redis 订阅与发布

Reference: https://redisbook.readthedocs.io/en/latest/feature/pubsub.html

Redis 的 SUBSCRIBE 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"];
}

当有新消息通过 PUBLISH 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"];
}

在后面的内容中, 我们将探讨 SUBSCRIBE 和 PUBLISH 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;
}

当客户端调用 SUBSCRIBE 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;
}

SUBSCRIBE 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 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;
}

PUBLISH 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 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 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"];
}

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"];


}

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"];



}

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;
}

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;
}

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 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 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 命令的行为, 因为 PUBLISH 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 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;
}

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 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。

posted on 2019-08-10 01:24  alex.shu  阅读(210)  评论(0编辑  收藏  举报

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