# 订阅与发布
[TOC=2,3]
Redis 通过 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 、 [SUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/subscribe.html#subscribe "(in Redis 命令参考 v2.8)") 等命令实现了订阅与发布模式，这个功能提供两种信息机制，分别是订阅/发布到频道和订阅/发布到模式，下文先讨论订阅/发布到频道的实现，再讨论订阅/发布到模式的实现。
### 频道的订阅与信息发送
Redis 的 [SUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/subscribe.html#subscribe "(in Redis 命令参考 v2.8)") 命令可以让客户端订阅任意数量的频道，每当有新信息发送到被订阅的频道时，信息就会被发送给所有订阅指定频道的客户端。
作为例子，下图展示了频道 `channel1` ，以及订阅这个频道的三个客户端 —— `client2` 、 `client5` 和 `client1` 之间的关系：
![digraph pubsub_relation { rankdir = BT; node [style = filled]; edge [style = bold]; channel1 [label = "channel1", fillcolor = "#A8E270"]; node [shape = box, fillcolor = "#95BBE3"]; client2 [label = "client2"]; client5 [label = "client5"]; client1 [label = "client1"]; client2 -> channel1 [label = "subscribe"]; client5 -> channel1 [label = "subscribe"]; client1 -> channel1 [label = "subscribe"];}](https://box.kancloud.cn/2015-09-13_55f4effd60979.svg)
当有新消息通过 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令发送给频道 `channel1` 时，这个消息就会被发送给订阅它的三个客户端：
![digraph send_message_to_subscriber { node [style = filled]; edge [style = "dashed, bold"]; message [label = "PUBLISH channel1 message", shape = plaintext, fillcolor = "#FADCAD"]; message -> channel1 [color = "#B22222]"]; channel1 [label = "channel1", fillcolor = "#A8E270"]; node [shape = box]; client2 [label = "client2", fillcolor = "#95BBE3"]; client5 [label = "client5", fillcolor = "#95BBE3"]; client1 [label = "client1", fillcolor = "#95BBE3"]; /* client2 -> channel1 [label = "subscribe"]; client5 -> channel1 [label = "subscribe"]; client1 -> channel1 [label = "subscribe"]; */ channel1 -> client2 [label = "message", color = "#B22222"]; channel1 -> client5 [label = "message", color = "#B22222"]; channel1 -> client1 [label = "message", color = "#B22222"];}](https://box.kancloud.cn/2015-09-13_55f4effd69833.svg)
在后面的内容中，我们将探讨 [SUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/subscribe.html#subscribe "(in Redis 命令参考 v2.8)") 和 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令的实现，以及这套订阅与发布机制的运作原理。
### 订阅频道
每个 Redis 服务器进程都维持着一个表示服务器状态的 `redis.h/redisServer` 结构，结构的 `pubsub_channels` 属性是一个字典，这个字典就用于保存订阅频道的信息：
~~~
struct redisServer {
// ...
dict *pubsub_channels;
// ...
};
~~~
其中，字典的键为正在被订阅的频道，而字典的值则是一个链表，链表中保存了所有订阅这个频道的客户端。
比如说，在下图展示的这个 `pubsub_channels` 示例中， `client2` 、 `client5` 和 `client1` 就订阅了 `channel1` ，而其他频道也分别被别的客户端所订阅：
![digraph pubsub { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; // keys pubsub [label = "pubsub_channels |<channel1> channel1 |<channel2> channel2 |<channel3> channel3 | ... |<channelN> channelN", fillcolor = "#A8E270"]; // clients blocking for channel1 client1 [label = "client1", fillcolor = "#95BBE3"]; client5 [label = "client5", fillcolor = "#95BBE3"]; client2 [label = "client2", fillcolor = "#95BBE3"]; null_1 [label = "NULL", shape = plaintext]; pubsub:channel1 -> client2; client2 -> client5; client5 -> client1; client1 -> null_1; // clients blocking for channel2 client7 [label = "client7", fillcolor = "#95BBE3"]; null_2 [label = "NULL", shape = plaintext]; pubsub:channel2 -> client7; client7 -> null_2; // channel client3 [label = "client3", fillcolor = "#95BBE3"]; client4 [label = "client4", fillcolor = "#95BBE3"]; client6 [label = "client6", fillcolor = "#95BBE3"]; null_3 [label = "NULL", shape = plaintext]; pubsub:channel3 -> client3; client3 -> client4; client4 -> client6; client6 -> null_3;}](https://box.kancloud.cn/2015-09-13_55f4effd71e2f.svg)
