Redis 设计与实现（第十四章） -- 服务器

概述

---

Redis服务器负责与客户端建立网络连接，之前的数据结构部分已经看过了，本章主要从下面三个方面讲解。

1.服务器执行命令的过程

2.serverCron函数的执行

3.服务器的初始化

服务器执行命令的过程

一个客户端请求命令的基本过程大致如下：

1.客户端发送请求命令给服务器，比如set key value；

2.服务器端接受命令并处理，在数据库中进行设置操作，并返回ok；

3.客户端接受服务器返回的ok，并将这个回复打印给用户看。

接下来会根据这个大致的流程来讲一些细节的处理：

1.发送命令请求：

    当用户在客户端键入一个命令后，客户端会将这个命令转为相应的协议格式，并通过套接字，将命令请求的协议格式发送给服务器；

2.读取命令请求：

    服务器接受到套接字发送来的协议请求后，将其保存在客户端状态的输入缓冲区中（之前的看过redisServer和client的数据结构可以知道），如下；

    对输入缓冲区的协议格式进行解析，提前命令请求中的命令参数，以及参数的个数，分别将参数和参数个数保存到客户端状态的argv和argc属性中，如下；

    调用命令执行器，执行客户端指定的命令。

命令执行器的执行

查找命令实现

命令执行器首先查找argv[0]的参数，并在命令表中查找所指定的指令，并将指令保存到客户端状态的cmd属性中。

命令表是一个字典，键为一个命令名字，比如set、get、del等，而字典的值则是一个redisCommand的数据结构，每个RedisCommand记录一个命令的实现信息，数据结构如下：

struct redisCommand {
    char *name;  //命令的名字，比如set、get等
    redisCommandProc *proc;  //执行命令的实现函数
    int arity;  //命令参数的个数，用于校验命令请求的格式是否正确
    char *sflags; /* Flags as string representation, one char per flag. ，记录命令的属性，比如是读还是写等*/
    int flags;    /* The actual flags, obtained from the 'sflags' field.，对sflags的分析得出的二进制，服务器自动生成 */
    /* Use a function to determine keys arguments in a command line.
     * Used for Redis Cluster redirect. */
    redisGetKeysProc *getkeys_proc;  //
    /* What keys should be loaded in background when calling this command? */
    int firstkey; /* The first argument that's a key (0 = no keys) */
    int lastkey;  /* The last argument that's a key */
    int keystep;  /* The step between first and last key */
    long long microseconds, calls;  //服务器执行的总时长，和执行了多少次
};

下图分别用set和get展示了RedisCommand的结构：

set命令表示，name为set；参数arity未-3，表示接受三个或三个以上的参数；命令标识为'wm'，表示这是一个写入命令，并且在执行前要先对服务器占用内存检查，因为这个命令可能占用大量内存。（命令的大小写不限制）

查找到命令表后，将redisClient中的cmd指针指向redisCommand对应的结构。

 执行预备操作

服务器通过上述步骤，已经将执行命令所需函数、参数、参数个数都收集到了，在真正执行前，还需要执行一个进行一些预备操作，才能保证命令被正确执行，这些操作包括：

1.判断cmd的执行是否指向null，如果指向null，则说明命令不存在，这时给客户端返回错误信息；

2.根据artiy属性的值，判断argc中的参数个数是否正确，如果不正确，则返回错误；

3.检测客户端是否已经通过了认证，未通过认证的只能指向auth命令，如果未通过认证执行auth以为的命令会返回给客户端错误信息；

4.如果服务器打开了maxmemory命令，在命令执行前，先检查服务器的内存占用情况，并在有需要时进行回收，如果回收失败，则给客户端返回错误信息；

5.如果服务器上一次执行bgsave命令错误，并且服务器打开了stop-write-on-bgsave-error功能，如果此时执行的命令是写命令，那么则会返回错误；

6.如果当前客户端正在用subscribe命令订阅频道，或者正则用psubscribe命令订阅模式，那么服务器只会执行订阅有关的四个命令（subscribe，psubscribe，unsubscribe，punsubscribe），其他命令会被拒绝；

7.如果服务器正在执行数据载入，那么客户端发送的命令必须带有1标识（比如info，shutdown，publish等等）才能被服务器执行，其他命令会被拒绝；

8.如果服务器正在执行lua脚本而超时进入阻塞状态，那么服务器只会执行客户端发来的shutdown nosave和script kill命令，其他命令都会被拒绝；

9.如果客户端正在执行事务，那么服务器只会执行客户端发来的事务命令，exec，discard，multi，watch四个命令，其他命令都会被放入事务队列中；

10.如果服务器打开了监视器功能，那么服务器会将就要将执行的命令及参数发送给监视器。

上述步骤都完成后，就开始执行命令了（这里这讲了单机模式的，集群模式下还会有更多一些步骤）。

下面是commandProc的源码，上述步骤的判断与集群模式的处理都在里面：

/* If this function gets called we already read a whole
 * command, arguments are in the client argv/argc fields.
 * processCommand() execute the command or prepare the
 * server for a bulk read from the client.
 *
 * If 1 is returned the client is still alive and valid and
 * other operations can be performed by the caller. Otherwise
 * if 0 is returned the client was destroyed (i.e. after QUIT). */
int processCommand(redisClient *c) {
    /* The QUIT command is handled separately. Normal command procs will
     * go through checking for replication and QUIT will cause trouble
     * when FORCE_REPLICATION is enabled and would be implemented in
     * a regular command proc. */
    if (!strcasecmp(c->argv[0]->ptr,"quit")) {
        addReply(c,shared.ok);
        c->flags |= REDIS_CLOSE_AFTER_REPLY;
        return REDIS_ERR;
    }

    /* Now lookup the command and check ASAP about trivial error conditions
     * such as wrong arity, bad command name and so forth. */
    c->cmd = c->lastcmd = lookupCommand(c->argv[0]->ptr);
    if (!c->cmd) {
        flagTransaction(c);
        addReplyErrorFormat(c,"unknown command '%s'",
            (char*)c->argv[0]->ptr);
        return REDIS_OK;
    } else if ((c->cmd->arity > 0 && c->cmd->arity != c->argc) ||
               (c->argc < -c->cmd->arity)) {
        flagTransaction(c);
        addReplyErrorFormat(c,"wrong number of arguments for '%s' command",
            c->cmd->name);
        return REDIS_OK;
    }

