Redis源碼分析（三十）--- pubsub發布訂閱模式

        今天學習了Redis中比較高大上的名詞，“發布訂閱模式”，發布訂閱模式這個詞在我最開始接觸聽說的時候是在JMS(Java Message Service)javaMessage Service中聽說的。這個名次用通俗的一點話說，就是我訂閱了這類訊息，當只有這類的訊息進行廣播發送的時候，我才會，其他的訊息直接過濾，保證了一個高效的傳輸效率。下面切入正題，學習一下Redis是如何?這個發布訂閱模式的。先看看裡面的簡單的API構造；

/*----------------------------------------------------------------------------- * Pubsub low level API *----------------------------------------------------------------------------*/void freePubsubPattern(void *p) /* 釋放發布訂閱的模式 */int listMatchPubsubPattern(void *a, void *b) /* 發布訂閱模式是否匹配 */int clientSubscriptionsCount(redisClient *c) /* 返回用戶端的所訂閱的數量，包括channels + patterns管道和模式 */int pubsubSubscribeChannel(redisClient *c, robj *channel) /* Client訂閱一個Channel管道 */int pubsubUnsubscribeChannel(redisClient *c, robj *channel, int notify) /* 取消訂閱Client中的Channel */int pubsubSubscribePattern(redisClient *c, robj *pattern) /* Client客訂閱一種模式 */int pubsubUnsubscribePattern(redisClient *c, robj *pattern, int notify) /* Client用戶端取消訂閱pattern模式 */int pubsubUnsubscribeAllChannels(redisClient *c, int notify) /* 用戶端取消自身訂閱的所有Channel */int pubsubUnsubscribeAllPatterns(redisClient *c, int notify) /* 用戶端取消訂閱所有的pattern模式 */int pubsubPublishMessage(robj *channel, robj *message) /* 為所有訂閱了Channel的Client發送訊息message *//* ------------PUB/SUB API ---------------- */void subscribeCommand(redisClient *c) /* 訂閱Channel的命令 */void unsubscribeCommand(redisClient *c) /* 取消訂閱Channel的命令 */void psubscribeCommand(redisClient *c) /* 訂閱模式命令 */void punsubscribeCommand(redisClient *c) /* 取消訂閱模式命令 */void publishCommand(redisClient *c) /* 發布訊息命令 */void pubsubCommand(redisClient *c) /* 發布訂閱命令 */

在這裡面出現了高頻的詞Pattern(模式)和Channel(頻道,叫管道比較彆扭)，也就是說，後續所有的關於發布訂閱的東東都是基於這2者展開進行的。現在大致講解一下在Redis中是如何?此中模式的:

1.在RedisClient 內部維護了一個pubsub_channels的Channel列表，記錄了此用戶端所訂閱的頻道

2.在Server服務端，同樣維護著一個類似的變數叫做，pubsub_channels，這是一個dict字典變數，每一個Channel對應著一批訂閱了此頻道的Client，也就是Channel-->list of Clients

3.當一個Client publish一個message的時候，會先去服務端的pubsub_channels找相應的Channel，遍曆裡面的Client，然後發送通知，即完成了整個發布訂閱模式。

    我們可以簡單的看一下Redis訂閱一個Channel的方法實現；

/* Subscribe a client to a channel. Returns 1 if the operation succeeded, or * 0 if the client was already subscribed to that channel. *//* Client訂閱一個Channel管道 */int pubsubSubscribeChannel(redisClient *c, robj *channel) {    struct dictEntry *de;    list *clients = NULL;    int retval = 0;    /* Add the channel to the client -> channels hash table */    //在Client的字典pubsub_channels中添加Channel    if (dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {        retval = 1;        incrRefCount(channel);        /* Add the client to the channel -> list of clients hash table */        //添加Clietn到server中的pubsub_channels，對應的列表中        de = dictFind(server.pubsub_channels,channel);        if (de == NULL) {        //如果此頻道的Client列表為空白，則建立新列表並添加            clients = listCreate();            dictAdd(server.pubsub_channels,channel,clients);            incrRefCount(channel);        } else {        //否則，擷取這個頻道的用戶端列表，在尾部添加新的用戶端            clients = dictGetVal(de);        }        listAddNodeTail(clients,c);    }    /* Notify the client */    //添加給回複用戶端    addReply(c,shared.mbulkhdr[3]);    addReply(c,shared.subscribebulk);    addReplyBulk(c,channel);    addReplyLongLong(c,clientSubscriptionsCount(c));    return retval;}

