（三十）— pubsub发布订阅模式
最后更新于：2022-04-01 20:21:19
今天学习了Redis中比较高大上的名词，“发布订阅模式”，发布订阅模式这个词在我最开始接触听说的时候是在JMS(Java Message Service)java消息服务中听说的。这个名次用通俗的一点话说，就是我订阅了这类消息，当只有这类的消息进行广播发送的时候，我才会，其他的消息直接过滤，保证了一个高效的传输效率。下面切入正题，学习一下Redis是如何实现这个发布订阅模式的。先看看里面的简单的API构造；
~~~
/*-----------------------------------------------------------------------------
* Pubsub low level API
*----------------------------------------------------------------------------*/
void freePubsubPattern(void *p) /* 释放发布订阅的模式 */
int listMatchPubsubPattern(void *a, void *b) /* 发布订阅模式是否匹配 */
int clientSubscriptionsCount(redisClient *c) /* 返回客户端的所订阅的数量，包括channels + patterns管道和模式 */
int pubsubSubscribeChannel(redisClient *c, robj *channel) /* Client订阅一个Channel管道 */
int pubsubUnsubscribeChannel(redisClient *c, robj *channel, int notify) /* 取消订阅Client中的Channel */
int pubsubSubscribePattern(redisClient *c, robj *pattern) /* Client客户端订阅一种模式 */
int pubsubUnsubscribePattern(redisClient *c, robj *pattern, int notify) /* Client客户端取消订阅pattern模式 */
int pubsubUnsubscribeAllChannels(redisClient *c, int notify) /* 客户端取消自身订阅的所有Channel */
int pubsubUnsubscribeAllPatterns(redisClient *c, int notify) /* 客户端取消订阅所有的pattern模式 */
int pubsubPublishMessage(robj *channel, robj *message) /* 为所有订阅了Channel的Client发送消息message */
/* ------------PUB/SUB API ---------------- */
void subscribeCommand(redisClient *c) /* 订阅Channel的命令 */
void unsubscribeCommand(redisClient *c) /* 取消订阅Channel的命令 */
void psubscribeCommand(redisClient *c) /* 订阅模式命令 */
void punsubscribeCommand(redisClient *c) /* 取消订阅模式命令 */
void publishCommand(redisClient *c) /* 发布消息命令 */
void pubsubCommand(redisClient *c) /* 发布订阅命令 */
~~~
在这里面出现了高频的词Pattern(模式)和Channel(频道,叫管道比较别扭)，也就是说，后续所有的关于发布订阅的东东都是基于这2者展开进行的。现在大致讲解一下在Redis中是如何实现此中模式的:
**1.在RedisClient 内部维护了一个pubsub_channels的Channel列表，记录了此客户端所订阅的频道**
**2.在Server服务端，同样维护着一个类似的变量叫做，pubsub_channels，这是一个dict字典变量，每一个Channel对应着一批订阅了此频道的Client，也就是Channel-->list of Clients**
**3.当一个Client publish一个message的时候，会先去服务端的pubsub_channels找相应的Channel，遍历里面的Client，然后发送通知，即完成了整个发布订阅模式。**
我们可以简单的看一下Redis订阅一个Channel的方法实现；
~~~
/* Subscribe a client to a channel. Returns 1 if the operation succeeded, or
* 0 if the client was already subscribed to that channel. */
/* Client订阅一个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，对应的列表中
de = dictFind(server.pubsub_channels,channel);
if (de == NULL) {
//如果此频道的Client列表为空，则创建新列表并添加
clients = listCreate();
dictAdd(server.pubsub_channels,channel,clients);
incrRefCount(channel);
} else {
//否则，获取这个频道的客户端列表，在尾部添加新的客户端
clients = dictGetVal(de);
}
listAddNodeTail(clients,c);
}
/* Notify the client */
//添加给回复客户端
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自身的内部维护的pubsub_channels的添加，是一个dict字典对象，然后，是server端维护的pubsub_channels中的client列表的添加。在进行Channel频道的删除的时候，也是执行的这2步骤操作:
~~~
/* Unsubscribe a client from a channel. Returns 1 if the operation succeeded, or
* 0 if the client was not subscribed to the specified channel. */
/* 取消订阅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... */
//字典删除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对应的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;
}
~~~
里面还有对应的模式的订阅和取消订阅的操作，原理和channel完全一致，二者的区别在于，pattern是用来匹配的Channel的，这个是什么意思呢。在后面会做出答案，接着看。最后看一个最最核心的方法，客户端发步消息方法:
~~~
/* Publish a message */
/* 为所有订阅了Channel的Client发送消息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所对应的dictEntry
de = dictFind(server.pubsub_channels,channel);
if (de) {
//获取此Channel对应的客户单列表
list *list = dictGetVal(de);
listNode *ln;
listIter li;
listRewind(list,&li);
while ((ln = listNext(&li)) != NULL) {
//依次取出List中的客户单，添加消息回复
redisClient *c = ln->value;
addReply(c,shared.mbulkhdr[3]);
addReply(c,shared.messagebulk);
addReplyBulk(c,channel);
//添加消息回复
addReplyBulk(c,message);
receivers++;
}
}
/* Send to clients listening to matching channels */
/* 发送给尝试匹配该Channel的客户端消息 */
if (listLength(server.pubsub_patterns)) {
listRewind(server.pubsub_patterns,&li);
channel = getDecodedObject(channel);
while ((ln = listNext(&li)) != NULL) {
pubsubPattern *pat = ln->value;
//客户端的模式如果匹配了Channel，也会发送消息
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的作用就在上面体现了，如果某种pattern匹配了Channel频道，则模式的客户端也会接收消息。在server->pubsub_patterns中，pubsub_patterns是一个list列表，里面的每一个pattern只对应一个Client,就是上面的pat->client，这一点和Channel还是有本质的区别的。讲完发布订阅模式的基本操作后，顺便把与此相关的notify通知类也稍稍讲讲，通知只有3个方法，
~~~
/* ----------------- API ------------------- */
int keyspaceEventsStringToFlags(char *classes) /* 键值字符类型转为对应的Class类型 */
sds keyspaceEventsFlagsToString(int flags) /* 通过输入的flag值类，转为字符类型*/
void notifyKeyspaceEvent(int type, char *event, robj *key, int dbid) /* 发布通知方法，分为2类，keySpace的通知，keyEvent的通知 */
~~~
涉及到string To flag 和flag To String 的转换，也不知道这个会在哪里用到；
~~~
/* 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. */
/* 键值字符类型转为对应的Class类型 */
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;
}
~~~
应该是响应键盘输入的类型和Redis类型之间的转换。在notify的方法还有一个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. */
/* 发布通知方法，分为2类，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种的通知形式，略有差别
/* __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);
//上述几步操作，组件格式字符串，最后发布消息，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种事件通知。具体怎么用，等后面碰到的时候在看看。
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