    /* Check if the user is authenticated */
    if (server.requirepass && !c->authenticated && c->cmd->proc != authCommand)
    {
        flagTransaction(c);
        addReply(c,shared.noautherr);
        return REDIS_OK;
    }

    /* If cluster is enabled perform the cluster redirection here.
     * However we don't perform the redirection if:
     * 1) The sender of this command is our master.
     * 2) The command has no key arguments. */
    if (server.cluster_enabled &&
        !(c->flags & REDIS_MASTER) &&
        !(c->flags & REDIS_LUA_CLIENT &&
          server.lua_caller->flags & REDIS_MASTER) &&
        !(c->cmd->getkeys_proc == NULL && c->cmd->firstkey == 0))
    {
        int hashslot;

        if (server.cluster->state != REDIS_CLUSTER_OK) {
            flagTransaction(c);
            clusterRedirectClient(c,NULL,0,REDIS_CLUSTER_REDIR_DOWN_STATE);
            return REDIS_OK;
        } else {
            int error_code;
            clusterNode *n = getNodeByQuery(c,c->cmd,c->argv,c->argc,&hashslot,&error_code);
            if (n == NULL || n != server.cluster->myself) {
                flagTransaction(c);
                clusterRedirectClient(c,n,hashslot,error_code);
                return REDIS_OK;
            }
        }
    }

    /* Handle the maxmemory directive.
     *
     * First we try to free some memory if possible (if there are volatile
     * keys in the dataset). If there are not the only thing we can do
     * is returning an error. */
    if (server.maxmemory) {
        int retval = freeMemoryIfNeeded();
        /* freeMemoryIfNeeded may flush slave output buffers. This may result
         * into a slave, that may be the active client, to be freed. */
        if (server.current_client == NULL) return REDIS_ERR;

        /* It was impossible to free enough memory, and the command the client
         * is trying to execute is denied during OOM conditions? Error. */
        if ((c->cmd->flags & REDIS_CMD_DENYOOM) && retval == REDIS_ERR) {
            flagTransaction(c);
            addReply(c, shared.oomerr);
            return REDIS_OK;
        }
    }

    /* Don't accept write commands if there are problems persisting on disk
     * and if this is a master instance. */
    if (((server.stop_writes_on_bgsave_err &&
          server.saveparamslen > 0 &&
          server.lastbgsave_status == REDIS_ERR) ||
          server.aof_last_write_status == REDIS_ERR) &&
        server.masterhost == NULL &&
        (c->cmd->flags & REDIS_CMD_WRITE ||
         c->cmd->proc == pingCommand))
    {
        flagTransaction(c);
        if (server.aof_last_write_status == REDIS_OK)
            addReply(c, shared.bgsaveerr);
        else
            addReplySds(c,
                sdscatprintf(sdsempty(),
                "-MISCONF Errors writing to the AOF file: %s
",
                strerror(server.aof_last_write_errno)));
        return REDIS_OK;
    }

    /* Don't accept write commands if there are not enough good slaves and
     * user configured the min-slaves-to-write option. */
    if (server.masterhost == NULL &&
        server.repl_min_slaves_to_write &&
        server.repl_min_slaves_max_lag &&
        c->cmd->flags & REDIS_CMD_WRITE &&
        server.repl_good_slaves_count < server.repl_min_slaves_to_write)
    {
        flagTransaction(c);
        addReply(c, shared.noreplicaserr);
        return REDIS_OK;
    }

    /* Don't accept write commands if this is a read only slave. But
     * accept write commands if this is our master. */
    if (server.masterhost && server.repl_slave_ro &&
        !(c->flags & REDIS_MASTER) &&
        c->cmd->flags & REDIS_CMD_WRITE)
    {
        addReply(c, shared.roslaveerr);
        return REDIS_OK;
    }

    /* Only allow SUBSCRIBE and UNSUBSCRIBE in the context of Pub/Sub */
    if (c->flags & REDIS_PUBSUB &&
        c->cmd->proc != pingCommand &&
        c->cmd->proc != subscribeCommand &&
        c->cmd->proc != unsubscribeCommand &&
        c->cmd->proc != psubscribeCommand &&
        c->cmd->proc != punsubscribeCommand) {
        addReplyError(c,"only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in this context");
        return REDIS_OK;
    }

    /* Only allow INFO and SLAVEOF when slave-serve-stale-data is no and
     * we are a slave with a broken link with master. */
    if (server.masterhost && server.repl_state != REDIS_REPL_CONNECTED &&
        server.repl_serve_stale_data == 0 &&
        !(c->cmd->flags & REDIS_CMD_STALE))
    {
        flagTransaction(c);
        addReply(c, shared.masterdownerr);
        return REDIS_OK;
    }

    /* Loading DB? Return an error if the command has not the
     * REDIS_CMD_LOADING flag. */
    if (server.loading && !(c->cmd->flags & REDIS_CMD_LOADING)) {
        addReply(c, shared.loadingerr);
        return REDIS_OK;
    }

    /* Lua script too slow? Only allow a limited number of commands. */
    if (server.lua_timedout &&
          c->cmd->proc != authCommand &&
          c->cmd->proc != replconfCommand &&
        !(c->cmd->proc == shutdownCommand &&
          c->argc == 2 &&
          tolower(((char*)c->argv[1]->ptr)[0]) == 'n') &&
        !(c->cmd->proc == scriptCommand &&
          c->argc == 2 &&
          tolower(((char*)c->argv[1]->ptr)[0]) == 'k'))
    {
        flagTransaction(c);
        addReply(c, shared.slowscripterr);
        return REDIS_OK;
    }

    /* Exec the command */
    if (c->flags & REDIS_MULTI &&
        c->cmd->proc != execCommand && c->cmd->proc != discardCommand &&
        c->cmd->proc != multiCommand && c->cmd->proc != watchCommand)
    {
        queueMultiCommand(c);
        addReply(c,shared.queued);
    } else {
        call(c,REDIS_CALL_FULL);
        c->woff = server.master_repl_offset;
        if (listLength(server.ready_keys))
            handleClientsBlockedOnLists();
    }
    return REDIS_OK;
}

