订阅与发布

Redis 通过 PUBLISHSUBSCRIBE 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 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 —— client2client5client1 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 SUBSCRIBEPUBLISH 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, client2client5client1 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 client2client5client1 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 clientXclientY 之外, 还会发送给订阅 tweet.shop.* 模式的 client123client256

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 client123client256 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 client123client256 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 channelpubsub_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."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 client123client256 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。