Redis系列（八）：数据结构List双向链表中阻塞版本之BLPOP、BRPOP和LINDEX、LINSERT、LRANGE命令详解

1.BRPOP、BLPOP

BLPOP：

BLPOP 是阻塞式列表的弹出原语。 它是命令 LPOP 的阻塞版本，这是因为当给定列表内没有任何元素可供弹出的时候，

连接将被 BLPOP 命令阻塞。 当给定多个 key 参数时，按参数 key 的先后顺序依次检查各个列表，弹出第一个非空列表的头元素。

BRPOP:

BRPOP 是一个阻塞的列表弹出原语。 它是 RPOP 的阻塞版本，因为这个命令会在给定list无法弹出任何元素的时候阻塞连接。 该命令会按照给出的 key 顺序查看 list，并在找到的第一个非空 list 的尾部弹出一个元素。

请在 BLPOP 文档 中查看该命令的准确语义，因为 BRPOP 和 BLPOP 基本是完全一样的，除了它们一个是从尾部弹出元素，而另一个是从头部弹出元素。

时间复杂度 :O(1)

127.0.0.1:6379> rpush order 10001
(integer) 1
127.0.0.1:6379> rpush order 10002
(integer) 2
127.0.0.1:6379> rpush order 10003
(integer) 3
127.0.0.1:6379> rpush order 10004
(integer) 4
127.0.0.1:6379> rpush order 10005
(integer) 5
127.0.0.1:6379> rpush order 10006
(integer) 1
127.0.0.1:6379>

127.0.0.1:6379> brpop order 0
1) "order"
2) "10005"
127.0.0.1:6379> brpop order 0
1) "order"
2) "10004"
127.0.0.1:6379> brpop order 0
1) "order"
2) "10003"
127.0.0.1:6379> brpop order 0
1) "order"
2) "10002"
127.0.0.1:6379> brpop order 0
1) "order"
2) "10001"
127.0.0.1:6379> brpop order 0
1) "order"
2) "10006"
(2765.54s)
127.0.0.1:6379>

 源码解析

void blpopCommand(client *c) {
    blockingPopGenericCommand(c,LIST_HEAD);
}

void brpopCommand(client *c) {
    blockingPopGenericCommand(c,LIST_TAIL);
}

blockingPopGenericCommand

/* Blocking RPOP/LPOP */
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);

                    addReplyArrayLen(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) {
                        dbDelete(c->db,c->argv[j]);
                        notifyKeyspaceEvent(NOTIFY_GENERIC,"del",
                                            c->argv[j],c->db->id);
                    }
                    signalModifiedKey(c,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) {
        addReplyNullArray(c);
        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);
}

blockForKeys

void blockForKeys(client *c, int btype, robj **keys, int numkeys, mstime_t timeout, robj *target, streamID *ids) {
    dictEntry *de;
    list *l;
    int j;

    c->bpop.timeout = timeout;
    c->bpop.target = target;

    if (target != NULL) incrRefCount(target);

    for (j = 0; j < numkeys; j++) {
        /* Allocate our bkinfo structure, associated to each key the client
         * is blocked for. */
        bkinfo *bki = zmalloc(sizeof(*bki));
        if (btype == BLOCKED_STREAM)
            bki->stream_id = ids[j];

        /* If the key already exists in the dictionary ignore it. */
        if (dictAdd(c->bpop.keys,keys[j],bki) != DICT_OK) {
            zfree(bki);
            continue;
        }
        incrRefCount(keys[j]);

        /* And in the other "side", to map keys -> clients */
        de = dictFind(c->db->blocking_keys,keys[j]);
        if (de == NULL) {
            int retval;

            /* For every key we take a list of clients blocked for it */
            l = listCreate();
            retval = dictAdd(c->db->blocking_keys,keys[j],l);
            incrRefCount(keys[j]);
            serverAssertWithInfo(c,keys[j],retval == DICT_OK);
        } else {
            l = dictGetVal(de);
        }
        listAddNodeTail(l,c);
        bki->listnode = listLast(l);
    }
    blockClient(c,btype);
}

blockClient

void blockClient(client *c, int btype) {
    c->flags |= CLIENT_BLOCKED;
    c->btype = btype;
    server.blocked_clients++;
    server.blocked_clients_by_type[btype]++;
    addClientToTimeoutTable(c);
}

addClientToTimeoutTable

void addClientToTimeoutTable(client *c) {
    if (c->bpop.timeout == 0) return;
    uint64_t timeout = c->bpop.timeout;
    unsigned char buf[CLIENT_ST_KEYLEN];
    encodeTimeoutKey(buf,timeout,c);
    if (raxTryInsert(server.clients_timeout_table,buf,sizeof(buf),NULL,NULL))
        c->flags |= CLIENT_IN_TO_TABLE;
}

2.LINDEX

返回列表里的元素的索引 index 存储在 key 里面。 下标是从0开始索引的，所以 0 是表示第一个元素，

1 表示第二个元素，并以此类推。 负数索引用于指定从列表尾部开始索引的元素。在这种方法下，-1 表示最后一个元素，-2 表示倒数第二个元素，并以此往前推。

当 key 位置的值不是一个列表的时候，会返回一个error。

时间复杂度：O(N)

127.0.0.1:6379> lpush mylist "World" "Hello"
(integer) 2
127.0.0.1:6379> lindex mylist 0
"Hello"
127.0.0.1:6379> lindex mylist 1
"World"
127.0.0.1:6379>

