redis學習筆記(七): pubsub

redis簡單地實現了訂閱發布功能。

pubsub涉及到的結構主要是下面兩個:

typedef struct redisClient {
    ...
    dict *pubsub_channels;  //該client訂閱的channels，以channel為key用dict的方式組織
    list *pubsub_patterns;  //該client訂閱的pattern，以list的方式組織
    ...
} redisClient;

struct redisServer {
    ...
    dict *pubsub_channels;      //redis server進程中維護的channel dict，它以channel為key，訂閱channel的client list為value
    list *pubsub_patterns;      //redis server進程中維護的pattern list
    int notify_keyspace_events; 
    ...
};

沒搞懂的是在redisClient中，為什么channel和pattern一個用dict一個用list？

對應的command:

struct redisCommand redisCommandTable[] = {
    ...
    {"subscribe",    subscribeCommand,    -2,"rpslt",0,NULL,0,0,0,0,0},    //channel訂閱命令
    {"unsubscribe",  unsubscribeCommand,  -1,"rpslt",0,NULL,0,0,0,0,0},    //channel退訂命令
    {"psubscribe",   psubscribeCommand,   -2,"rpslt",0,NULL,0,0,0,0,0},    //pattern訂閱命令
    {"punsubscribe", punsubscribeCommand, -1,"rpslt",0,NULL,0,0,0,0,0},    //pattern退訂命令
    {"publish",      publishCommand,       3,"pltrF",0,NULL,0,0,0,0,0},    //消息發布命令
    {"pubsub",       pubsubCommand,       -2,"pltrR",0,NULL,0,0,0,0,0},    //pubsub命令，用於輸出channel相關的統計信息
    ...
}

pattern的匹配，里面調用的equalStringObjects就是redis實現的正則匹配：

int listMatchPubsubPattern(void *a, void *b) {
    pubsubPattern *pa = a, *pb = b;

    return (pa->client == pb->client) &&
           (equalStringObjects(pa->pattern,pb->pattern));
}

訂閱某個channel的核心操作

int pubsubSubscribeChannel(redisClient *c, robj *channel) {
    dictEntry *de;
    list *clients = NULL;
    int retval = 0;

    /* Add the channel to the client -> channels hash table */
    /* 以channel為key加到c->pubsub_channels當中 */
    if (dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {
        retval = 1;
        incrRefCount(channel);
        /* Add the client to the channel -> list of clients hash table */
        /* server.pubsub_channels記錄了所有被訂閱的channel以及訂閱特定channel的clients, 這里把c加到該channel對應的列表當中 */
        de = dictFind(server.pubsub_channels,channel);
        if (de == NULL) {
            clients = listCreate();
            dictAdd(server.pubsub_channels,channel,clients);
            incrRefCount(channel);
        } else {
            clients = dictGetVal(de);
        }
        listAddNodeTail(clients,c);
    }
    /* Notify the client */
    /* 給客戶端生成答復 */
    addReply(c,shared.mbulkhdr[3]);
    addReply(c,shared.subscribebulk);
    addReplyBulk(c,channel);
    addReplyLongLong(c,clientSubscriptionsCount(c));
    return retval;
}

退訂某個channel的核心操作

int pubsubUnsubscribeChannel(redisClient *c, robj *channel, int notify) {
    dictEntry *de;
    list *clients;
    listNode *ln;
    int retval = 0;

    /* Remove the channel from the client -> channels hash table */
    /* 為什么需要保護起來？
     * 考慮到redis本身是單進程單線程的，所以這里不是為了防止在別的地方被free掉     
     */
    incrRefCount(channel); /* channel may be just a pointer to the same object
                            we have in the hash tables. Protect it... */
    /* 從該client的訂閱dict中移除該channel */
    if (dictDelete(c->pubsub_channels,channel) == DICT_OK) {
        retval = 1;
        /* Remove the client from the channel -> clients list hash table */
        /* 從server.pubsub_channels中該channel維護的client列表上刪除該client */
        de = dictFind(server.pubsub_channels,channel);
        redisAssertWithInfo(c,NULL,de != NULL);
        clients = dictGetVal(de);
        ln = listSearchKey(clients,c);
        redisAssertWithInfo(c,NULL,ln != NULL);
        listDelNode(clients,ln);
        /* 如果該channel上沒有其它訂閱者，則從server.pubsub_channels上刪除該channel */
        if (listLength(clients) == 0) {
            /* Free the list and associated hash entry at all if this was
             * the latest client, so that it will be possible to abuse
             * Redis PUBSUB creating millions of channels. */
            dictDelete(server.pubsub_channels,channel);
        }
    }
    /* Notify the client */
    /* 如果需要的話，生成回復 */
    if (notify) {
        addReply(c,shared.mbulkhdr[3]);
        addReply(c,shared.unsubscribebulk);
        addReplyBulk(c,channel);
        addReplyLongLong(c,dictSize(c->pubsub_channels)+
                       listLength(c->pubsub_patterns));

