Calculating how many visitors are on the same webpage using rolling average

In this example we show how to count how many visitors are on same webpage URL.

There are many methods that you can use to track the users on same URL. The easiest method is to make assumption that if someone hit an URL, he staying there for 5 minutes. The assumption does not sounds good :), however this is how most of popular counters do it.

Evidences suggests that Google doing it same way.

One of the ways to implement this in Redis is with sorted sets (zset). Another, more clear way is to use normal sets and to "roll" them. This technique is sometimes called rolling average or moving average.

$r = new Redis();
$r->connect("127.0.0.1", "6379");

$expire = 5; // min, good values are 3 to 10.

$ip     = $_SERVER["REMOTE_ADDR"]; // you can put session or user id here
$page   = md5($_SERVER["REQUEST_URI"]);

$ts     = time();

$count = 0;

for($i = 0; $i < $expire; $i++){
	$time = date("Hi", $ts + $i * 60);
	$r->sadd(  "page:$time:$page", $ip);
	$r->expire("page:$time:$page", $expire * 60 + 1); 
	// we want set to be live at least $expire minutes.
	// we can optimize it with TTL = $i * 60 + 60
	
	if ($i == 0)
		$count = $r->scard("page:$time:$page");
}

echo $count . " visitors looking at this page right now!";

In case you put user names in the redis set, you can easily do smembers() or srandmember() to show some usernames as this is done in some forum softwares.


Redis connection note

The example uses generic connection to the Redis server. To make the example work with our service you will need to use code similar to this one:

// change these parameter according to the information in your instance list
$host = '50.30.35.9';
$port = 1234;
$password = 'somehashcode';
$db = 0;

$r = new Redis();
$r->connect($host, $port);
$r->auth($password);
$r->select($db);

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Code library

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Because we want this page to be useful for memcached users, we tagged with memcached all examples that may be "recreated" for memcached server.


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