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)java消息服務中聽說的。這個名次用通俗的一點話說，就是我訂閱了這類消息，當只有這類的消息進行廣播發送的時候，我才會，其他的消息直接過濾，保證了一個高效的傳輸效率。下面切入正題，學習一下Redis是如何實現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) /* 返回客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客戶端訂閱一種模式 */ int pubsubUnsubscribePattern(redisClient *c, robj *pattern, int notify) /* Client客戶端取消訂閱pattern模式 */ int pubsubUnsubscribeAllChannels(redisClient *c, int notify) /* 客戶端取消自身訂閱的所有Channel */ int pubsubUnsubscribeAllPatterns(redisClient *c, int notify) /* 客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發送消息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) /* 發布消息命令 */ 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,叫管道比較別扭)，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中是如何實現此中模式的:
1.在RedisClient 內部維護了一個pubsub_channels的Channel列表，記錄了此客戶端所訂閱的頻道
2.在Server服務端，同樣維護著一個類似的變量叫做，pubsub_channels，這是一個dict字典變量，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，遍歷裡面的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，則創建新列表並添加
clients = listCreate();
dictAdd(server.pubsub_channels,channel,clients);
incrRefCount(channel);
} else {
//否則，獲取這個頻道的客戶端列表，在尾部添加新的客戶端
clients = dictGetVal(de);
}
listAddNodeTail(clients,c);
}
/* Notify the client */
//添加給回復客戶端
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，客戶端發步消息方法:
/* Publish a message */
/* 為所有訂閱了Channel的Client發送消息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) {
//獲取此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，添加消息回復
redisClient *c = ln->value;
addReply(c,shared.mbulkhdr[3]);
addReply(c,shared.messagebulk);
addReplyBulk(c,channel);
//添加消息回復
addReplyBulk(c,message);
receivers++;
}
}
/* Send to clients listening to matching channels */
/* 發送給嘗試匹配該Channel的客戶端消息 */
if (listLength(server.pubsub_patterns)) {
listRewind(server.pubsub_patterns,&li);
channel = getDecodedObject(channel);
while ((ln = listNext(&li)) != NULL) {
pubsubPattern *pat = ln->value;
//客戶端的模式如果匹配了Channel，也會發送消息
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，則模式的客戶端也會接收消息。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) /* 鍵值字符類型轉為對應的Class類型 */ sds keyspaceEventsFlagsToString(int flags) /* 通過輸入的flag值類，轉為字符類型*/ 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. */
/* 鍵值字符類型轉為對應的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@__: 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，組件格式字符串，最後發布消息，下面keyEvent的通知同理
pubsubPublishMessage(chanobj, eventobj);
decrRefCount(chanobj);
}
/* __keyevente@__: 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。