当客户端调用 [SUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/subscribe.html#subscribe "(in Redis 命令参考 v2.8)") 命令时，程序就将客户端和要订阅的频道在 `pubsub_channels` 字典中关联起来。
举个例子，如果客户端 `client10086` 执行命令 `SUBSCRIBE channel1 channel2 channel3` ，那么前面展示的 `pubsub_channels` 将变成下面这个样子：
![digraph new_subscribe { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; // keys pubsub [label = "pubsub_channels |<channel1> channel1 |<channel2> channel2 |<channel3> channel3 | ... |<channelN> channelN", fillcolor = "#A8E270"]; // clients blocking for channel1 client1 [label = "client1", fillcolor = "#95BBE3"]; client5 [label = "client5", fillcolor = "#95BBE3"]; client2 [label = "client2", fillcolor = "#95BBE3"]; client10086 [label = "client10086", fillcolor = "#FFC1C1"]; client10086_1 [label = "client10086", fillcolor = "#FFC1C1"]; client10086_2 [label = "client10086", fillcolor = "#FFC1C1"]; null_1 [label = "NULL", shape = plaintext]; null_2 [label = "NULL", shape = plaintext]; null_3 [label = "NULL", shape = plaintext]; pubsub:channel1 -> client2; client2 -> client5; client5 -> client1; client1 -> client10086; client10086 -> null_1; // clients blocking for channel2 client7 [label = "client7", fillcolor = "#95BBE3"]; pubsub:channel2 -> client7; client7 -> client10086_1; client10086_1 -> null_2; // channel client3 [label = "client3", fillcolor = "#95BBE3"]; client4 [label = "client4", fillcolor = "#95BBE3"]; client6 [label = "client6", fillcolor = "#95BBE3"]; pubsub:channel3 -> client3; client3 -> client4; client4 -> client6; client6 -> client10086_2; client10086_2 -> null_3;}](https://box.kancloud.cn/2015-09-13_55f4effd7add8.svg)
[SUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/subscribe.html#subscribe "(in Redis 命令参考 v2.8)") 命令的行为可以用伪代码表示如下：
~~~
def SUBSCRIBE(client, channels):
# 遍历所有输入频道
for channel in channels:
# 将客户端添加到链表的末尾
redisServer.pubsub_channels[channel].append(client)
~~~
通过 `pubsub_channels` 字典，程序只要检查某个频道是否为字典的键，就可以知道该频道是否正在被客户端订阅；只要取出某个键的值，就可以得到所有订阅该频道的客户端的信息。
### 发送信息到频道
了解了 `pubsub_channels` 字典的结构之后，解释 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令的实现就非常简单了：当调用 `PUBLISH channel message` 命令，程序首先根据 `channel` 定位到字典的键，然后将信息发送给字典值链表中的所有客户端。
比如说，对于以下这个 `pubsub_channels` 实例，如果某个客户端执行命令 `PUBLISH channel1 "hello moto"` ，那么 `client2` 、 `client5` 和 `client1` 三个客户端都将接收到 `"hello moto"` 信息：
![digraph pubsub { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; // keys pubsub [label = "pubsub_channels |<channel1> channel1 |<channel2> channel2 |<channel3> channel3 | ... |<channelN> channelN", fillcolor = "#A8E270"]; // clients blocking for channel1 client1 [label = "client1", fillcolor = "#95BBE3"]; client5 [label = "client5", fillcolor = "#95BBE3"]; client2 [label = "client2", fillcolor = "#95BBE3"]; null_1 [label = "NULL", shape = plaintext]; pubsub:channel1 -> client2; client2 -> client5; client5 -> client1; client1 -> null_1; // clients blocking for channel2 client7 [label = "client7", fillcolor = "#95BBE3"]; null_2 [label = "NULL", shape = plaintext]; pubsub:channel2 -> client7; client7 -> null_2; // channel client3 [label = "client3", fillcolor = "#95BBE3"]; client4 [label = "client4", fillcolor = "#95BBE3"]; client6 [label = "client6", fillcolor = "#95BBE3"]; null_3 [label = "NULL", shape = plaintext]; pubsub:channel3 -> client3; client3 -> client4; client4 -> client6; client6 -> null_3;}](https://box.kancloud.cn/2015-09-13_55f4effd84159.svg)
[PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令的实现可以用以下伪代码来描述：
~~~
def PUBLISH(channel, message):
# 遍历所有订阅频道 channel 的客户端
for client in server.pubsub_channels[channel]:
# 将信息发送给它们
send_message(client, message)
~~~
### 退订频道
使用 [UNSUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/unsubscribe.html#unsubscribe "(in Redis 命令参考 v2.8)") 命令可以退订指定的频道，这个命令执行的是订阅的反操作：它从 `pubsub_channels` 字典的给定频道（键）中，删除关于当前客户端的信息，这样被退订频道的信息就不会再发送给这个客户端。
### 模式的订阅与信息发送
当使用 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令发送信息到某个频道时，不仅所有订阅该频道的客户端会收到信息，如果有某个/某些模式和这个频道匹配的话，那么所有订阅这个/这些频道的客户端也同样会收到信息。
下图展示了一个带有频道和模式的例子，其中 `tweet.shop.*` 模式匹配了 `tweet.shop.kindle` 频道和 `tweet.shop.ipad` 频道，并且有不同的客户端分别订阅它们三个：
![digraph pattern_relation { rankdir = BT; node [style = filled]; edge [style = bold]; kindle [label = "tweet.shop.kindle", fillcolor = "#A8E270"]; ipad [label = "tweet.shop.ipad", fillcolor = "#A8E270"]; node [shape = octagon]; pattern [label = "tweet.shop.*"]; pattern -> kindle [label = "match"]; pattern -> ipad [label = "match"]; node [shape = box]; client123 [fillcolor = "#95BBE3"]; client256 [fillcolor = "#95BBE3"]; clientX [fillcolor = "#95BBE3"]; clientY [fillcolor = "#95BBE3"]; client3333 [fillcolor = "#95BBE3"]; client4444 [fillcolor = "#95BBE3"]; client5555 [fillcolor = "#95BBE3"]; client123 -> pattern [label = "subscribe"]; client256 -> pattern [label = "subscribe"]; clientX -> kindle [label = "subscribe"]; clientY -> kindle [label = "subscribe"]; client3333 -> ipad [label = "subscribe"]; client4444 -> ipad [label = "subscribe"]; client5555 -> ipad [label = "subscribe"];}](https://box.kancloud.cn/2015-09-13_55f4effd8d5d6.svg)
当有信息发送到 `tweet.shop.kindle` 频道时，信息除了发送给 `clientX` 和 `clientY` 之外，还会发送给订阅 `tweet.shop.*` 模式的 `client123` 和 `client256` ：
![digraph send_message_to_pattern { node [style = filled]; edge [style = bold]; // tweet.shop.ipad ipad [label = "tweet.shop.ipad", fillcolor = "#A8E270"]; ipad -> pattern [label = "match", dir = back]; node [shape = box]; ipad -> client3333 [label = "subscribe", dir = back]; ipad -> client4444 [label = "subscribe", dir = back]; ipad -> client5555 [label = "subscribe", dir = back]; node [shape = plaintext]; message [label = "PUBLISH tweet.shop.kindle message", fillcolor = "#FADCAD"]; kindle [label = "tweet.shop.kindle", shape = ellipse, fillcolor = "#A8E270"]; pattern [label = "tweet.shop.*", shape = octagon]; message -> kindle [style = "bold, dashed", color = "#B22222"]; kindle -> pattern [style = "bold, dashed", color = "#B22222"]; node [shape = box]; kindle -> clientX [style = "bold, dashed", color = "#B22222", label = "message"]; kindle -> clientY [style = "bold, dashed", color = "#B22222", label = "message"]; pattern -> client123 [label = "message", style = "bold, dashed", color = "#B22222"]; pattern -> client256 [label = "message", style = "bold, dashed", color = "#B22222"]; // client color client123 [fillcolor = "#95BBE3"]; client256 [fillcolor = "#95BBE3"]; clientX [fillcolor = "#95BBE3"]; clientY [fillcolor = "#95BBE3"]; client3333 [fillcolor = "#95BBE3"]; client4444 [fillcolor = "#95BBE3"]; client5555 [fillcolor = "#95BBE3"];}](https://box.kancloud.cn/2015-09-13_55f4effd98208.svg)