命令的执行

服务器将要执行的命令保存到了客户端状态的cmd属性中，且命令及参数个数保存到了argv和argc属性中，所以服务器执行的时候，只需要调用函数即可：

c->cmd->proc(c); //proc函数只需要c指针作为参数即可获取到所需要的数据，如下：

执行成功后，产生相应的回复，并将回复的内容放入到客户端状态的输出缓冲区中（buf和reply属性），之后函数还会为客户端的套接字关联回复处理器，回复处理器将命令返回给客户端，当执行set命令后，会返回一个ok，如下：

执行后续工作

 在执行完命令后，服务器还需要一些后续工作：

1.如果服务器开启了慢查询日志，那么服务器会检查是否需要为刚才执行的命令产生一条慢查询日志；

2.根据刚才命令执行的耗时，更新被执行命令的rediscommand属性中的milliseconds属性，并将calls属性值+1；

3.如果服务器开启了AOF持久化，会将刚才执行的命令写入到AOF缓冲区；

4.如果有其他从服务器正在复制当前这个服务器，服务器会将刚才这个命令传播给所有从服务器；

到此，服务器的执行完毕，下面是call函数的实现：

/* Call() is the core of Redis execution of a command */
void call(redisClient *c, int flags) {
    long long dirty, start, duration;
    int client_old_flags = c->flags;

    /* Sent the command to clients in MONITOR mode, only if the commands are
     * not generated from reading an AOF. */
    if (listLength(server.monitors) &&
        !server.loading &&
        !(c->cmd->flags & (REDIS_CMD_SKIP_MONITOR|REDIS_CMD_ADMIN)))
    {
        replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc);
    }

    /* Call the command. */
    c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL);
    redisOpArrayInit(&server.also_propagate);
    dirty = server.dirty;
    start = ustime();
    c->cmd->proc(c);
    duration = ustime()-start;
    dirty = server.dirty-dirty;
    if (dirty < 0) dirty = 0;

    /* When EVAL is called loading the AOF we don't want commands called
     * from Lua to go into the slowlog or to populate statistics. */
    if (server.loading && c->flags & REDIS_LUA_CLIENT)
        flags &= ~(REDIS_CALL_SLOWLOG | REDIS_CALL_STATS);

    /* If the caller is Lua, we want to force the EVAL caller to propagate
     * the script if the command flag or client flag are forcing the
     * propagation. */
    if (c->flags & REDIS_LUA_CLIENT && server.lua_caller) {
        if (c->flags & REDIS_FORCE_REPL)
            server.lua_caller->flags |= REDIS_FORCE_REPL;
        if (c->flags & REDIS_FORCE_AOF)
            server.lua_caller->flags |= REDIS_FORCE_AOF;
    }

    /* Log the command into the Slow log if needed, and populate the
     * per-command statistics that we show in INFO commandstats. */
    if (flags & REDIS_CALL_SLOWLOG && c->cmd->proc != execCommand) {
        char *latency_event = (c->cmd->flags & REDIS_CMD_FAST) ?
                              "fast-command" : "command";
        latencyAddSampleIfNeeded(latency_event,duration/1000);
        slowlogPushEntryIfNeeded(c->argv,c->argc,duration);
    }
    if (flags & REDIS_CALL_STATS) {
        c->cmd->microseconds += duration;
        c->cmd->calls++;
    }

    /* Propagate the command into the AOF and replication link */
    if (flags & REDIS_CALL_PROPAGATE) {
        int flags = REDIS_PROPAGATE_NONE;

        if (c->flags & REDIS_FORCE_REPL) flags |= REDIS_PROPAGATE_REPL;
        if (c->flags & REDIS_FORCE_AOF) flags |= REDIS_PROPAGATE_AOF;
        if (dirty)
            flags |= (REDIS_PROPAGATE_REPL | REDIS_PROPAGATE_AOF);
        if (flags != REDIS_PROPAGATE_NONE)
            propagate(c->cmd,c->db->id,c->argv,c->argc,flags);
    }

    /* Restore the old FORCE_AOF/REPL flags, since call can be executed
     * recursively. */
    c->flags &= ~(REDIS_FORCE_AOF|REDIS_FORCE_REPL);
    c->flags |= client_old_flags & (REDIS_FORCE_AOF|REDIS_FORCE_REPL);

    /* Handle the alsoPropagate() API to handle commands that want to propagate
     * multiple separated commands. */
    if (server.also_propagate.numops) {
        int j;
        redisOp *rop;

        for (j = 0; j < server.also_propagate.numops; j++) {
            rop = &server.also_propagate.ops[j];
            propagate(rop->cmd, rop->dbid, rop->argv, rop->argc, rop->target);
        }
        redisOpArrayFree(&server.also_propagate);
    }
    server.stat_numcommands++;
}

serverCron函数的执行

serverCron函数默认100ms执行一次，负责服务器资源的管理。主要操作如下：

1.更新服务器时间缓存

    Redis服务器中有不少功能都需要调用系统时间，为了减少系统时间的调用次数，服务器状态中的unixtime和mstime属性保存了系统时间，即系统时间缓存；

time_t unixtime;        /* Unix time sampled every cron cycle.秒级 */
    long long mstime;       /* Like 'unixtime' but with milliseconds resolution. 毫秒级*/

因为serverCron每100毫秒执行一次，所以这两个属性记录的时间精确度并不高：

• 服务器只会在打印日志、更新服务器的LRU时钟、决定是否执行持久化、计算服务器上线时间这类对精确度要求不高的操作，才会调用缓存属性；
• 为键设置过期时间、为慢查询添加日志这些高精确度的计算上，会再次调用执行系统调用获取精确的时间。

    /* Update the time cache. */
    updateCachedTime();

void updateCachedTime(void) {
    server.unixtime = time(NULL);
    server.mstime = mstime();
}

2.更新LRU时钟

redisServer{
    unsigned lruclock:REDIS_LRU_BITS; /* Clock for LRU eviction */

lruclock也是一个服务器时间缓存。默认10s更新一次，用于计算键的空转时长。

每个redis对象都有一个lru属性，记录了对象最后一次被命令访问的时间

typedef struct redisObject {
    unsigned type:4;
    unsigned encoding:4;
    unsigned lru:REDIS_LRU_BITS; /* lru time (relative to server.lruclock)，需要与server的lruclock计算 */
    int refcount;
    void *ptr;
} robj;

