Redis阻塞队列原理学习

1.redis介绍

Redis是一个开源的使用ANSI C语言编写、支持网络、可基于内存亦可持久化的日志型、Key-Value数据库，并提供多种语言的API。

2.阻塞队列

使用非阻塞队列的时候有一个很大问题就是：它不会对当前线程产生阻塞，那么在面对类似消费者-生产者的模型时，就必须额外地实现同步策略以及线程间唤醒策略，这个实现起来就非常麻烦。但是有了阻塞队列就不一样了，它会对当前线程产生阻塞，比如一个线程从一个空的阻塞队列中取元素，此时线程会被阻塞直到阻塞队列中有了元素。当队列中有元素后，被阻塞的线程会自动被唤醒（不需要我们编写代码去唤醒）。这样提供了极大的方便性。

3.redis的阻塞队列

redis提供远程访问模式，本文以redis远程阻塞从队列中取数据为例，介绍redis阻塞实现原理。

/*-----------------------------------------------------------------------------
 * Blocking POP operations
 *----------------------------------------------------------------------------*/

/* This is how the current blocking POP works, we use BLPOP as example:
 * - If the user calls BLPOP and the key exists and contains a non empty list
 *   then LPOP is called instead. So BLPOP is semantically the same as LPOP
 *   if blocking is not required.
 * - If instead BLPOP is called and the key does not exists or the list is
 *   empty we need to block. In order to do so we remove the notification for
 *   new data to read in the client socket (so that we‘ll not serve new
 *   requests if the blocking request is not served). Also we put the client
 *   in a dictionary (db->blocking_keys) mapping keys to a list of clients
 *   blocking for this keys.
 * - If a PUSH operation against a key with blocked clients waiting is
 *   performed, we mark this key as "ready", and after the current command,
 *   MULTI/EXEC block, or script, is executed, we serve all the clients waiting
 *   for this list, from the one that blocked first, to the last, accordingly
 *   to the number of elements we have in the ready list.
 */

　　在调用阻塞队列取操作时，队列中无数据时才会真正的触发代码的阻塞分支。在客户端，redis对新来的读请求删除了标记(通知)，这样知道阻塞的请求在获得服务前，新来的读请求都不能够被正常的服务。

blpop

void blpopCommand(redisClient *c) {
    blockingPopGenericCommand(c,REDIS_HEAD);
}

　　队列的左右出队对应队列（list）的表头和表尾。

blockingPopGenericCommand

/* Blocking RPOP/LPOP */
void blockingPopGenericCommand(redisClient *c, int where) {
    robj *o;
    mstime_t timeout;
    int j;

    if (getTimeoutFromObjectOrReply(c,c->argv[c->argc-1],&timeout,UNIT_SECONDS)
        != REDIS_OK) return;

    for (j = 1; j < c->argc-1; j++) {
        o = lookupKeyWrite(c->db,c->argv[j]);
        if (o != NULL) {
            if (o->type != REDIS_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 == REDIS_HEAD) ? "lpop" : "rpop";
                    robj *value = listTypePop(o,where);
                    redisAssert(value != NULL);

                    addReplyMultiBulkLen(c,2);
                    addReplyBulk(c,c->argv[j]);
                    addReplyBulk(c,value);
                    decrRefCount(value);
                    notifyKeyspaceEvent(REDIS_NOTIFY_LIST,event,
                                        c->argv[j],c->db->id);
                    if (listTypeLength(o) == 0) {
                        dbDelete(c->db,c->argv[j]);
                        notifyKeyspaceEvent(REDIS_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 == REDIS_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 & REDIS_MULTI) {
        addReply(c,shared.nullmultibulk);
        return;
    }

    /* If the list is empty or the key does not exists we must block */
    //这里才是阻塞的关键
    blockForKeys(c, c->argv + 1, c->argc - 2, timeout, NULL);
}

阻塞

/* Set a client in blocking mode for the specified key, with the specified
 * timeout */
void blockForKeys(redisClient *c, robj **keys, int numkeys, mstime_t timeout, robj *target) {
    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++) {
        /* If the key already exists in the dict ignore it. */
        if (dictAdd(c->bpop.keys,keys[j],NULL) != DICT_OK) 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]);
            redisAssertWithInfo(c,keys[j],retval == DICT_OK);
        } else {
            l = dictGetVal(de);
        }
        listAddNodeTail(l,c);
    }
    blockClient(c,REDIS_BLOCKED_LIST);
}