Redis深挖之-DEL

命令简介

用法：

DEL key [key ...]

删除给定的一个或多个 key 。

不存在的 key 会被忽略。

返回值：

被删除 key 的数量。

例子：

redis> SET name huangz
OK

redis> DEL name
(integer) 1


# 删除一个不存在的 key

redis> EXISTS phone
(integer) 0

redis> DEL phone # 失败，没有 key 被删除
(integer) 0

源码解析

命令所在文件：src/db.c

del 和 unlink相似，unlink (有条件的)会另起一个线程（在bio.c中）进行空间回收。而DEL会阻塞。

• delGenericCommand

void delCommand(client *c) {
    delGenericCommand(c,0);
}


void unlinkCommand(client *c) {
    delGenericCommand(c,1);
}


.
.
.

//del传参`lazy` = 0
void delGenericCommand(client *c, int lazy) {
    int numdel = 0, j;
    for (j = 1; j < c->argc; j++) {
        //检查是否过期，如果过期server.lazyfree_lazy_expire = 1是 也是异步删除
        expireIfNeeded(c->db,c->argv[j]);
        //如果 lazy 是 1 异步删除
        int deleted  = lazy ? dbAsyncDelete(c->db,c->argv[j]) :
                              dbSyncDelete(c->db,c->argv[j]);
        if (deleted) {
            signalModifiedKey(c->db,c->argv[j]);
            notifyKeyspaceEvent(NOTIFY_GENERIC,
                "del",c->argv[j],c->db->id);
            server.dirty++;
            numdel++;
        }
    }
    addReplyLongLong(c,numdel);
}

• dbAsyncDelete

其实这里的异步也是有条件的

/* Delete a key, value, and associated expiration entry if any, from the DB.
 * If there are enough allocations to free the value object may be put into
 * a lazy free list instead of being freed synchronously. The lazy free list
 * will be reclaimed in a different bio.c thread. */
#define LAZYFREE_THRESHOLD 64
int dbAsyncDelete(redisDb *db, robj *key) {
    /* Deleting an entry from the expires dict will not free the sds of
    o
     * the key, because it is shared with the main dictionary. */
    if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);

    /* If the value is composed of a few allocations, to free in a lazy way
     * is actually just slower... So under a certain limit we just free
     * the object synchronously. */
    dictEntry *de = dictUnlink(db->dict,key->ptr);
    if (de) {
        robj *val = dictGetVal(de);
        size_t free_effort = lazyfreeGetFreeEffort(val);

        /* If releasing the object is too much work, do it in the background
         * by adding the object to the lazy free list.
         * Note that if the object is shared, to reclaim it now it is not
         * possible. This rarely happens, however sometimes the implementation
         * of parts of the Redis core may call incrRefCount() to protect
         * objects, and then call dbDelete(). In this case we'll fall
         * through and reach the dictFreeUnlinkedEntry() call, that will be
         * equivalent to just calling decrRefCount(). */
        if (free_effort > LAZYFREE_THRESHOLD && val->refcount == 1) {
            atomicIncr(lazyfree_objects,1);
            //释放的太多了 后台处理
            bioCreateBackgroundJob(BIO_LAZY_FREE,val,NULL,NULL);
            dictSetVal(db->dict,de,NULL);
        }
    }

    /* Release the key-val pair, or just the key if we set the val
     * field to NULL in order to lazy free it later. */
    if (de) {
        dictFreeUnlinkedEntry(db->dict,de);
        if (server.cluster_enabled) slotToKeyDel(key);
        return 1;
    } else {
        return 0;
    }
}

• dbSyncDelete

/* Delete a key, value, and associated expiration entry if any, from the DB */
int dbSyncDelete(redisDb *db, robj *key) {
    /* Deleting an entry from the expires dict will not free the sds of
     * the key, because it is shared with the main dictionary. */
    if (dictSize(db->expires) > 0) dictDelete(db->expires,key->ptr);
    if (dictDelete(db->dict,key->ptr) == DICT_OK) {
        if (server.cluster_enabled) slotToKeyDel(key);
        return 1;
    } else {
        return 0;
    }
}