服务器在计算空转时长，就将server的lruclock减去对象的lru，得到的值就是对象的空转时长。通过info server命令可以看到服务器的lruclock值

127.0.0.1:6379> info server 

# Server

redis_version:3.2.1

hz:10

lru_clock:14359453

/* We have just REDIS_LRU_BITS bits per object for LRU information.
     * So we use an (eventually wrapping) LRU clock.
     *
     * Note that even if the counter wraps it's not a big problem,
     * everything will still work but some object will appear younger
     * to Redis. However for this to happen a given object should never be
     * touched for all the time needed to the counter to wrap, which is
     * not likely.
     *
     * Note that you can change the resolution altering the
     * REDIS_LRU_CLOCK_RESOLUTION define. */
    server.lruclock = getLRUClock();

3.更新服务器每秒执行命令数

   serverCron函数中的执行时，有个函数以100毫秒频率执行，采用抽样计算方式计算最近1s内服务器处理的命令请求数量，通过info stats命令可以查看到：

127.0.0.1:6379> info stats

# Stats

total_connections_received:8021544

total_commands_processed:89091769

instantaneous_ops_per_sec:7

    run_with_period(100) {
        trackInstantaneousMetric(REDIS_METRIC_COMMAND,server.stat_numcommands);
        trackInstantaneousMetric(REDIS_METRIC_NET_INPUT,
                server.stat_net_input_bytes);
        trackInstantaneousMetric(REDIS_METRIC_NET_OUTPUT,
                server.stat_net_output_bytes);
    }

4.更新服务器的内存使用峰值

如果当前使用值大于峰值则更新

通过info memory可以看到峰值

127.0.0.1:6379> info memory

# Memory

used_memory:126646712

used_memory_human:120.78M

used_memory_rss:149495808

used_memory_rss_human:142.57M

used_memory_peak:302379288

used_memory_peak_human:288.37M

    /* Record the max memory used since the server was started. */
    if (zmalloc_used_memory() > server.stat_peak_memory)
        server.stat_peak_memory = zmalloc_used_memory();

 5.处理sigterm信号

在启动服务器时，Redis会为服务器进程的SIGTERM信号关联sigshutdownHandler函数，这个函数负责在服务器接受到sigterm信号时，打开服务器的shutdown_asap标识：

每次serverCron执行时，会判断shutdown_asap属性值，决定是否需要关闭服务器。

struct redisServer {
    unsigned lruclock:REDIS_LRU_BITS; /* Clock for LRU eviction */
    int shutdown_asap;          /* SHUTDOWN needed ASAP */

static void sigShutdownHandler(int sig) {
    char *msg;

    switch (sig) {
    case SIGINT:
        msg = "Received SIGINT scheduling shutdown...";
        break;
    case SIGTERM:
        msg = "Received SIGTERM scheduling shutdown...";
        break;
    default:
        msg = "Received shutdown signal, scheduling shutdown...";
    };

    /* SIGINT is often delivered via Ctrl+C in an interactive session.
     * If we receive the signal the second time, we interpret this as
     * the user really wanting to quit ASAP without waiting to persist
     * on disk. */
    if (server.shutdown_asap && sig == SIGINT) {
        redisLogFromHandler(REDIS_WARNING, "You insist... exiting now.");
        rdbRemoveTempFile(getpid());
        exit(1); /* Exit with an error since this was not a clean shutdown. */
    } else if (server.loading) {
        exit(0);
    }

    redisLogFromHandler(REDIS_WARNING, msg);
    server.shutdown_asap = 1;

/* We received a SIGTERM, shutting down here in a safe way, as it is
     * not ok doing so inside the signal handler. */
    if (server.shutdown_asap) {
        if (prepareForShutdown(0) == REDIS_OK) exit(0);
        redisLog(REDIS_WARNING,"SIGTERM received but errors trying to shut down the server, check the logs for more information");
        server.shutdown_asap = 0;
    }

6.管理客户端资源

serverCron函数会调用clientCron函数，client函数会对一定数量的客户端做以下检查：

如果客户端与服务器端连接已经超时，那么程序释放这个客户端；

如果客户端在上一次命令执行之后，输入缓冲区超过了一定长度，那么程序会释放客户端的缓冲区，并重新创建一块默认大小的输入缓冲区。

void clientsCron(void) {
    /* Make sure to process at least numclients/server.hz of clients
     * per call. Since this function is called server.hz times per second
     * we are sure that in the worst case we process all the clients in 1
     * second. */
    int numclients = listLength(server.clients);
    int iterations = numclients/server.hz;
    mstime_t now = mstime();

    /* Process at least a few clients while we are at it, even if we need
     * to process less than CLIENTS_CRON_MIN_ITERATIONS to meet our contract
     * of processing each client once per second. */
    if (iterations < CLIENTS_CRON_MIN_ITERATIONS)
        iterations = (numclients < CLIENTS_CRON_MIN_ITERATIONS) ?
                     numclients : CLIENTS_CRON_MIN_ITERATIONS;

    while(listLength(server.clients) && iterations--) {
        redisClient *c;
        listNode *head;

        /* Rotate the list, take the current head, process.
         * This way if the client must be removed from the list it's the
         * first element and we don't incur into O(N) computation. */
        listRotate(server.clients);
        head = listFirst(server.clients);
        c = listNodeValue(head);
        /* The following functions do different service checks on the client.
         * The protocol is that they return non-zero if the client was
         * terminated. */
        if (clientsCronHandleTimeout(c,now)) continue;  //释放超时连接
        if (clientsCronResizeQueryBuffer(c)) continue;  //resize缓冲区
    }
}

7.管理数据库资源

serverCron每次执行都会调用databasesCron函数，这个函数会对数据库中的一部分数据进行检查，释放过期键，对字典进行收缩操作；

void databasesCron(void) {
    /* Expire keys by random sampling. Not required for slaves
     * as master will synthesize DELs for us. */
    if (server.active_expire_enabled && server.masterhost == NULL)
        activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);