源码解析

void lindexCommand(client *c) {
    robj *o = lookupKeyReadOrReply(c,c->argv[1],shared.null[c->resp]);
    if (o == NULL || checkType(c,o,OBJ_LIST)) return;
    long index;
    robj *value = NULL;

    if ((getLongFromObjectOrReply(c, c->argv[2], &index, NULL) != C_OK))
        return;

    if (o->encoding == OBJ_ENCODING_QUICKLIST) {
        quicklistEntry entry;
        if (quicklistIndex(o->ptr, index, &entry)) {
            if (entry.value) {
                value = createStringObject((char*)entry.value,entry.sz);
            } else {
                value = createStringObjectFromLongLong(entry.longval);
            }
            addReplyBulk(c,value);
            decrRefCount(value);
        } else {
            addReplyNull(c);
        }
    } else {
        serverPanic("Unknown list encoding");
    }
}

/* Populate 'entry' with the element at the specified zero-based index
 * where 0 is the head, 1 is the element next to head
 * and so on. Negative integers are used in order to count
 * from the tail, -1 is the last element, -2 the penultimate
 * and so on. If the index is out of range 0 is returned.
 *
 * Returns 1 if element found
 * Returns 0 if element not found */
int quicklistIndex(const quicklist *quicklist, const long long idx,
                   quicklistEntry *entry) {
    quicklistNode *n;
    unsigned long long accum = 0;
    unsigned long long index;
    int forward = idx < 0 ? 0 : 1; /* < 0 -> reverse, 0+ -> forward */

    initEntry(entry);
    entry->quicklist = quicklist;

    if (!forward) {
        index = (-idx) - 1;
        n = quicklist->tail;
    } else {
        index = idx;
        n = quicklist->head;
    }

    if (index >= quicklist->count)
        return 0;

    while (likely(n)) {
        if ((accum + n->count) > index) {
            break;
        } else {
            D("Skipping over (%p) %u at accum %lld", (void *)n, n->count,
              accum);
            accum += n->count;
            n = forward ? n->next : n->prev;
        }
    }

    if (!n)
        return 0;

    D("Found node: %p at accum %llu, idx %llu, sub+ %llu, sub- %llu", (void *)n,
      accum, index, index - accum, (-index) - 1 + accum);

    entry->node = n;
    if (forward) {
        /* forward = normal head-to-tail offset. */
        entry->offset = index - accum;
    } else {
        /* reverse = need negative offset for tail-to-head, so undo
         * the result of the original if (index < 0) above. */
        entry->offset = (-index) - 1 + accum;
    }

    quicklistDecompressNodeForUse(entry->node);
    entry->zi = ziplistIndex(entry->node->zl, entry->offset);
    ziplistGet(entry->zi, &entry->value, &entry->sz, &entry->longval);
    /* The caller will use our result, so we don't re-compress here.
     * The caller can recompress or delete the node as needed. */
    return 1;
}

3.LINSERT

把 value 插入存于 key 的列表中在基准值 pivot 的前面或后面。

当 key 不存在时，这个list会被看作是空list，任何操作都不会发生。

当 key 存在，但保存的不是一个list的时候，会返回error。

时间复杂度：O(N)

127.0.0.1:6379> linsert mylist before "World" "There"
(integer) 3
127.0.0.1:6379> lrange mylist 0 -1
1) "Hello"
2) "There"
3) "World"
127.0.0.1:6379>

4.LRANGE

返回存储在 key 的列表里指定范围内的元素。 start 和 end 偏移量都是基于0的下标，即list的第一个元素下标是0（list的表头），第二个元素下标是1，以此类推。

偏移量也可以是负数，表示偏移量是从list尾部开始计数。 例如， -1 表示列表的最后一个元素，-2 是倒数第二个，以此类推。

时间复杂度：O(S+N)

127.0.0.1:6379> lrange mylist 0 -1
1) "Hello"
2) "There"
3) "World"
127.0.0.1:6379> lrange mylist 0 1
1) "Hello"
2) "There"
127.0.0.1:6379>

源码解析

void lrangeCommand(client *c) {
    robj *o;
    long start, end, llen, rangelen;

    if ((getLongFromObjectOrReply(c, c->argv[2], &start, NULL) != C_OK) ||
        (getLongFromObjectOrReply(c, c->argv[3], &end, NULL) != C_OK)) return;

    if ((o = lookupKeyReadOrReply(c,c->argv[1],shared.emptyarray)) == NULL
         || checkType(c,o,OBJ_LIST)) return;
    llen = listTypeLength(o);

    /* convert negative indexes */
    if (start < 0) start = llen+start;
    if (end < 0) end = llen+end;
    if (start < 0) start = 0;

    /* Invariant: start >= 0, so this test will be true when end < 0.
     * The range is empty when start > end or start >= length. */
    if (start > end || start >= llen) {
        addReply(c,shared.emptyarray);
        return;
    }
    if (end >= llen) end = llen-1;
    rangelen = (end-start)+1;

    /* Return the result in form of a multi-bulk reply */
    addReplyArrayLen(c,rangelen);
    if (o->encoding == OBJ_ENCODING_QUICKLIST) {
        listTypeIterator *iter = listTypeInitIterator(o, start, LIST_TAIL);

        while(rangelen--) {
            listTypeEntry entry;
            listTypeNext(iter, &entry);
            quicklistEntry *qe = &entry.entry;
            if (qe->value) {
                addReplyBulkCBuffer(c,qe->value,qe->sz);
            } else {
                addReplyBulkLongLong(c,qe->longval);
            }
        }
        listTypeReleaseIterator(iter);
    } else {
        serverPanic("List encoding is not QUICKLIST!");
    }
}