添加操作主要分2部，Client自身的內部維護的pubsub_channels的添加，是一個dict字典對象，然後，是server端維護的pubsub_channels中的client列表的添加。在進行Channel頻道的刪除的時候，也是執行的這2步驟操作:

/* Unsubscribe a client from a channel. Returns 1 if the operation succeeded, or * 0 if the client was not subscribed to the specified channel. *//* 取消訂閱Client中的Channel */int pubsubUnsubscribeChannel(redisClient *c, robj *channel, int notify) {    struct dictEntry *de;    list *clients;    listNode *ln;    int retval = 0;    /* Remove the channel from the client -> channels hash table */    incrRefCount(channel); /* channel may be just a pointer to the same object                            we have in the hash tables. Protect it... */    //字典刪除Client中pubsub_channels中的Channel    if (dictDelete(c->pubsub_channels,channel) == DICT_OK) {        retval = 1;        /* Remove the client from the channel -> clients list hash table */        //再移除Channel對應的Client列表        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);        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 */    if (notify) {        addReply(c,shared.mbulkhdr[3]);        addReply(c,shared.unsubscribebulk);        addReplyBulk(c,channel);        addReplyLongLong(c,dictSize(c->pubsub_channels)+                       listLength(c->pubsub_patterns));    }    decrRefCount(channel); /* it is finally safe to release it */    return retval;}

裡面還有對應的模式的訂閱和取消訂閱的操作，原理和channel完全一致，二者的區別在於，pattern是用來匹配的Channel的，這個是什麼意思呢。在後面會做出答案，接著看。最後看一個最最核心的方法，用戶端發步訊息方法:

/* Publish a message *//* 為所有訂閱了Channel的Client發送訊息message */int pubsubPublishMessage(robj *channel, robj *message) {    int receivers = 0;    struct dictEntry *de;    listNode *ln;    listIter li;    /* Send to clients listening for that channel */    //找到Channel所對應的dictEntry    de = dictFind(server.pubsub_channels,channel);    if (de) {    //擷取此Channel對應的客戶單列表        list *list = dictGetVal(de);        listNode *ln;        listIter li;        listRewind(list,&li);        while ((ln = listNext(&li)) != NULL) {        //依次取出List中的客戶單，添加訊息回複            redisClient *c = ln->value;            addReply(c,shared.mbulkhdr[3]);            addReply(c,shared.messagebulk);            addReplyBulk(c,channel);            //添加訊息回複            addReplyBulk(c,message);            receivers++;        }    }    /* Send to clients listening to matching channels */    /* 發送給嘗試匹配該Channel的用戶端訊息 */    if (listLength(server.pubsub_patterns)) {        listRewind(server.pubsub_patterns,&li);        channel = getDecodedObject(channel);        while ((ln = listNext(&li)) != NULL) {            pubsubPattern *pat = ln->value;//用戶端的模式如果匹配了Channel，也會發送訊息            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++;            }        }        decrRefCount(channel);    }    return receivers;}

pattern的作用就在上面體現了，如果某種pattern匹配了Channel頻道，則模式的用戶端也會接收訊息。在server->pubsub_patterns中， pubsub_patterns是一個list列表，裡面的每一個pattern只對應一個Client,就是上面的pat->client，這一點和Channel還是有本質的區別的。講完發布訂閱模式的基本操作後，順便把與此相關的notify通知類也稍稍講講，通知只有3個方法，