    /* Perform hash tables rehashing if needed, but only if there are no
     * other processes saving the DB on disk. Otherwise rehashing is bad
     * as will cause a lot of copy-on-write of memory pages. */
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1) {
        /* We use global counters so if we stop the computation at a given
         * DB we'll be able to start from the successive in the next
         * cron loop iteration. */
        static unsigned int resize_db = 0;
        static unsigned int rehash_db = 0;
        int dbs_per_call = REDIS_DBCRON_DBS_PER_CALL;
        int j;

        /* Don't test more DBs than we have. */
        if (dbs_per_call > server.dbnum) dbs_per_call = server.dbnum;

        /* Resize */
        for (j = 0; j < dbs_per_call; j++) {
            tryResizeHashTables(resize_db % server.dbnum);
            resize_db++;
        }

        /* Rehash */
        if (server.activerehashing) {
            for (j = 0; j < dbs_per_call; j++) {
                int work_done = incrementallyRehash(rehash_db % server.dbnum);
                rehash_db++;
                if (work_done) {
                    /* If the function did some work, stop here, we'll do
                     * more at the next cron loop. */
                    break;
                }
            }
        }
    }
}

8.执行被延时的bgrewriteaof

在执行bgsave命令期间，如果客户端发送bgrewriteaof命令，那么服务器会将bgrewriteaof命令延迟到bgsave命令后执行。

服务器的aof_rewrite_scheduled属性记录了是否为延时，如果值为1，代表bgrewriteaof被延时了。

int aof_rewrite_scheduled; /* Rewrite once BGSAVE terminates. */

    /* Start a scheduled AOF rewrite if this was requested by the user while
     * a BGSAVE was in progress. */
    if (server.rdb_child_pid == -1 && server.aof_child_pid == -1 &&
        server.aof_rewrite_scheduled)
    {
        rewriteAppendOnlyFileBackground();
    }

9.检查持久化操作的运行状态

服务器用下面两个属性分别记录了bgsave和bgrewriteaof命令的子进程ID，这两个属性也可以查询这两个命令是否正在执行，如果id为-1，说明没有在执行；

pid_t rdb_child_pid; /* PID of RDB saving child */

pid_t aof_child_pid; /* PID if rewriting process */

每次serverCron函数执行，都会检查rdb_child_pid和aof_child_pid的值，如果有一个为-1，程序就会执行一次wait3函数，检查子进程是否有信号发送来服务器进程：

• 如果有，表示新的RBD文件已经生产完成或AOF文件已经重写完毕，服务器需要进行相应的后续操作，比如用新的RDB文件替换现有的RDB文件，或者用重写后的AOF文件替换现有的AOF文件；
• 如果没有，表示持久化操作没有完成，程序不做动作。

如果两个值均为-1，表示服务器没有在执行持久化操作，那么执行以下步骤： 

• 检查bgrewriteaof是否延时了，如果有的话，执行新的bgrewriteaof操作；
• 检查服务器自动保存的条件是否满足，如果满足，并且服务器没有在执行其他持久化操作，那么服务器开始一次新的bgsave操作（条件1可能会引起一次持久化，所以在这个检查中，程序会再次确认服务器已经在执行持久化了。）；
• 检查服务器自身设置的aof重新条件是否满足，如果条件满足，并且服务器没有在执行其他持久化操作，那么服务器开始一次新的bgrewriteaof操作。

下面图展示了这一过程：

/* Check if a background saving or AOF rewrite in progress terminated. */
    if (server.rdb_child_pid != -1 || server.aof_child_pid != -1) {
        int statloc;
        pid_t pid;

        if ((pid = wait3(&statloc,WNOHANG,NULL)) != 0) {
            int exitcode = WEXITSTATUS(statloc);
            int bysignal = 0;

            if (WIFSIGNALED(statloc)) bysignal = WTERMSIG(statloc);

            if (pid == -1) {
                redisLog(LOG_WARNING,"wait3() returned an error: %s. "
                    "rdb_child_pid = %d, aof_child_pid = %d",
                    strerror(errno),
                    (int) server.rdb_child_pid,
                    (int) server.aof_child_pid);
            } else if (pid == server.rdb_child_pid) {
                backgroundSaveDoneHandler(exitcode,bysignal);
            } else if (pid == server.aof_child_pid) {
                backgroundRewriteDoneHandler(exitcode,bysignal);
            } else {
                redisLog(REDIS_WARNING,
                    "Warning, detected child with unmatched pid: %ld",
                    (long)pid);
            }
            updateDictResizePolicy();
        }
    } else {
        /* If there is not a background saving/rewrite in progress check if
         * we have to save/rewrite now */
         for (j = 0; j < server.saveparamslen; j++) {
            struct saveparam *sp = server.saveparams+j;

            /* Save if we reached the given amount of changes,
             * the given amount of seconds, and if the latest bgsave was
             * successful or if, in case of an error, at least
             * REDIS_BGSAVE_RETRY_DELAY seconds already elapsed. */
            if (server.dirty >= sp->changes &&
                server.unixtime-server.lastsave > sp->seconds &&
                (server.unixtime-server.lastbgsave_try >
                 REDIS_BGSAVE_RETRY_DELAY ||
                 server.lastbgsave_status == REDIS_OK))
            {
                redisLog(REDIS_NOTICE,"%d changes in %d seconds. Saving...",
                    sp->changes, (int)sp->seconds);
                rdbSaveBackground(server.rdb_filename);
                break;
            }
         }

         /* Trigger an AOF rewrite if needed */
         if (server.rdb_child_pid == -1 &&
             server.aof_child_pid == -1 &&
             server.aof_rewrite_perc &&
             server.aof_current_size > server.aof_rewrite_min_size)
         {
            long long base = server.aof_rewrite_base_size ?
                            server.aof_rewrite_base_size : 1;
            long long growth = (server.aof_current_size*100/base) - 100;
            if (growth >= server.aof_rewrite_perc) {
                redisLog(REDIS_NOTICE,"Starting automatic rewriting of AOF on %lld%% growth",growth);
                rewriteAppendOnlyFileBackground();
            }
         }
    }

 10.将AOF缓冲区的内存写入到AOF文件

如果服务器开启了AOF功能，并且AOF缓冲区还有数据，serverCron会将缓冲区数据写入到AOF文件中。

 /* AOF postponed flush: Try at every cron cycle if the slow fsync
     * completed. */
    if (server.aof_flush_postponed_start) flushAppendOnlyFile(0);

