Custom Redis

Every connection gets its own parser, and every parsed command goes through one table of handlers. A handler reads or changes the keyspace — a single in-memory object — and returns its reply already encoded as RESP, or nothing, when it will answer later.

A parser that expects TCP to be messy

TCP delivers bytes, not commands: one read can hold half a command, or five pipelined ones. So each connection adds what arrives to a buffer, and the parser takes whole values off the front, one at a time. When the next one is cut short, it stops and waits for more bytes rather than guess. Malformed input is logged and skipped, up to the next byte that can start a RESP value — in a loop, not by recursion, so a flood of junk can't overflow the stack.

Each command also keeps the exact bytes it was parsed from. Replication forwards those bytes as they are, and counts its progress in them.

One event loop, so every command is atomic

NodeJS runs one callback at a time, and no handler waits on anything: each reads, writes and replies in one go. No other client can get in between, so there's nothing to lock.

BLPOP and XREAD BLOCK wait for data without holding up anyone else. The handler puts a callback in a queue for the key and returns nothing, so no reply goes out yet. When RPUSH or XADD adds to that key, it schedules a check with setImmediate: the writer gets its own reply first, then the longest-waiting client gets the value. A client that gave a timeout has a timer, which takes its callback out of the queue and answers with a null.

Started with --replicaof, a server connects to its master, introduces itself and asks to sync from scratch. The master answers FULLRESYNC with its replication id and offset, then sends a snapshot of its data as an RDB file. That arrives like a bulk string, but without the closing CRLF, so the parser has a mode for it.

From then on the master forwards each write to every replica as the bytes it received, and both sides count their offset in those bytes. The replica runs them through the same router as any client's commands — the master is just another connection to it, one that gets no replies.

WAIT

WAIT 2 500 asks: have at least two replicas caught up, within 500 ms? The master sends every replica REPLCONF GETACK *, counts those whose acknowledged offset matches its own, and replies as soon as enough have — or, when the time runs out, with however many had.