Redis系列7-订阅发布机制

订阅模式

Redis 提供两个订阅模式:频道(channel)订阅glob-style 模式(pattern)频道订阅

  • 频道订阅: CA(client A)向服务器订阅了频道 news,当 CB 向 news 发布消息的时候,CA 便能收到
  • glob-style频道订阅:举例,CA(client A)向服务器订阅了频道*.news:当 CB 向 cc.news、aa.news 发布消息的时候,CA 都能收到

数据结构

struct redisServer 和 struct redisClient 都维护了频道和模式频道,前者维护了所有频道和订阅频道的客户端,后者维护了客户端自己订阅的频道。

struct redisServer {
   ......
   /* Pubsub */
   dict *pubsub_channels; /* Map channels to list of subscribed clients */
   list *pubsub_patterns; /* A list of pubsub_patterns */
   ......
}
typedef struct redisClient {
   ......
   // 用户感兴趣的频道
   dict *pubsub_channels; /* channels a client is interested in (SUBSCRIBE) */
   // 用户感兴趣的模式
   list *pubsub_patterns; /* patterns a client is interested in (SUBSCRIBE) */
   ......
} redisClient;
   // 模式频道数据结构,list *pubsub_patterns 里的每个节点数据都是struct
   // pubsubPattern。
typedef struct pubsubPattern {
   redisClient *client;
   robj *pattern;
} pubsubPattern;
  • 频道订阅是一个 dict,每个 channel 被哈希进相应的桶,每个 channel 对应一个 clients,clients 都订阅了此 channel。当有消息发布的时候,检索 channel,遍历 clients,发布消息。
    Redis系列7-订阅发布机制

  • 模式频道订阅是一个 list。当有消息发布的时候,channel 与 glob-style pattern 匹配,发布消息。
    Redis系列7-订阅发布机制

订阅过程

两种订阅模式是维护上述两种数据结构的过程,

// 订阅频道
/* 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;
   // redisClient.pubsub_channels 中保存客户端订阅的所有频道,可以查看客户端
   // 订阅了多少频道以及客户端是否订阅某个频道
   // server.pubsub_channels 中保存所有的频道和每个频道的订阅客户端,可以将
   // 消息发布到订阅客户端
   // 将频道加入redisClient.pubsub_channels
   /* Add the channel to the client -> channels hash table */
   if (dictAdd(c->pubsub_channels,channel,NULL) == DICT_OK) {
       retval = 1;
       incrRefCount(channel);
       // 在服务器负责维护的channel->clients 哈希表中寻找指定的频道
       /* Add the client to the channel -> list of clients hash table */
       de = dictFind(server.pubsub_channels,channel);
       // 未找到客户端指定的频道,需要创建
   if (de == NULL) {
       clients = listCreate();
       // 将频道加入server.pubsub_channels
       dictAdd(server.pubsub_channels,channel,clients);
       incrRefCount(channel);
       // 找到客户端指定的频道,直接获取这个频道
   } else {
       clients = dictGetVal(de);
   }
       // 将客户端添加到链表的尾部
       listAddNodeTail(clients,c);
   }
   // 通知客户端
   /* 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;
}
// 订阅模式频道
/* 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;
   // redisClient.pubsub_patterns 中保存客户端订阅的所有模式频道,可以查看
   // 客户端订阅了多少频道以及客户端是否订阅某个频道
   // server.pubsub_patterns 中保存所有的模式频道和每个模式频道的订阅客户端
   // ,可以将消息发布到订阅客户端
   // 未订阅模式频道,插入
   if (listSearchKey(c->pubsub_patterns,pattern) == NULL) {
       retval = 1;
       pubsubPattern *pat;
       // 将模式频道加入redisClient.pubsub_patterns
       listAddNodeTail(c->pubsub_patterns,pattern);
       incrRefCount(pattern);
       // 将模式频道加入server.pubsub_patterns
       pat = zmalloc(sizeof(*pat));
       pat->pattern = getDecodedObject(pattern);
       pat->client = c;
       listAddNodeTail(server.pubsub_patterns,pat);
   }
   // 通知客户端
   /* 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;
}

消息发布

发布消息的过程则遍历上述两个数据结构(dict 和list),并将消息发布到匹配频道的所有客户端。

// 发布消息
/* Publish a message */
int pubsubPublishMessage(robj *channel, robj *message) {
   int receivers = 0;
   struct dictEntry *de;
   listNode *ln;
   listIter li;
   // 发布消息有两个步骤,
   // 指定频道的所有订阅者发布消息
   // 指定模式频道的所有订阅者发布消息
   //
   // 寻找频道
   /* Send to clients listening for that channel */
   de = dictFind(server.pubsub_channels,channel);
   // 向频道所有订阅者发布信息
   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++;
   }
}
//
// 进行glob-style 模式匹配
/* Send to clients listening to matching channels */
if (listLength(server.pubsub_patterns)) {
   listRewind(server.pubsub_patterns,&li);
   channel = getDecodedObject(channel);
   while ((ln = listNext(&li)) != NULL) {
       pubsubPattern *pat = ln->value;
       // 匹配成功,向订阅者发布消息
       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;
}

注意, 只要客户端订阅了频道, 除了SUBCRIBE,UNSUBCRIBE,PSUBCRIBE,PSUBCRIBE,就不能执行其他命令。

int processCommand(redisClient *c) {
   ......
   // 在订阅发布模式下,只允许处理SUBSCRIBE 或者UNSUBSCRIBE 命令
   // 从下面的检测条件可以看出:只要存在redisClient.pubsub_channels 或者
   // redisClient.pubsub_patterns,就代表处于订阅发布模式下
   /* Only allow SUBSCRIBE and UNSUBSCRIBE in the context of Pub/Sub */
   if ((dictSize(c->pubsub_channels) > 0 || listLength(c->pubsub_patterns) > 0)
       &&
       c->cmd->proc != subscribeCommand &&
       c->cmd->proc != unsubscribeCommand &&
       c->cmd->proc != psubscribeCommand &&
       c->cmd->proc != punsubscribeCommand) {
       addReplyError(c,"only (P)SUBSCRIBE / (P)UNSUBSCRIBE / QUIT allowed
               "in this context");
       return REDIS_OK;
       }
   ......
}
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