    → "abc123" (or nil if expired)

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Lists

Command	Syntax	Description
RPUSH	RPUSH <key> <value> [value ...]	Appends one or more values to the tail of a list. Returns new list length.
LPUSH	LPUSH <key> <value> [value ...]	Prepends one or more values to the head of a list. Returns new list length.
LPOP	LPOP <key> [count]	Removes and returns element(s) from the head of the list.
LLEN	LLEN <key>	Returns the length of the list.
LRANGE	LRANGE <key> <start> <stop>	Returns a range of elements. Supports negative indices (-1 = last element).
BLPOP	BLPOP <key> <timeout>	Blocking pop — waits up to timeout seconds for an element. 0 blocks indefinitely. Woken up by RPUSH/LPUSH from another client.

Examples

RPUSH queue "job1" "job2"   → 2
LPUSH queue "job0"          → 3
LRANGE queue 0 -1           → ["job0", "job1", "job2"]
LPOP queue                  → "job0"
BLPOP queue 5               → blocks up to 5s, returns next pushed element

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Streams

Streams are append-only logs. Each entry has a unique <millisecondsTime>-<sequenceNumber> ID and one or more field-value pairs.

Command	Syntax	Description
XADD	XADD <key> <id> <field> <value> [field value ...]	Appends a new entry to a stream. The ID must be strictly greater than the last entry's ID. Returns the entry ID on success.

ID validation rules:

• time component must be ≥ last entry's time
• If time is equal, seq must be strictly greater
• Violating either returns: -ERR The ID specified in XADD is equal or smaller than the target stream top item Examples

XADD events 1-1 action "login" user "alice"   → "1-1"
XADD events 1-2 action "click"               → "1-2"
XADD events 1-2 action "duplicate"           → ERR (equal ID)
XADD events 0-9 action "past"               → ERR (smaller ID)

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Architecture Notes

• Boost.Asio — networking is raw POSIX (socket, bind, listen, accept, recv, send)
• Parser — a simple \r\n-delimited tokenizer that handles RESP2 arrays
• Thread model — one std::thread per client, detached; shared state protected by std::mutex
• BLPOP wake-up — RPUSH/LPUSH directly checks the waiter list under the same mutex and sends the response to the blocked client's fd

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Limitations

• No persistence (RDB/AOF)
• No replication
• No XREAD, XRANGE, or other stream query commands yet
• Single-buffer recv (max 1023 bytes per command — pipelining not supported)
• No AUTH or ACL