11.关闭异步客户端

服务器会关闭那些输出缓冲区超过限制大小的客户端

/* Close clients that need to be closed asynchronous */
    freeClientsInAsyncFreeQueue();

    /* Clear the paused clients flag if needed. */
    clientsArePaused(); /* Don't check return value, just use the side effect. */

    /* Replication cron function -- used to reconnect to master and
     * to detect transfer failures. */
    run_with_period(1000) replicationCron();

    /* Run the Redis Cluster cron. */
    run_with_period(100) {
        if (server.cluster_enabled) clusterCron();
    }

    /* Run the Sentinel timer if we are in sentinel mode. */
    run_with_period(100) {
        if (server.sentinel_mode) sentinelTimer();
    }

    /* Cleanup expired MIGRATE cached sockets. */
    run_with_period(1000) {
        migrateCloseTimedoutSockets();
    }

12.增加loopcount计数

服务器会增加serverCron执行的次数，在服务器的cronloops属性中记录：

int cronloops; /* Number of times the cron function run */

    server.cronloops++;

服务器的初始化

initServerConfig初始化

服务器的初始化第一步就是初始化一个redisServer的struct，并设置各个属性的默认值：

void initServerConfig(void) {
    int j;

    getRandomHexChars(server.runid,REDIS_RUN_ID_SIZE);
    server.configfile = NULL;
    server.hz = REDIS_DEFAULT_HZ;
    server.runid[REDIS_RUN_ID_SIZE] = '';
    server.arch_bits = (sizeof(long) == 8) ? 64 : 32;
    server.port = REDIS_SERVERPORT;
    server.tcp_backlog = REDIS_TCP_BACKLOG;
    server.bindaddr_count = 0;
    server.unixsocket = NULL;
    server.unixsocketperm = REDIS_DEFAULT_UNIX_SOCKET_PERM;
    server.ipfd_count = 0;
    server.sofd = -1;
    server.dbnum = REDIS_DEFAULT_DBNUM;
    server.verbosity = REDIS_DEFAULT_VERBOSITY;
    server.maxidletime = REDIS_MAXIDLETIME;
    server.tcpkeepalive = REDIS_DEFAULT_TCP_KEEPALIVE;
    server.active_expire_enabled = 1;
    server.client_max_querybuf_len = REDIS_MAX_QUERYBUF_LEN;
    server.saveparams = NULL;
    server.loading = 0;
    server.logfile = zstrdup(REDIS_DEFAULT_LOGFILE);
    server.syslog_enabled = REDIS_DEFAULT_SYSLOG_ENABLED;
    server.syslog_ident = zstrdup(REDIS_DEFAULT_SYSLOG_IDENT);
    server.syslog_facility = LOG_LOCAL0;
    server.daemonize = REDIS_DEFAULT_DAEMONIZE;
    server.aof_state = REDIS_AOF_OFF;
    server.aof_fsync = REDIS_DEFAULT_AOF_FSYNC;
    server.aof_no_fsync_on_rewrite = REDIS_DEFAULT_AOF_NO_FSYNC_ON_REWRITE;
    server.aof_rewrite_perc = REDIS_AOF_REWRITE_PERC;
    server.aof_rewrite_min_size = REDIS_AOF_REWRITE_MIN_SIZE;
    server.aof_rewrite_base_size = 0;
    server.aof_rewrite_scheduled = 0;
    server.aof_last_fsync = time(NULL);
    server.aof_rewrite_time_last = -1;
    server.aof_rewrite_time_start = -1;
    server.aof_lastbgrewrite_status = REDIS_OK;
    server.aof_delayed_fsync = 0;
    server.aof_fd = -1;
    server.aof_selected_db = -1; /* Make sure the first time will not match */
    server.aof_flush_postponed_start = 0;
    server.aof_rewrite_incremental_fsync = REDIS_DEFAULT_AOF_REWRITE_INCREMENTAL_FSYNC;
    server.aof_load_truncated = REDIS_DEFAULT_AOF_LOAD_TRUNCATED;
    server.pidfile = zstrdup(REDIS_DEFAULT_PID_FILE);
    server.rdb_filename = zstrdup(REDIS_DEFAULT_RDB_FILENAME);
    server.aof_filename = zstrdup(REDIS_DEFAULT_AOF_FILENAME);
    server.requirepass = NULL;
    server.rdb_compression = REDIS_DEFAULT_RDB_COMPRESSION;
    server.rdb_checksum = REDIS_DEFAULT_RDB_CHECKSUM;
    server.stop_writes_on_bgsave_err = REDIS_DEFAULT_STOP_WRITES_ON_BGSAVE_ERROR;
    server.activerehashing = REDIS_DEFAULT_ACTIVE_REHASHING;
    server.notify_keyspace_events = 0;
    server.maxclients = REDIS_MAX_CLIENTS;
    server.bpop_blocked_clients = 0;
    server.maxmemory = REDIS_DEFAULT_MAXMEMORY;
    server.maxmemory_policy = REDIS_DEFAULT_MAXMEMORY_POLICY;
    server.maxmemory_samples = REDIS_DEFAULT_MAXMEMORY_SAMPLES;
    server.hash_max_ziplist_entries = REDIS_HASH_MAX_ZIPLIST_ENTRIES;
    server.hash_max_ziplist_value = REDIS_HASH_MAX_ZIPLIST_VALUE;
    server.list_max_ziplist_entries = REDIS_LIST_MAX_ZIPLIST_ENTRIES;
    server.list_max_ziplist_value = REDIS_LIST_MAX_ZIPLIST_VALUE;
    server.set_max_intset_entries = REDIS_SET_MAX_INTSET_ENTRIES;
    server.zset_max_ziplist_entries = REDIS_ZSET_MAX_ZIPLIST_ENTRIES;
    server.zset_max_ziplist_value = REDIS_ZSET_MAX_ZIPLIST_VALUE;
    server.hll_sparse_max_bytes = REDIS_DEFAULT_HLL_SPARSE_MAX_BYTES;
    server.shutdown_asap = 0;
    server.repl_ping_slave_period = REDIS_REPL_PING_SLAVE_PERIOD;
    server.repl_timeout = REDIS_REPL_TIMEOUT;
    server.repl_min_slaves_to_write = REDIS_DEFAULT_MIN_SLAVES_TO_WRITE;
    server.repl_min_slaves_max_lag = REDIS_DEFAULT_MIN_SLAVES_MAX_LAG;
    server.cluster_enabled = 0;
    server.cluster_node_timeout = REDIS_CLUSTER_DEFAULT_NODE_TIMEOUT;
    server.cluster_migration_barrier = REDIS_CLUSTER_DEFAULT_MIGRATION_BARRIER;
    server.cluster_slave_validity_factor = REDIS_CLUSTER_DEFAULT_SLAVE_VALIDITY;
    server.cluster_require_full_coverage = REDIS_CLUSTER_DEFAULT_REQUIRE_FULL_COVERAGE;
    server.cluster_configfile = zstrdup(REDIS_DEFAULT_CLUSTER_CONFIG_FILE);
    server.lua_caller = NULL;
    server.lua_time_limit = REDIS_LUA_TIME_LIMIT;
    server.lua_client = NULL;
    server.lua_timedout = 0;
    server.migrate_cached_sockets = dictCreate(&migrateCacheDictType,NULL);
    server.next_client_id = 1; /* Client IDs, start from 1 .*/
    server.loading_process_events_interval_bytes = (1024*1024*2);