    }
    /* 減少引用計數 */
    decrRefCount(channel); /* it is finally safe to release it */
    return retval;
}

訂閱/退訂pattern的操作也很類似，就不貼代碼了。

它還提供了pubsubUnsubscribeAllChannels和pubsubUnsubscribeAllPatterns，用於一次性退訂所有的channels/patters。實現上就是循環調用相應的退訂函數。

發布消息的核心操作

int pubsubPublishMessage(robj *channel, robj *message) {
    int receivers = 0;
    dictEntry *de;
    listNode *ln;
    listIter li;

    /* Send to clients listening for that channel */
    /* 先找到該channel */
    de = dictFind(server.pubsub_channels,channel);
    if (de) {
        list *list = dictGetVal(de);
        listNode *ln;
        listIter li;

        listRewind(list,&li);
        /* 對所有訂閱該頻道的client發送消息 */
        while ((ln = listNext(&li)) != NULL) {
            redisClient *c = ln->value;

            addReply(c,shared.mbulkhdr[3]);
            addReply(c,shared.messagebulk);
            addReplyBulk(c,channel);
            addReplyBulk(c,message);
            receivers++;
        }
    }
    /* Send to clients listening to matching channels */
    /* 如果pattern也有client在訂閱，那么還要進行模式的匹配並發送消息給相應的client */
    if (listLength(server.pubsub_patterns)) {
        listRewind(server.pubsub_patterns,&li);
        
        channel = getDecodedObject(channel);
        while ((ln = listNext(&li)) != NULL) {
            pubsubPattern *pat = ln->value;

            if (stringmatchlen((char*)pat->pattern->ptr,
                                sdslen(pat->pattern->ptr),
                                (char*)channel->ptr,
                                sdslen(channel->ptr),0)) {
                addReply(pat->client,shared.mbulkhdr[4]);
                addReply(pat->client,shared.pmessagebulk);
                addReplyBulk(pat->client,pat->pattern);
                addReplyBulk(pat->client,channel);
                addReplyBulk(pat->client,message);
                receivers++;
            }
        }
        /* 釋放getDecodedObject返回的對象 */
        decrRefCount(channel);
    }
    return receivers;
}

有了上面的幾個核心操作，subscribe/unsubscribe, psubscribe/punsubscribe, publish幾個操作基本上就是直接調用上面的函數，這里就不貼代碼了。

有一點提一下：發布/訂閱跟具體的db無關，只跟client和具體的channel/pattern有關。

redis還提供一個pubsub命令，用於輸出當前訂閱的總體情況：

void pubsubCommand(redisClient *c) {
    if (!strcasecmp(c->argv[1]->ptr,"channels") &&
        (c->argc == 2 || c->argc ==3))
    {
        /* PUBSUB CHANNELS [<pattern>] */
        /* 如果沒有指定pattern，則輸出所有channel的信息
         * 如果指定了pattern，則輸出跟pattern匹配的所有channel信息
         */
        sds pat = (c->argc == 2) ? NULL : c->argv[2]->ptr;
        dictIterator *di = dictGetIterator(server.pubsub_channels);
        dictEntry *de;
        long mblen = 0;
        void *replylen;

        replylen = addDeferredMultiBulkLength(c);
        while((de = dictNext(di)) != NULL) {
            robj *cobj = dictGetKey(de);
            sds channel = cobj->ptr;

            if (!pat || stringmatchlen(pat, sdslen(pat),
                                       channel, sdslen(channel),0))
            {
                addReplyBulk(c,cobj);
                mblen++;
            }
        }
        dictReleaseIterator(di);
        setDeferredMultiBulkLength(c,replylen,mblen);
    } else if (!strcasecmp(c->argv[1]->ptr,"numsub") && c->argc >= 2) {
        /* PUBSUB NUMSUB [Channel_1 ... Channel_N] */
        int j;
        /* 輸出指定channel上client的數量 */
        addReplyMultiBulkLen(c,(c->argc-2)*2);
        for (j = 2; j < c->argc; j++) {
            list *l = dictFetchValue(server.pubsub_channels,c->argv[j]);

            addReplyBulk(c,c->argv[j]);
            addReplyLongLong(c,l ? listLength(l) : 0);
        }
    } else if (!strcasecmp(c->argv[1]->ptr,"numpat") && c->argc == 2) {
        /* PUBSUB NUMPAT */
        /* 輸出pattern的數量 */
        addReplyLongLong(c,listLength(server.pubsub_patterns));
    } else {
        addReplyErrorFormat(c,
            "Unknown PUBSUB subcommand or wrong number of arguments for '%s'",
            (char*)c->argv[1]->ptr);
    }
}