redis-2.6.16源码分析之pub-sub系统

redis实现的发送订阅系统，即pub-sub，这部分的的代码比较少，也方便分析。在这只将会分析下普通的pub-sub(会忽略掉Pattern-matching subscriptions)，以此来简述一个pubsub系统是如何实现的。
在redis主要有介绍redis的pub-sub，在开始之前， 需要知道redis的pubsub的几个命令：
SUBSCRIBE first second //订阅两个channel，分别是first和second

PUBLISH secondHello //发送方向channel是second的发送"hello"消息

UNSUBSCRIBE //取消之前订阅的所有channel

下面来看看在redis的Pubsub.c代码中对这些命令的实现。
首先看看subscribe的实现:

voidsubscribeCommand(redisClient *c) {
int j;

for (j = 1; j < c->argc; j++)
//客户端订阅多个channel，对于需要订阅的每个channel都执行该函数
pubsubSubscribeChannel(c,c->argv[j]);
}

/* Subscribe aclient to a channel. Returns 1 if the operation succeeded, or
* 0 if the client was already subscribed tothat channel. */
intpubsubSubscribeChannel(redisClient *c, robj *channel) {
struct dictEntry *de;
list *clients = NULL;
int retval = 0;

//c->pubsub_channels是一个redis自己实现的hash表，这不是我们的重点，就不展开说了
//dictAdd向hash表中增加一个key为channel，value为null的键值对
/* Add the channel to the client ->channels hash table */
if(dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {
retval = 1;
incrRefCount(channel);
//从服务端查找当前需要订阅的channel是否已经在服务端登记过
//没有登记过，则创建一个列表作为channel的值，并记录到服务端
//也就是意味着服务端是通过这个列表来得知这个channel是被哪些客户端订阅过了
//登记过，则取出在服务端，channel所对应的客户端列表
/* Add the client to the channel ->list of clients hash table */
de =dictFind(server.pubsub_channels,channel);
if (de == NULL) {
clients = listCreate();
dictAdd(server.pubsub_channels,channel,clients);
incrRefCount(channel);
} else {
clients = dictGetVal(de);
}
//向这个channel所对应的记录客户端的列表的尾部插入当前客户端，
//这样服务端就当前客户端订阅了这个channel
listAddNodeTail(clients,c);
}
/* Notify the client */
addReply(c,shared.mbulkhdr[3]);
addReply(c,shared.subscribebulk);
addReplyBulk(c,channel);
addReplyLongLong(c,dictSize(c->pubsub_channels)+listLength(c->pubsub_patterns));
return retval;
}

//在看看pub的实现：
voidpublishCommand(redisClient *c) {
//发送则向某个channel发送消息,如 PUBLISH second Hello
//c->args[1]就是channel second，第二个参数就是要发送的消息Hello
int receivers =pubsubPublishMessage(c->argv[1],c->argv[2]);
addReplyLongLong(c,receivers);
}

/*Publish a message */
intpubsubPublishMessage(robj *channel, robj *message) {
int receivers = 0;
struct dictEntry *de;
listNode *ln;
listIter li;

//从服务端找到订阅了这个channel的客户端列表
//遍历这个列表，将消息发送给每个客户端
/* Send to clients listening for that channel */
de = dictFind(server.pubsub_channels,channel);
if (de) {
list *list = dictGetVal(de);
listNode *ln;
listIter li;

listRewind(list,&li);
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 */
发送到模式匹配的订阅方的处理...

return receivers;
}

//最后看看unsubscribe的处理：
void unsubscribeCommand(redisClient *c) {
if (c->argc == 1) {
//取消这个客户端的所有订阅
pubsubUnsubscribeAllChannels(c,1);
} else {
int j;

for (j = 1; j < c->argc; j++)
//取消这个客户端的关于某个channel的订阅
pubsubUnsubscribeChannel(c,c->argv[j],1);
}
}
/* Unsubscribe from all the channels. Return the number of channels the
* client was subscribed from. */
int pubsubUnsubscribeAllChannels(redisClient *c, int notify) {
//取得这个客户端的所有订阅的channel的迭代器
dictIterator *di =dictGetSafeIterator(c->pubsub_channels);
dictEntry *de;
int count = 0;
//通过遍历迭代器来获取在客户端记录的每个channel记录，并对每个记录取消订阅
while((de = dictNext(di)) != NULL) {
robj *channel = dictGetKey(de);

count +=pubsubUnsubscribeChannel(c,channel,notify);
}
/* We were subscribed to nothing? Still reply to the client.*/
if (notify && count == 0) {
addReply(c,shared.mbulkhdr[3]);
addReply(c,shared.unsubscribebulk);
addReply(c,shared.nullbulk);
addReplyLongLong(c,dictSize(c->pubsub_channels)+
listLength(c->pubsub_patterns));
}
dictReleaseIterator(di);
return count;
}

* Unsubscribe a client from a channel. Returns 1 if the operationsucceeded, or
* 0 if the client was not subscribed to the specified channel. */
int pubsubUnsubscribeChannel(redisClient *c, robj *channel, int notify) {
struct dictEntry *de;
list *clients;
listNode *ln;
int retval = 0;

//从客户端记录的hash表中删除这个channel记录，以此来取消客户对这个channel的订阅
//从服务端查找channel所对应客户端列表，从这个列表中删除这个客户端的记录
//这样就完成了取消订阅
/* Remove the channel from the client -> channels hashtable */
incrRefCount(channel); /* channel may be just a pointer tothe same object
we have in the hash tables. Protect it... */
if (dictDelete(c->pubsub_channels,channel) == DICT_OK) {
retval = 1;
/* Remove the client from the channel ->clients list hash table */
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 associatedhash entry at all if this was
* the latest client, sothat it will be possible to abuse
* Redis PUBSUB creatingmillions 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;
}