Chapter 5: hiredis — The C Redis Client Library Internals

What hiredis Is

hiredis is the official C client library for Redis. Both db-proxy and db-mux use it internally. It handles:

• RESP protocol encoding/decoding
• Connection management
• Async command dispatch (non-blocking)
• Output buffering

Async Mode (What PCG Uses)

// Create async connection
redisAsyncContext *ctx = redisAsyncConnect("127.0.0.1", 6379);

// Register event loop adapter (hooks hiredis into EVL)
redisAsyncSetConnectCallback(ctx, on_connect);
redisAsyncSetDisconnectCallback(ctx, on_disconnect);

// Send a command (NON-BLOCKING)
redisAsyncCommand(ctx, reply_callback, privdata, "SET %s %b", key, val, val_len);
// This does NOT send over network immediately!
// It formats the RESP command and appends to ctx->obuf (output buffer)
// Then calls the "addWrite" hook to signal "I have data to send"

The Output Buffer (obuf)

The addWrite/delWrite Callbacks

hiredis doesn't know about your event loop. It uses callbacks to signal when it needs to write:

// hiredis calls this when it has data to send:
void addWrite(void *privdata) {
    // "I have data in obuf, please enable POLLOUT for my fd"
    evl_fd_add_events(fd, POLLOUT);
}

// hiredis calls this when obuf is empty:
void delWrite(void *privdata) {
    // "obuf is empty, no more data to send, disable POLLOUT"
    evl_fd_del_events(fd, POLLOUT);
}

redisAsyncFormattedCommand (What db-mux Uses)

// Normal path (db-proxy uses this internally):
redisAsyncCommandArgv(ctx, cb, priv, argc, argv, argvlen);
// hiredis formats RESP internally (allocates SDS string, snprintf, etc.)
// = 2-3 heap allocations per command

// Pre-formatted path (db-mux uses this):
char *cmd = preformatted_resp_buffer;  // already in RESP format
redisAsyncFormattedCommand(ctx, cb, priv, cmd, cmd_len);
// hiredis just appends to obuf — NO formatting, NO allocations
// = 0 heap allocations per command

The Hidden API (From Today's Meeting)

Patrik mentioned a "hidden API" in hiredis to push to the output buffer without triggering a flush. This likely refers to directly appending to ctx->obuf (or using internal functions) while suppressing the addWrite callback — giving the application full control over when to flush.

Summary: Data Flow Through hiredis