redis源代码分析17–持久化之快照

redis的持久化支持快照方式。快照方式会将整个db dump到磁盘上。

client 可以发布save/bgsave命令让server将db dump到磁盘上。其中bgsave会执行后台dump（新建子进程执行dump），而save是阻塞式的dump db，会影响其他client的命令执行。除了发布命令执行快照保存外，redis的serverCron也会按照配置的参数执行后台dump，另外 slave建立连接时，master也会执行一个后台dump，然后才发送数据给slave（这在主从复制一节中介绍）。

static int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
    ---
/* Check if a background saving or AOF rewrite in progress terminated */
    if (server.bgsavechildpid != -1 || server.bgrewritechildpid != -1) {
      ---
    }
else {
        /* If there is not a background saving in progress check if
         * we have to save now */
         time_t now = time(NULL);
         for (j = 0; j < server.saveparamslen; j++) {
            struct saveparam *sp = server.saveparams+j;

            if (server.dirty >= sp->changes &&
                now-server.lastsave > sp->seconds) {
                redisLog(REDIS_NOTICE,"%d changes in %d seconds. Saving...",
                    sp->changes, sp->seconds);
                rdbSaveBackground(server.dbfilename);
                break;
            }
         }
}
---
}

无论是新建子进程还是阻塞式的执行快照方式（新建子进程方式会先调用rdbSaveBackground），最终都会调用rdbSave来保存db。

在rdbSave中可以看到，redis是按type、key、val方式来保存db中的数据的。

rdbLoad是快照方式保存数据后server启动时加载数据的函数，是rdbSave的逆过程。

static int rdbSave(char *filename) {

    ---
    for (j = 0; j < server.dbnum; j++) {
        redisDb *db = server.db+j;
       ---
        /* Iterate this DB writing every entry */
        while((de = dictNext(di)) != NULL) {
            robj *key = dictGetEntryKey(de);
            robj *o = dictGetEntryVal(de);
            time_t expiretime = getExpire(db,key);
            ---
            /* Save type, key, value */
           if (rdbSaveType(fp,o->type) == -1) goto werr;
          if (rdbSaveStringObject(fp,key) == -1) goto werr;
          if (rdbSaveObject(fp,o) == -1) goto werr;
          ---
        }
        dictReleaseIterator(di);
    }
    ---
    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. */
    if (rename(tmpfile,filename) == -1) {
        redisLog(REDIS_WARNING,"Error moving temp DB file on the final destination: %s", strerror(errno));
        unlink(tmpfile);
        return REDIS_ERR;
    }
    ---
}