Scene 02

One thread, one command at a time — the Redis event loop and slow-command stalls

Redis runs every command on a single event-loop thread, so atomicity is free but one slow command stalls every client.

Previously

You've named the parts. Here's the first thing that defines how Redis behaves — there is exactly ONE event loop between every client and every byte of data.

Scene 02

One thread, one command at a time

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What to watch for

Three clients dispatch GETs and SETs into the loop. One thread pulls one command at a time, runs it to completion, then takes the next. With fast commands, every client gets sub-millisecond latency.

Implementation

Highlighted lines are the ones running in the diagram right now.

EventLoop.runOnce

the ae loop: one command, start to finish, before the next

1while server.running:
2    ready = epoll_wait(fds)
3    for fd in ready:
4        readQueryFromClient(fd)  # parses RESP
5    cmd = nextReadyCommand()
6    if cmd is not None:
7        reply = execute(cmd)     # touches the dataset
8        addReplyToClient(cmd.client, reply)
9    # nothing else runs until execute() returns
10    flushPendingWrites()

Command.KEYS

scans the whole keyspace in one command — no yielding

1def keysCommand(pattern):
2    matches = []
3    for key in server.db.dict:        # every key, every time
4        if stringmatch(pattern, key):
5            matches.append(key)
6    # loop is held the entire time;
7    # no other client is served until we return
8    return matches
9 
10# contrast: SCAN returns a cursor after ~COUNT keys
11# so the loop is free between batches

Sources

Where this sits in Build Redis

Scene 02 of 10, in the The loop act — One thread, one command — and the encoding under it.. Why a single event loop is fast — and why one slow command (KEYS *, big LRANGE, slow Lua) stalls every client.

Up next. Encodings — if one loop runs every command, the SHAPE of the value it's touching matters. Some shapes are cheap; some quietly aren't.

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