redis7源码分析：redis 单线程模型解析，一条get命令执行流程

有了下文的梳理后

redis 启动流程

再来解析redis 在单线程模式下解析并处理客户端发来的命令

1. 当 client fd 可读时，会回调readQueryFromClient函数

void readQueryFromClient(connection *conn) {
client *c = connGetPrivateData(conn);
    int nread, big_arg = 0;
    size_t qblen, readlen;

    /* Check if we want to read from the client later when exiting from
     * the event loop. This is the case if threaded I/O is enabled. */
    // 这里是多线程模型走的地方，被文分析单线程模式，不走这里
    if (postponeClientRead(c)) return;
    ......
    /* There is more data in the client input buffer, continue parsing it
     * and check if there is a full command to execute. */
    //  走到这里，开始处理客户端发来的命令
    if (processInputBuffer(c) == C_ERR)
         c = NULL;
}

2. 分析processInputBuffer

int processInputBuffer(client *c) {
    /* Keep processing while there is something in the input buffer */
    while(c->qb_pos < sdslen(c->querybuf)) {
        if (c->flags & CLIENT_BLOCKED) break;
        if (c->flags & CLIENT_PENDING_COMMAND) break;
        if (isInsideYieldingLongCommand() && c->flags & CLIENT_MASTER) break;

        if (c->flags & (CLIENT_CLOSE_AFTER_REPLY|CLIENT_CLOSE_ASAP)) break;

        ......

        /* Multibulk processing could see a <= 0 length. */
        if (c->argc == 0) {
            resetClient(c);
        } else {
            if (io_threads_op != IO_THREADS_OP_IDLE) {
                serverAssert(io_threads_op == IO_THREADS_OP_READ);
                c->flags |= CLIENT_PENDING_COMMAND;
                break;
            }

            /* We are finally ready to execute the command. */
            // 最终在这里执行命令
            if (processCommandAndResetClient(c) == C_ERR) {
                return C_ERR;
            }
        }
    }
    ......

    return C_OK;
}

2.1 分析 processCommandAndResetClient

int processCommandAndResetClient(client *c) {
    int deadclient = 0;
    client *old_client = server.current_client;
    server.current_client = c;
    // 这里处理命令
    if (processCommand(c) == C_OK) {
        commandProcessed(c);
        /* Update the client's memory to include output buffer growth following the
         * processed command. */
        updateClientMemUsageAndBucket(c);
    }

    if (server.current_client == NULL) deadclient = 1;
    /*
     * Restore the old client, this is needed because when a script
     * times out, we will get into this code from processEventsWhileBlocked.
     * Which will cause to set the server.current_client. If not restored
     * we will return 1 to our caller which will falsely indicate the client
     * is dead and will stop reading from its buffer.
     */
    server.current_client = old_client;
    /* performEvictions may flush slave output buffers. This may
     * result in a slave, that may be the active client, to be
     * freed. */
    return deadclient ? C_ERR : C_OK;
}

2.2 分析 processCommand

int processCommand(client *c) {
    ......
    /* Now lookup the command and check ASAP about trivial error conditions
     * such as wrong arity, bad command name and so forth. */
    // 这里搜索命令，sentinel的命令也是在这里处理
    // 不像老版本，6.0以前，使用的是命令覆盖的方式，改用搜索
    c->cmd = c->lastcmd = c->realcmd = lookupCommand(c->argv,c->argc);

    ......
    /* Exec the command */
    if (c->flags & CLIENT_MULTI &&
        c->cmd->proc != execCommand &&
        c->cmd->proc != discardCommand &&
        c->cmd->proc != multiCommand &&
        c->cmd->proc != watchCommand &&
        c->cmd->proc != quitCommand &&
        c->cmd->proc != resetCommand)
    {
        queueMultiCommand(c, cmd_flags);
        addReply(c,shared.queued);
    } else {
        //最终普通命令调用 call函数执行
        call(c,CMD_CALL_FULL);
        c->woff = server.master_repl_offset;
        if (listLength(server.ready_keys) && !isInsideYieldingLongCommand())
            handleClientsBlockedOnKeys();
    }
}

2.3 call函数在server.c中

void call(client *c, int flags) {
......
    server.in_nested_call++;
    c->cmd->proc(c);
    server.in_nested_call--;
......
}

最终调用cmd 对应的proc

假设是get 请求，那么在lookupCommand时就会找到对应的command 对象，由redisCommandTable可知，对应方法为getCommand

t_string.c

void getCommand(client *c) {
    getGenericCommand(c);
}

int getGenericCommand(client *c) {
    robj *o;

    // 搜索db中数据
    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.null[c->resp])) == NULL)
        return C_OK;

    if (checkType(c,o,OBJ_STRING)) {
        return C_ERR;
    }

    // 将数据写入到输出缓存中，待eventloop 发现可写后发送
    addReplyBulk(c,o);
    return C_OK;
}