redis的list为空时为什么键被删除

待我称王封你为后i 2022-02-19 05:17 415阅读 0赞

### 问题 ###

一个协程brpop在一个redis list队列上，当list为空时，使用key查询redis db，发现此队列的key不存在；只有当list不空时才能keys到。以至于以为list push不成功。

### 原因 ###

命令字：

struct redisCommand redisCommandTable[] = { 
       { "brpop",brpopCommand,-3,"ws",0,NULL,1,-2,1,0,0},
       { "brpoplpush",brpoplpushCommand,4,"wms",0,NULL,1,2,1,0,0},
       { "blpop",blpopCommand,-3,"ws",0,NULL,1,-2,1,0,0},
    
    }
    
    void blpopCommand(client *c) { 
        blockingPopGenericCommand(c,LIST_HEAD);
    }
    
    void brpopCommand(client *c) { 
        blockingPopGenericCommand(c,LIST_TAIL);
    }

功能实现：

void blockingPopGenericCommand(client *c, int where) { 
        robj *o;
        mstime_t timeout;
        int j;
    
        if (getTimeoutFromObjectOrReply(c,c->argv[c->argc-1],&timeout,UNIT_SECONDS)
            != C_OK) return;
    
        for (j = 1; j < c->argc-1; j++) { 
            o = lookupKeyWrite(c->db,c->argv[j]);
            if (o != NULL) { 
                if (o->type != OBJ_LIST) { 
                    addReply(c,shared.wrongtypeerr);
                    return;
                } else { 
                    if (listTypeLength(o) != 0) { 
                        /* Non empty list, this is like a non normal [LR]POP. */
                        char *event = (where == LIST_HEAD) ? "lpop" : "rpop";
                        robj *value = listTypePop(o,where);
                        serverAssert(value != NULL);
    
                        addReplyMultiBulkLen(c,2);
                        addReplyBulk(c,c->argv[j]);
                        addReplyBulk(c,value);
                        decrRefCount(value);
                        notifyKeyspaceEvent(NOTIFY_LIST,event,
                                            c->argv[j],c->db->id);
                        if (listTypeLength(o) == 0) {  //这里将key删掉了
                            dbDelete(c->db,c->argv[j]);
                            notifyKeyspaceEvent(NOTIFY_GENERIC,"del",
                                                c->argv[j],c->db->id);
                        }
                        signalModifiedKey(c->db,c->argv[j]);
                        server.dirty++;
    
                        /* Replicate it as an [LR]POP instead of B[LR]POP. */
                        rewriteClientCommandVector(c,2,
                            (where == LIST_HEAD) ? shared.lpop : shared.rpop,
                            c->argv[j]);
                        return;
                    }
                }
            }
        }
    
        /* If we are inside a MULTI/EXEC and the list is empty the only thing * we can do is treating it as a timeout (even with timeout 0). */
        if (c->flags & CLIENT_MULTI) { 
            addReply(c,shared.nullmultibulk);
            return;
        }
    
        /* If the list is empty or the key does not exists we must block */
        blockForKeys(c,BLOCKED_LIST,c->argv + 1,c->argc - 2,timeout,NULL,NULL);
    }

unsigned long listTypeLength(const robj *subject) { 
        if (subject->encoding == OBJ_ENCODING_QUICKLIST) { 
            return quicklistCount(subject->ptr);
        } else { 
            serverPanic("Unknown list encoding");
        }
    }
    
    unsigned long quicklistCount(const quicklist *ql) {  return ql->count; }

看以上源码其中当list为空时执行了删除

if (listTypeLength(o) == 0) {  //这里将key删掉了
        dbDelete(c->db,c->argv[j]);
        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",c->argv[j],c->db->id);
    }

每次当list为空时，都执行一次删除调用，删除有两个选择，看实现

/* This is a wrapper whose behavior depends on the Redis lazy free * configuration. Deletes the key synchronously or asynchronously. */
    int dbDelete(redisDb *db, robj *key) { 
        return server.lazyfree_lazy_server_del ? dbAsyncDelete(db,key) : dbSyncDelete(db,key);
    }

分为同步删除和异步删除，具体决定于用户配置是否开启 lazyfree\_lazy\_server\_del。  
虽然异步删除，但是不是不删除，也不是懒惰删除，而是拉起一个进程执行非阻塞删除，这样做只是为了优化单线程redis的删除延迟问题。

### 结论 ###

**如此看来，将list作为缓冲队列，对于读大于写请求的情况下，必然会导致写一次删除一次list key的资源浪费。**