/* ----------------- API ------------------- */int keyspaceEventsStringToFlags(char *classes) /* 索引值字元類型轉為對應的Class類型 */sds keyspaceEventsFlagsToString(int flags) /* 通過輸入的flag值類，轉為字元類型*/void notifyKeyspaceEvent(int type, char *event, robj *key, int dbid) /* 發布通知方法，分為2類，keySpace的通知，keyEvent的通知 */

涉及到string To flag 和flag To String 的轉換，也不知道這個會在哪裡用到；

/* Turn a string representing notification classes into an integer * representing notification classes flags xored. * * The function returns -1 if the input contains characters not mapping to * any class. *//* 索引值字元類型轉為對應的Class類型 */int keyspaceEventsStringToFlags(char *classes) {    char *p = classes;    int c, flags = 0;    while((c = *p++) != '\0') {        switch(c) {        case 'A': flags |= REDIS_NOTIFY_ALL; break;        case 'g': flags |= REDIS_NOTIFY_GENERIC; break;        case '$': flags |= REDIS_NOTIFY_STRING; break;        case 'l': flags |= REDIS_NOTIFY_LIST; break;        case 's': flags |= REDIS_NOTIFY_SET; break;        case 'h': flags |= REDIS_NOTIFY_HASH; break;        case 'z': flags |= REDIS_NOTIFY_ZSET; break;        case 'x': flags |= REDIS_NOTIFY_EXPIRED; break;        case 'e': flags |= REDIS_NOTIFY_EVICTED; break;        case 'K': flags |= REDIS_NOTIFY_KEYSPACE; break;        case 'E': flags |= REDIS_NOTIFY_KEYEVENT; break;        default: return -1;        }    }    return flags;}

應該是響應鍵盤輸入的類型和Redis類型之間的轉換。在notify的方法還有一個event事件的通知方法:

/* The API provided to the rest of the Redis core is a simple function: * * notifyKeyspaceEvent(char *event, robj *key, int dbid); * * 'event' is a C string representing the event name. * 'key' is a Redis object representing the key name. * 'dbid' is the database ID where the key lives.  *//* 發布通知方法，分為2類，keySpace的通知，keyEvent的通知 */ void notifyKeyspaceEvent(int type, char *event, robj *key, int dbid) {    sds chan;    robj *chanobj, *eventobj;    int len = -1;    char buf[24];    /* If notifications for this class of events are off, return ASAP. */    if (!(server.notify_keyspace_events & type)) return;    eventobj = createStringObject(event,strlen(event));        //2種的通知形式，略有差別    /* __keyspace@<db>__:<key> <event> notifications. */    if (server.notify_keyspace_events & REDIS_NOTIFY_KEYSPACE) {        chan = sdsnewlen("__keyspace@",11);        len = ll2string(buf,sizeof(buf),dbid);        chan = sdscatlen(chan, buf, len);        chan = sdscatlen(chan, "__:", 3);        chan = sdscatsds(chan, key->ptr);        chanobj = createObject(REDIS_STRING, chan);        //上述幾步操作，組件格式字串，最後發布訊息，下面keyEvent的通知同理        pubsubPublishMessage(chanobj, eventobj);        decrRefCount(chanobj);    }    /* __keyevente@<db>__:<event> <key> notifications. */    if (server.notify_keyspace_events & REDIS_NOTIFY_KEYEVENT) {        chan = sdsnewlen("__keyevent@",11);        if (len == -1) len = ll2string(buf,sizeof(buf),dbid);        chan = sdscatlen(chan, buf, len);        chan = sdscatlen(chan, "__:", 3);        chan = sdscatsds(chan, eventobj->ptr);        chanobj = createObject(REDIS_STRING, chan);        pubsubPublishMessage(chanobj, key);        decrRefCount(chanobj);    }    decrRefCount(eventobj);}

有keySpace和keyEvent的2種事件通知。具體怎麼用，等後面碰到的時候在看看。