Redis AOF重写时如何与主进程通信

Redis在进行AOF重写时，主进程仍然在接收客户端写操作，那么这些新写操作会记录如何写到AOF重写日志中？是否会和正在进行中的AOF写记录冲突？如何保证顺序？

(1) AOF重写的基本过程

/* ------------------------
 * AOF background rewrite
 * ------------------------
 */

/* This is how rewriting of the append only file in background works:
 *
 * 1) The user calls BGREWRITEAOF
 * 2) Redis calls this function, that forks():
 *    2a) the child rewrite the append only file in a temp file.
 *    2b) the parent accumulates differences in server.aof_rewrite_buf.
 * 3) When the child finished '2a' exists.
 * 4) The parent will trap the exit code, if it's OK, will append the
 *    data accumulated into server.aof_rewrite_buf into the temp file, and
 *    finally will rename(2) the temp file in the actual file name.
 *    The the new file is reopened as the new append only file. Profit!
 */
int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;

    if (hasActiveChildProcess()) return C_ERR;

    // 创建管道，用于父子进程通信
    if (aofCreatePipes() != C_OK) return C_ERR;
    openChildInfoPipe();
    if ((childpid = redisFork(CHILD_TYPE_AOF)) == 0) {
        char tmpfile[256];

        /* Child */
        redisSetProcTitle("redis-aof-rewrite");
        redisSetCpuAffinity(server.aof_rewrite_cpulist);
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
        if (rewriteAppendOnlyFile(tmpfile) == C_OK) {
            sendChildCOWInfo(CHILD_TYPE_AOF, "AOF rewrite");
            exitFromChild(0);
        } else {
            exitFromChild(1);
        }
    } else {
        /* Parent */
        if (childpid == -1) {
            closeChildInfoPipe();
            serverLog(LL_WARNING,
                "Can't rewrite append only file in background: fork: %s",
                strerror(errno));
            aofClosePipes();
            return C_ERR;
        }
        serverLog(LL_NOTICE,
            "Background append only file rewriting started by pid %d",childpid);
        server.aof_rewrite_scheduled = 0;
        server.aof_rewrite_time_start = time(NULL);
        server.aof_child_pid = childpid;
        updateDictResizePolicy();
        /* We set appendseldb to -1 in order to force the next call to the
         * feedAppendOnlyFile() to issue a SELECT command, so the differences
         * accumulated by the parent into server.aof_rewrite_buf will start
         * with a SELECT statement and it will be safe to merge. */
        server.aof_selected_db = -1;
        replicationScriptCacheFlush();
        return C_OK;
    }
    return C_OK; /* unreached */
}

(1.1) 如何使用管道进行父子进程间通信？

在AOF重写过程中，主进程和AOF子进程通过管道通信。

/* Create the pipes used for parent - child process IPC during rewrite.
 * We have a data pipe used to send AOF incremental diffs to the child,
 * and two other pipes used by the children to signal it finished with
 * the rewrite so no more data should be written, and another for the
 * parent to acknowledge it understood this new condition. */
int aofCreatePipes(void) {
    int fds[6] = {-1, -1, -1, -1, -1, -1};
    int j;

    if (pipe(fds) == -1) goto error; /* parent -> children data. */
    if (pipe(fds+2) == -1) goto error; /* children -> parent ack. */
    if (pipe(fds+4) == -1) goto error; /* parent -> children ack. */


    /* Parent -> children data is non blocking. */
    // 
    if (anetNonBlock(NULL,fds[0]) != ANET_OK) goto error;
    if (anetNonBlock(NULL,fds[1]) != ANET_OK) goto error;

    // 创建文件事件
    if (aeCreateFileEvent(server.el, fds[2], AE_READABLE, aofChildPipeReadable, NULL) == AE_ERR) goto error;

    // 主进程和重写子进程间用于传递操作命令的管道-写描述符
    server.aof_pipe_write_data_to_child = fds[1];
    // 主进程和重写子进程间用于传递操作命令的管道-读描述符
    server.aof_pipe_read_data_from_parent = fds[0];
    // 重写子进程向父进程发送ACK信息的管道-写描述符
    server.aof_pipe_write_ack_to_parent = fds[3];
    // 重写子进程向父进程发送ACK信息的管道-读描述符
    server.aof_pipe_read_ack_from_child = fds[2];
    // 父进程向重写子进程发送ACK信息的管道-写描述符
    server.aof_pipe_write_ack_to_child = fds[5];
    // 父进程向重写子进程发送ACK信息的管道-读描述符
    server.aof_pipe_read_ack_from_parent = fds[4];
    server.aof_stop_sending_diff = 0;
    return C_OK;

error:
    serverLog(LL_WARNING,"Error opening /setting AOF rewrite IPC pipes: %s",
        strerror(errno));
    for (j = 0; j < 6; j++) if(fds[j] != -1) close(fds[j]);
    return C_ERR;
}

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