    server.lruclock = getLRUClock();
    resetServerSaveParams();

    appendServerSaveParams(60*60,1);  /* save after 1 hour and 1 change */
    appendServerSaveParams(300,100);  /* save after 5 minutes and 100 changes */
    appendServerSaveParams(60,10000); /* save after 1 minute and 10000 changes */
    /* Replication related */
    server.masterauth = NULL;
    server.masterhost = NULL;
    server.masterport = 6379;
    server.master = NULL;
    server.cached_master = NULL;
    server.repl_master_initial_offset = -1;
    server.repl_state = REDIS_REPL_NONE;
    server.repl_syncio_timeout = REDIS_REPL_SYNCIO_TIMEOUT;
    server.repl_serve_stale_data = REDIS_DEFAULT_SLAVE_SERVE_STALE_DATA;
    server.repl_slave_ro = REDIS_DEFAULT_SLAVE_READ_ONLY;
    server.repl_down_since = 0; /* Never connected, repl is down since EVER. */
    server.repl_disable_tcp_nodelay = REDIS_DEFAULT_REPL_DISABLE_TCP_NODELAY;
    server.repl_diskless_sync = REDIS_DEFAULT_REPL_DISKLESS_SYNC;
    server.repl_diskless_sync_delay = REDIS_DEFAULT_REPL_DISKLESS_SYNC_DELAY;
    server.slave_priority = REDIS_DEFAULT_SLAVE_PRIORITY;
    server.master_repl_offset = 0;

    /* Replication partial resync backlog */
    server.repl_backlog = NULL;
    server.repl_backlog_size = REDIS_DEFAULT_REPL_BACKLOG_SIZE;
    server.repl_backlog_histlen = 0;
    server.repl_backlog_idx = 0;
    server.repl_backlog_off = 0;
    server.repl_backlog_time_limit = REDIS_DEFAULT_REPL_BACKLOG_TIME_LIMIT;
    server.repl_no_slaves_since = time(NULL);

    /* Client output buffer limits */
    for (j = 0; j < REDIS_CLIENT_TYPE_COUNT; j++)
        server.client_obuf_limits[j] = clientBufferLimitsDefaults[j];

    /* Double constants initialization */
    R_Zero = 0.0;
    R_PosInf = 1.0/R_Zero;
    R_NegInf = -1.0/R_Zero;
    R_Nan = R_Zero/R_Zero;

    /* Command table -- we initiialize it here as it is part of the
     * initial configuration, since command names may be changed via
     * redis.conf using the rename-command directive. */
    server.commands = dictCreate(&commandTableDictType,NULL);
    server.orig_commands = dictCreate(&commandTableDictType,NULL);
    populateCommandTable();
    server.delCommand = lookupCommandByCString("del");
    server.multiCommand = lookupCommandByCString("multi");
    server.lpushCommand = lookupCommandByCString("lpush");
    server.lpopCommand = lookupCommandByCString("lpop");
    server.rpopCommand = lookupCommandByCString("rpop");

    /* Slow log */
    server.slowlog_log_slower_than = REDIS_SLOWLOG_LOG_SLOWER_THAN;
    server.slowlog_max_len = REDIS_SLOWLOG_MAX_LEN;

    /* Latency monitor */
    server.latency_monitor_threshold = REDIS_DEFAULT_LATENCY_MONITOR_THRESHOLD;

    /* Debugging */
    server.assert_failed = "<no assertion failed>";
    server.assert_file = "<no file>";
    server.assert_line = 0;
    server.bug_report_start = 0;
    server.watchdog_period = 0;
}

主要完成的工作是：

1.设置服务器的运行ID；

2.设置服务器的默认运行频率；

3.设置服务器的默认配置文件路径；

4.设置服务器的默认运行架构、端口号；

5.设置默认的RDB和AOF持久化条件；

6.设置服务器的LRU时钟；

7.创建命令表

加载配置文件

初始化工作完成后，开始加载配置文件（之前初始化的为一些默认配置，比如端口号默认6379，db数量默认16） ,并且更新redisServer中的之前设置的一些默认属性

