复制从节点过期键清理

在周期时间事件serverCron-->databasesCron函数中有如下代码：

/* This function handles 'background' operations we are required to do
 * incrementally in Redis databases, such as active key expiring, resizing,
 * rehashing. */
void databasesCron(void) {
    /* Expire keys by random sampling. Not required for slaves
     * as master will synthesize DELs for us. */
    if (server.active_expire_enabled) {
        if (server.masterhost == NULL) {
            activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);
        } else {
            expireSlaveKeys();
        }
    }
}

参考资料

https://help.aliyun.com/knowledge_detail/150185.html

对于复制主节点，会使用activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW)来进行清理过期键，标准源码中该方法每次调用最多执行25ms。

对于复制从节点，会使用expireSlaveKeys()来清理在读写模式下从库单独写入的过期键。

expireSlaveKeys函数

标准模式下，复制从节点不能单独进行写操作，复制从节点的所有数据都从复制主节点上同步过来，当主节点主动删除key或对过期键进行清理时，主节点操作会同步到从节点上执行，保证主从数据一致。

当复制从节点开启读写模式后，Redis使用全局变量slaveKeysWithExpire来记录直接写入到辅助从节点上带有过期事件的键：

/* The dictionary where we remember key names and database ID of keys we may
 * want to expire from the slave. Since this function is not often used we
 * don't even care to initialize the database at startup. We'll do it once
 * the feature is used the first time, that is, when rememberSlaveKeyWithExpire()
 * is called.
 *
 * The dictionary has an SDS string representing the key as the hash table
 * key, while the value is a 64 bit unsigned integer with the bits corresponding
 * to the DB where the keys may exist set to 1. Currently the keys created
 * with a DB id > 63 are not expired, but a trivial fix is to set the bitmap
 * to the max 64 bit unsigned value when we know there is a key with a DB
 * ID greater than 63, and check all the configured DBs in such a case. */
dict *slaveKeysWithExpire = NULL;

/* Track keys that received an EXPIRE or similar command in the context
 * of a writable slave. */
void rememberSlaveKeyWithExpire(redisDb *db, robj *key) {
    if (slaveKeysWithExpire == NULL) {
        static dictType dt = {
            dictSdsHash,                /* hash function */
            NULL,                       /* key dup */
            NULL,                       /* val dup */
            dictSdsKeyCompare,          /* key compare */
            dictSdsDestructor,          /* key destructor */
            NULL                        /* val destructor */
        };
        slaveKeysWithExpire = dictCreate(&dt,NULL);
    }
    if (db->id > 63) return;

    dictEntry *de = dictAddOrFind(slaveKeysWithExpire,key->ptr);
    /* If the entry was just created, set it to a copy of the SDS string
     * representing the key: we don't want to need to take those keys
     * in sync with the main DB. The keys will be removed by expireSlaveKeys()
     * as it scans to find keys to remove. */
    if (de->key == key->ptr) {
        de->key = sdsdup(key->ptr);
        dictSetUnsignedIntegerVal(de,0);
    }

    uint64_t dbids = dictGetUnsignedIntegerVal(de);
    dbids |= (uint64_t)1 << db->id;
    dictSetUnsignedIntegerVal(de,dbids);
}

由于全局变量slaveKeysWithExpire记录的过期键仅在复制从节点上存在，因此需要使用单独的expireSlaveKeys函数中基于全局变量slaveKeysWithExpire来进行过期数据清理。

/* Check the set of keys created by the master with an expire set in order to
 * check if they should be evicted. */
void expireSlaveKeys(void) {
    if (slaveKeysWithExpire == NULL ||
        dictSize(slaveKeysWithExpire) == 0) return;

    int cycles = 0, noexpire = 0;
    mstime_t start = mstime();
    while(1) {
        dictEntry *de = dictGetRandomKey(slaveKeysWithExpire);
        sds keyname = dictGetKey(de);
        uint64_t dbids = dictGetUnsignedIntegerVal(de);
        uint64_t new_dbids = 0;

        /* Check the key against every database corresponding to the
         * bits set in the value bitmap. */
        int dbid = 0;
        while(dbids && dbid < server.dbnum) {
            if ((dbids & 1) != 0) {
                redisDb *db = server.db+dbid;
                dictEntry *expire = dictFind(db->expires,keyname);
                int expired = 0;

                if (expire &&
                    activeExpireCycleTryExpire(server.db+dbid,expire,start))
                {
                    expired = 1;
                }

                /* If the key was not expired in this DB, we need to set the
                 * corresponding bit in the new bitmap we set as value.
                 * At the end of the loop if the bitmap is zero, it means we
                 * no longer need to keep track of this key. */
                if (expire && !expired) {
                    noexpire++;
                    new_dbids |= (uint64_t)1 << dbid;
                }
            }
            dbid++;
            dbids >>= 1;
        }

        /* Set the new bitmap as value of the key, in the dictionary
         * of keys with an expire set directly in the writable slave. Otherwise
         * if the bitmap is zero, we no longer need to keep track of it. */
        if (new_dbids)
            dictSetUnsignedIntegerVal(de,new_dbids);
        else
            dictDelete(slaveKeysWithExpire,keyname);

        /* Stop conditions: found 3 keys we cna't expire in a row or
         * time limit was reached. */
        cycles++;
        if (noexpire > 3) break;
        if ((cycles % 64) == 0 && mstime()-start > 1) break;
        if (dictSize(slaveKeysWithExpire) == 0) break;
    }
}