Redis源码分析--发布与订阅
发布与订阅：
一、介绍：
Redis提供了一个轻量的订阅机制：
| 命令 | 功能 |
|---|---|
| PUBLISH channel message | 将消息发送到指定的频道 |
| SUBSCRIBE channel [channel …] | 订阅给定的一个或多个频道的信息 |
| UNSUBSCRIBE [channel [channel …]] | 退订给定的频道 |
| PSUBSCRIBE pattern [pattern …] | 订阅一个或多个符合给定模式的频道 |
| PUNSUBSCRIBE [pattern [pattern …]] | 退订所有给定模式的频道 |
下面将介绍SUBCRIBE、PSUBSCRIBE以及PUBLISH，退订实现与订阅相似，不赘述；
二、SUBSCRIBE：
为了实现订阅关系，redisServer和 redisClient都有对应的结构：
struct redisServer {
// ...
// 保存所有频道的订阅关系
dict *pubsub_channels;
// ...
};
struct redisClient {
// ...
dict *pubsub_channels;
// ...
};
- L4：保存所有频道的订阅关系，key：channel，val：list of clients；
- L10：保存该客户端订阅的频道，key：channel，val：NULL；
订阅命令的实现：
void subscribeCommand(redisClient *c) {
int j;
/* 命令：SUBSCRIBE channel [channel …]，对所有channel参数进行subscribe*/
for (j = 1; j < c->argc; j++)
pubsubSubscribeChannel(c,c->argv[j]);
}
/* Subscribe a client to a channel. Returns 1 if the operation succeeded, or
* 0 if the client was already subscribed to that channel. */
int pubsubSubscribeChannel(redisClient *c, robj *channel) {
struct dictEntry *de;
list *clients = NULL;
int retval = 0;
/* Add the channel to the client -> channels hash table */
/*
* client端：保存所有本客户端订阅的频道
* pubsub_channels: {
* "chan1": NULL,
* "chan2": NULL,
* ...
* }
* server端：保存所有的订阅信息，key：channel，val：list of clients
* pubsub_channels: {
* "chan1": client3->client2,
* "chan2": client2->client1->client3,
* ...
* }
*/
if (dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {
retval = 1;
incrRefCount(channel);
/* Add the client to the channel -> list of clients hash table */
de = dictFind(server.pubsub_channels,channel);
if (de == NULL) {
/* 如果server保存的订阅信息找不到该频道，新建一个空client链表并插入channel对应的key中 */
clients = listCreate();
dictAdd(server.pubsub_channels,channel,clients);
incrRefCount(channel);
} else {
clients = dictGetVal(de);
}
/* 新客户端插到链表的尾部 */
listAddNodeTail(clients,c);
}
/* Notify the client */
/* 大概说一下，添加"*3\r\n"，下标3就是数字3，redis使用这些共享的sds来减少内存消耗 */
addReply(c,shared.mbulkhdr[3]);
/* "$9\r\nsubscribe\r\n" */
addReply(c,shared.subscribebulk);
addReplyBulk(c,channel);
addReplyLongLong(c,dictSize(c->pubsub_channels)+listLength(c->pubsub_patterns));
return retval;
}
- L9~L21： redisServer和 redisClient结构体中都有 dict *pubsub_channels字典，但是保存的val不同；
- L40~L46：执行结果返回个客户端；
三、PSUBSCRIBE：
为了实现订阅关系，redisServer和 redisClient都有对应的结构：
typedef struct pubsubPattern {
redisClient *client;
robj *pattern;
} pubsubPattern;
struct redisServer {
// ...
list *pubsub_patterns;
// ...
};
struct redisClient {
// ...
list *pubsub_patterns;
// ...
};
- L8：保存所有模式以及订阅的频道，val：pubsubPattern；
- L14：保存该客户端订阅的频道，val：pattern字符串；
void psubscribeCommand(redisClient *c) {
int j;
/* 命令：PSUBSCRIBE pattern [pattern …] */
for (j = 1; j < c->argc; j++)
pubsubSubscribePattern(c,c->argv[j]);
}
/* Subscribe a client to a pattern. Returns 1 if the operation succeeded, or 0 if the client was already subscribed to that pattern. */
int pubsubSubscribePattern(redisClient *c, robj *pattern) {
int retval = 0;
/* 如果找不到模式链表中对应的模式，新建并插入链表中 */
if (listSearchKey(c->pubsub_patterns,pattern) == NULL) {
retval = 1;
pubsubPattern *pat;
listAddNodeTail(c->pubsub_patterns,pattern);
incrRefCount(pattern);
pat = zmalloc(sizeof(*pat));
pat->pattern = getDecodedObject(pattern);
pat->client = c;
listAddNodeTail(server.pubsub_patterns,pat);
}
/* Notify the client */
addReply(c,shared.mbulkhdr[3]);
addReply(c,shared.psubscribebulk);
addReplyBulk(c,pattern);
addReplyLongLong(c,dictSize(c->pubsub_channels)+listLength(c->pubsub_patterns));
return retval;
}
- L6：pattern必须在中模式链表找到完全匹配的模式；
四、PUBLISH：
void publishCommand(redisClient *c) {
int receivers = pubsubPublishMessage(c->argv[1],c->argv[2]);
/* publish发送的消息要和从服务器一致，所以进行命令传播 */
forceCommandPropagation(c,REDIS_PROPAGATE_REPL);
addReplyLongLong(c,receivers);
}
- L4：publish发送的消息要和从服务器一致，所以进行命令传播；
/* Publish a message */
int pubsubPublishMessage(robj *channel, robj *message) {
int receivers = 0;
struct dictEntry *de;
listNode *ln;
listIter li;
/* Send to clients listening for that channel */
de = dictFind(server.pubsub_channels,channel);
if (de) {
/* 保存了所有订阅channel的client的链表 */
list *list = dictGetVal(de);
listNode *ln;
listIter li;
/* 从list->head开始遍历，将消息发送给订阅的client */
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 */
/* 发送给模式匹配的订阅者 */
if (listLength(server.pubsub_patterns)) {
listRewind(server.pubsub_patterns,&li);
channel = getDecodedObject(channel);
while ((ln = listNext(&li)) != NULL) {
pubsubPattern *pat = ln->value;
/* 正则匹配 */
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;
}
五、键空间通知：
2.8版本开始，Redis引入了键空间通知(keyspace notifications)，户端可以通过订阅/发布(Pub/Sub)机制，接收那些以某种方式改变了Redis数据空间的事件通知。具体看Redis键空间通知[1][2]，具体实现也是用 pubsubPublishMessage来发布消息的。
