Redis消息通知系统的实现
最近忙着用Redis实现一个消息通知系统，今天大概总结了一下技术细节，其中演示代码如果没有特殊说明，使用的都是PhpRedis扩展来实现的。 内存 比如要推送一条全局消息，如果真的给所有用户都推送一遍的话，那么会占用很大的内存，实际上不管粘性有多高的产品
最近忙着用Redis实现一个消息通知系统，今天大概总结了一下技术细节，其中演示代码如果没有特殊说明，使用的都是PhpRedis扩展来实现的。
内存
比如要推送一条全局消息，如果真的给所有用户都推送一遍的话，那么会占用很大的内存，实际上不管粘性有多高的产品，活跃用户同全部用户比起来，都会小很多，所以如果只处理登录用户的话，那么至少在内存消耗上是相当划算的，至于未登录用户，可以推迟到用户下次登录时再处理，如果用户一直不登录，就一了百了了。
队列
当大量用户同时登录的时候，如果全部都即时处理，那么很容易就崩溃了，此时可以使用一个队列来保存待处理的登录用户，如此一来顶多是反应慢点，但不会崩溃。
Redis的LIST数据类型可以很自然的创建一个队列，代码如下：
connect('/tmp/redis.sock');
$redis->lPush('usr',);
while ($usr = $redis->rPop('usr')) {
var_dump($usr);
}
?>出于类似的原因，我们还需要一个队列来保存待处理的消息。当然也可以使用LIST来实现，但LIST只能按照插入的先后顺序实现类似FIFO或LIFO形式的队列，然而消息实际上是有优先级的：比如说个人消息优先级高，全局消息优先级低。此时可以使用ZSET来实现，它里面分数的概念很自然的实现了优先级。
不过ZSET没有原生的POP操作，所以我们需要模拟实现，代码如下：
zsetPop($zset, self::POSITION_FIRST);
}
public function zRevPop($zset)
{
return $this->zsetPop($zset, self::POSITION_LAST);
}
private function zsetPop($zset, $position)
{
$this->watch($zset);
$element = $this->zRange($zset, $position, $position);
if (!isset($element[0])) {
return false;
}
if ($this->multi()->zRem($zset, $element[0])->exec()) {
return $element[0];
}
return $this->zsetPop($zset, $position);
}
}
?>模拟实现了POP操作后，我们就可以使用ZSET实现队列了，代码如下：
connect('/tmp/redis.sock');
$redis->zAdd('msg',,);
while ($msg = $redis->zRevPop('msg')) {
var_dump($msg);
}
?>推拉
以前微博架构中推拉选择的问题已经被大家讨论过很多次了。实际上消息通知系统和微博差不多，也存在推拉选择的问题，同样答案也是类似的，那就是应该推拉结合。具体点说：在登陆用户获取消息的时候，就是一个拉消息的过程；在把消息发送给登陆用户的时候，就是一个推消息的过程。
速度
假设要推送一百万条消息的话，那么最直白的实现就是不断的插入，代码如下：
<?php for ($msgid = 1; $msgid sAdd('usr::msg', $msgid); } ?>
说明：这里我使用了SET数据类型，当然你也可以视需求换成LIST或者ZSET。
Redis的速度是很快的，但是借助PIPELINE，会更快，代码如下：
<?php
for ($i = 1; $i multi(Redis::PIPELINE);
for ($j = 1; $j sAdd('usr::msg', $msgid);
}
$redis->exec();
}
?>说明：所谓PIPELINE，就是省略了无谓的折返跑，把命令打包给服务端统一处理。
前后两段代码在我的测试里，使用PIPELINE的速度大概是不使用PIPELINE的十倍。
查询
我们用Redis命令行来演示一下用户是如何查询消息的。
先插入三条消息，其
redis> HMSET msg:1 title title1 content content1 redis> HMSET msg:2 title title2 content content2 redis> HMSET msg:3 title title3 content content3
再把这三条消息发送给某个用户，其
redis> SADD usr:123:msg 1 redis> SADD usr:123:msg 2 redis> SADD usr:123:msg 3
此时如果简单查询用户有哪些消息的话，无疑只能查到一些
redis> SMEMBERS usr:123:msg 1) "1" 2) "2" 3) "3"
如果还需要用程序根据
redis> SORT usr:123:msg GET msg:*->title 1) "title1" 2) "title2" 3) "title3" redis> SORT usr:123:msg GET msg:*->content 1) "content1" 2) "content2" 3) "content3"
SORT的缺点是它只能GET出字符串类型的数据，如果你想要多个数据，就要多次GET：
redis> SORT usr:123:msg GET msg:*->title GET msg:*->content 1) "title1" 2) "content1" 3) "title2" 4) "content2" 5) "title3" 6) "content3"
很多情况下这显得不够灵活，好在我们可以采用其他一些方法平衡一下利弊，比如说新加一个字段，冗余保存完整消息的序列化，接着只GET这个字段就OK了。
实际暴露查询接口的时候，不会使用PHP等程序来封装，因为那会成倍降低RPS，推荐使用Webdis，它是一个Redis的Web代理，效率没得说。
…
最近Tumblr发表了一篇类似的文章：Staircar: Redis-powered notifications，介绍了他们使用Redis实现消息通知系统的一些情况，有兴趣的不妨一起看看。
原文地址：Redis消息通知系统的实现, 感谢原作者分享。
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