服务器载入用户配置后，更新服务器状态，就开始执行初始化数据结构操作,initServer函数，主要操作如下：
1.初始化数据结构；

2.为服务器进程设置信号处理器；

3.创建共享对象，这些对象包含服务器经常用到的一些值，比如OK，ERR的回复字符串对象，包含整数1-1000的字符串对象

4.打开监听端口，并设置套接字的事件处理器；

5.为serverCron创建时间时间，等待服务器执行；

6.如果打开了AOF，那么打开现有的AOF文件，如果AOF不存在，则新建一个，为AOF写入做准备；

7.初始化服务器后台的I/O模块，为I/O操作做准备；

当initServer执行完毕后，将会打印出Redis图表，服务器版本等信息。

initServer函数如下：

void initServer(void) {
    int j;

    signal(SIGHUP, SIG_IGN);
    signal(SIGPIPE, SIG_IGN);
    setupSignalHandlers();

    if (server.syslog_enabled) {
        openlog(server.syslog_ident, LOG_PID | LOG_NDELAY | LOG_NOWAIT,
            server.syslog_facility);
    }

    server.pid = getpid();
    server.current_client = NULL;
    server.clients = listCreate();
    server.clients_to_close = listCreate();
    server.slaves = listCreate();
    server.monitors = listCreate();
    server.slaveseldb = -1; /* Force to emit the first SELECT command. */
    server.unblocked_clients = listCreate();
    server.ready_keys = listCreate();
    server.clients_waiting_acks = listCreate();
    server.get_ack_from_slaves = 0;
    server.clients_paused = 0;

    createSharedObjects();
    adjustOpenFilesLimit();
    server.el = aeCreateEventLoop(server.maxclients+REDIS_EVENTLOOP_FDSET_INCR);
    server.db = zmalloc(sizeof(redisDb)*server.dbnum);

    /* Open the TCP listening socket for the user commands. */
    if (server.port != 0 &&
        listenToPort(server.port,server.ipfd,&server.ipfd_count) == REDIS_ERR)
        exit(1);

    /* Open the listening Unix domain socket. */
    if (server.unixsocket != NULL) {
        unlink(server.unixsocket); /* don't care if this fails */
        server.sofd = anetUnixServer(server.neterr,server.unixsocket,
            server.unixsocketperm, server.tcp_backlog);
        if (server.sofd == ANET_ERR) {
            redisLog(REDIS_WARNING, "Opening Unix socket: %s", server.neterr);
            exit(1);
        }
        anetNonBlock(NULL,server.sofd);
    }

    /* Abort if there are no listening sockets at all. */
    if (server.ipfd_count == 0 && server.sofd < 0) {
        redisLog(REDIS_WARNING, "Configured to not listen anywhere, exiting.");
        exit(1);
    }

    /* Create the Redis databases, and initialize other internal state. */
    for (j = 0; j < server.dbnum; j++) {
        server.db[j].dict = dictCreate(&dbDictType,NULL);
        server.db[j].expires = dictCreate(&keyptrDictType,NULL);
        server.db[j].blocking_keys = dictCreate(&keylistDictType,NULL);
        server.db[j].ready_keys = dictCreate(&setDictType,NULL);
        server.db[j].watched_keys = dictCreate(&keylistDictType,NULL);
        server.db[j].eviction_pool = evictionPoolAlloc();
        server.db[j].id = j;
        server.db[j].avg_ttl = 0;
    }
    server.pubsub_channels = dictCreate(&keylistDictType,NULL);
    server.pubsub_patterns = listCreate();
    listSetFreeMethod(server.pubsub_patterns,freePubsubPattern);
    listSetMatchMethod(server.pubsub_patterns,listMatchPubsubPattern);
    server.cronloops = 0;
    server.rdb_child_pid = -1;
    server.aof_child_pid = -1;
    server.rdb_child_type = REDIS_RDB_CHILD_TYPE_NONE;
    aofRewriteBufferReset();
    server.aof_buf = sdsempty();
    server.lastsave = time(NULL); /* At startup we consider the DB saved. */
    server.lastbgsave_try = 0;    /* At startup we never tried to BGSAVE. */
    server.rdb_save_time_last = -1;
    server.rdb_save_time_start = -1;
    server.dirty = 0;
    resetServerStats();
    /* A few stats we don't want to reset: server startup time, and peak mem. */
    server.stat_starttime = time(NULL);
    server.stat_peak_memory = 0;
    server.resident_set_size = 0;
    server.lastbgsave_status = REDIS_OK;
    server.aof_last_write_status = REDIS_OK;
    server.aof_last_write_errno = 0;
    server.repl_good_slaves_count = 0;
    updateCachedTime();

    /* Create the serverCron() time event, that's our main way to process
     * background operations. */
    if(aeCreateTimeEvent(server.el, 1, serverCron, NULL, NULL) == AE_ERR) {
        redisPanic("Can't create the serverCron time event.");
        exit(1);
    }

    /* Create an event handler for accepting new connections in TCP and Unix
     * domain sockets. */
    for (j = 0; j < server.ipfd_count; j++) {
        if (aeCreateFileEvent(server.el, server.ipfd[j], AE_READABLE,
            acceptTcpHandler,NULL) == AE_ERR)
            {
                redisPanic(
                    "Unrecoverable error creating server.ipfd file event.");
            }
    }
    if (server.sofd > 0 && aeCreateFileEvent(server.el,server.sofd,AE_READABLE,
        acceptUnixHandler,NULL) == AE_ERR) redisPanic("Unrecoverable error creating server.sofd file event.");

    /* Open the AOF file if needed. */
    if (server.aof_state == REDIS_AOF_ON) {
        server.aof_fd = open(server.aof_filename,
                               O_WRONLY|O_APPEND|O_CREAT,0644);
        if (server.aof_fd == -1) {
            redisLog(REDIS_WARNING, "Can't open the append-only file: %s",
                strerror(errno));
            exit(1);
        }
    }

    /* 32 bit instances are limited to 4GB of address space, so if there is
     * no explicit limit in the user provided configuration we set a limit
     * at 3 GB using maxmemory with 'noeviction' policy'. This avoids
     * useless crashes of the Redis instance for out of memory. */
    if (server.arch_bits == 32 && server.maxmemory == 0) {
        redisLog(REDIS_WARNING,"Warning: 32 bit instance detected but no memory limit set. Setting 3 GB maxmemory limit with 'noeviction' policy now.");
        server.maxmemory = 3072LL*(1024*1024); /* 3 GB */
        server.maxmemory_policy = REDIS_MAXMEMORY_NO_EVICTION;
    }

    if (server.cluster_enabled) clusterInit();
    replicationScriptCacheInit();
    scriptingInit();
    slowlogInit();
    latencyMonitorInit();
    bioInit();
}

还原数据库状态

在完成初始化后，需要对数据库进行还原，这时服务器需要载入RDB或AOF文件。

如果AOF开启，则加载AOF文件；否则加载RDB文件。

在加载完成后，服务器会打印DB load from disk。。。信息。

执行循环事件

这时服务器已经就绪，等待客户端连接，并进入事件循环，并处理文件事件或时间事件。