Redis: under the hood

How does the?Redis?server work?

I was curious to learn more about Redis’s internals, so I’ve been familiarizing myself with the source, largely by reading and jumping around in Emacs. After I had peeled back enough of the onion’s layers, I realized I was trying to keep track of too many details in my head, and it wasn’t clear how it all hung together. I decided to write out in narrative form how an instance of the Redis server starts up and initializes itself, and how it handles the request/response cycle with a client, as a way of explaining it to myself, hopefully in a clear fashion. Luckily, Redis has a nice, clean code base that is easy to read and follow along. Armed with a?TAGSfile, my?$EDITOR, and GDB, I set out to see how it all works under the hood. (Incidentally, I was working with the Redis code base as ofcommit b4f2e41. Of course, internals such as I outline below are subject to change. However, the broad architecture of the server is unlikely to change very much, and I tried to keep that in mind as I went along.)

This article examines server startup and takes a high-level view of the request/response processing cycle. In a subsequent article, I’ll dive in to greater detail and trace a simple?SET/GET?command pair as they make their way through Redis.

Startup

Let’s begin with the?main()?function in?redis.c.

Beginning global server state initialization

First,?initServerConfig()?is called. This partially initializes a variable?server, which has the type?struct redisServer, that serves as the global server state.

// redis.h:338
struct redisServer {
    pthread_t mainthread;
    int port;
    int fd;
    redisDb *db;
    // ...
};

// redis.c:69
struct redisServer server