另一方面，如果接收到信息的是频道 `tweet.shop.ipad` ，那么 `client123` 和 `client256` 同样会收到信息：
![digraph pattern_relation { rankdir = BT; node [style = filled]; edge [style = bold]; kindle [label = "tweet.shop.kindle", fillcolor = "#A8E270"]; ipad [label = "tweet.shop.ipad", fillcolor = "#A8E270"]; node [shape = octagon]; pattern [label = "tweet.shop.*"]; pattern -> kindle [label = "match"]; pattern -> ipad [style = "bold, dashed", color = "#B22222", dir = back]; node [shape = box]; client123 -> pattern [label = "message", dir = back, style= "bold, dashed", color = "#B22222"]; client256 -> pattern [label = "message", dir = back, style= "bold, dashed", color = "#B22222"]; clientX -> kindle [label = "subscribe"]; clientY -> kindle [label = "subscribe"]; client3333 -> ipad [label = "message", style = "bold, dashed", color = "#B22222", dir = back]; client4444 -> ipad [label = "message", style = "bold, dashed", color = "#B22222", dir = back]; client5555 -> ipad [label = "message", style = "bold, dashed", color = "#B22222", dir = back]; // new publish [label = "PUBLISH tweet.shop.ipad message", shape = plaintext, fillcolor = "#FADCAD"]; ipad -> publish [style = "bold, dashed", color = "#B22222", dir = back]; // client color client123 [fillcolor = "#95BBE3"]; client256 [fillcolor = "#95BBE3"]; clientX [fillcolor = "#95BBE3"]; clientY [fillcolor = "#95BBE3"]; client3333 [fillcolor = "#95BBE3"]; client4444 [fillcolor = "#95BBE3"]; client5555 [fillcolor = "#95BBE3"];}](https://box.kancloud.cn/2015-09-13_55f4effda0456.svg)
### 订阅模式
`redisServer.pubsub_patterns` 属性是一个链表，链表中保存着所有和模式相关的信息：
~~~
struct redisServer {
// ...
list *pubsub_patterns;
// ...
};
~~~
链表中的每个节点都包含一个 `redis.h/pubsubPattern` 结构：
~~~
typedef struct pubsubPattern {
redisClient *client;
robj *pattern;
} pubsubPattern;
~~~
`client` 属性保存着订阅模式的客户端，而 `pattern` 属性则保存着被订阅的模式。
每当调用 `PSUBSCRIBE` 命令订阅一个模式时，程序就创建一个包含客户端信息和被订阅模式的 `pubsubPattern` 结构，并将该结构添加到 `redisServer.pubsub_patterns` 链表中。
作为例子，下图展示了一个包含两个模式的 `pubsub_patterns` 链表，其中 `client123` 和 `client256` 都正在订阅 `tweet.shop.*` 模式：
![digraph publish_pattern { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; redisServer [label = "redisServer| ... |<pubsub_patterns> pubsub_patterns | ...", fillcolor = "#A8E270"]; pubsubPattern_1 [label = "pubsubPattern | client \n client123 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; pubsubPattern_2 [label = "pubsubPattern | client \n client256 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; redisServer:pubsub_patterns -> pubsubPattern_1; pubsubPattern_1 -> pubsubPattern_2;}](https://box.kancloud.cn/2015-09-13_55f4effda9d15.svg)
如果这时客户端 `client10086` 执行 `PSUBSCRIBE broadcast.list.*` ，那么 `pubsub_patterns` 链表将被更新成这样：
![digraph pubsub_pattern { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; redisServer [label = "redisServer| ... |<pubsub_patterns> pubsub_patterns | ...", fillcolor = "#A8E270"]; pubsubPattern_1 [label = "pubsubPattern | client \n client123 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; pubsubPattern_2 [label = "pubsubPattern | client \n client256 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; pubsubPattern_3 [label = "pubsubPattern | client \n client10086 | pattern \n broadcast.live.*", fillcolor = "#FFC1C1"]; redisServer:pubsub_patterns -> pubsubPattern_1; pubsubPattern_1 -> pubsubPattern_2; pubsubPattern_2 -> pubsubPattern_3;}](https://box.kancloud.cn/2015-09-13_55f4effdb1cd0.svg)
通过遍历整个 `pubsub_patterns` 链表，程序可以检查所有正在被订阅的模式，以及订阅这些模式的客户端。
### 发送信息到模式
发送信息到模式的工作也是由 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令进行的，在前面讲解频道的时候，我们给出了这样一段伪代码，说它定义了 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令的行为：
~~~
def PUBLISH(channel, message):
# 遍历所有订阅频道 channel 的客户端
for client in server.pubsub_channels[channel]:
# 将信息发送给它们
send_message(client, message)
~~~
但是，这段伪代码并没有完整描述 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 命令的行为，因为 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 除了将 `message` 发送到所有订阅 `channel` 的客户端之外，它还会将 `channel` 和 `pubsub_patterns` 中的模式进行对比，如果 `channel` 和某个模式匹配的话，那么也将 `message` 发送到订阅那个模式的客户端。
完整描述 [PUBLISH](http://redis.readthedocs.org/en/latest/pub_sub/publish.html#publish "(in Redis 命令参考 v2.8)") 功能的伪代码定于如下：
~~~
def PUBLISH(channel, message):
# 遍历所有订阅频道 channel 的客户端
for client in server.pubsub_channels[channel]:
# 将信息发送给它们
send_message(client, message)
# 取出所有模式，以及订阅模式的客户端
for pattern, client in server.pubsub_patterns:
# 如果 channel 和模式匹配
if match(channel, pattern):
# 那么也将信息发给订阅这个模式的客户端
send_message(client, message)
~~~
举个例子，如果 Redis 服务器的 `pubsub_patterns` 状态如下：
![digraph pubsub_pattern { rankdir = LR; node [shape = record, style = filled]; edge [style = bold]; redisServer [label = "redisServer| ... |<pubsub_patterns> pubsub_patterns | ...", fillcolor = "#A8E270"]; pubsubPattern_1 [label = "pubsubPattern | client \n client123 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; pubsubPattern_2 [label = "pubsubPattern | client \n client256 | pattern \n tweet.shop.*", fillcolor = "#95BBE3"]; pubsubPattern_3 [label = "pubsubPattern | client \n client10086 | pattern \n broadcast.live.*", fillcolor = "#FFC1C1"]; redisServer:pubsub_patterns -> pubsubPattern_1; pubsubPattern_1 -> pubsubPattern_2; pubsubPattern_2 -> pubsubPattern_3;}](https://box.kancloud.cn/2015-09-13_55f4effdbab5a.svg)
那么当某个客户端发送信息 `"Amazon Kindle, $69."` 到 `tweet.shop.kindle` 频道时，除了所有订阅了 `tweet.shop.kindle` 频道的客户端会收到信息之外，客户端 `client123` 和 `client256` 也同样会收到信息，因为这两个客户端订阅的 `tweet.shop.*` 模式和 `tweet.shop.kindle` 频道匹配。
### 退订模式
使用 [PUNSUBSCRIBE](http://redis.readthedocs.org/en/latest/pub_sub/punsubscribe.html#punsubscribe "(in Redis 命令参考 v2.8)") 命令可以退订指定的模式，这个命令执行的是订阅模式的反操作：程序会删除 `redisServer.pubsub_patterns` 链表中，所有和被退订模式相关联的 `pubsubPattern` 结构，这样客户端就不会再收到和模式相匹配的频道发来的信息。
### 小结
- 订阅信息由服务器进程维持的 `redisServer.pubsub_channels` 字典保存，字典的键为被订阅的频道，字典的值为订阅频道的所有客户端。
- 当有新消息发送到频道时，程序遍历频道（键）所对应的（值）所有客户端，然后将消息发送到所有订阅频道的客户端上。
- 订阅模式的信息由服务器进程维持的 `redisServer.pubsub_patterns` 链表保存，链表的每个节点都保存着一个 `pubsubPattern` 结构，结构中保存着被订阅的模式，以及订阅该模式的客户端。程序通过遍历链表来查找某个频道是否和某个模式匹配。
- 当有新消息发送到频道时，除了订阅频道的客户端会收到消息之外，所有订阅了匹配频道的模式的客户端，也同样会收到消息。
- 退订频道和退订模式分别是订阅频道和订阅模式的反操作。
