How to correctly calculate the user's online time?

Question

ruby

Tell me how, in your opinion, to correctly calculate the total time that the user was online?
For online, you can take a certain action that the user performs every 30 seconds.
It doesn't matter in what programming language - logic is interesting.

Answer 1

I think it would be more correct to store the beginning of authorization on the server side and record the "end" with each user action. On exit, consider the session ended. If there are no actions for more than n seconds, consider the last activity as "end".
It is wrong to consider any automatic requests as user activity IMHO.
However, everything very much depends on the specific case - both timeouts and storage method (still, current online and / or general). And even accounting for automatic requests.

Answer 2

Something like this comes to mind.

$mc = new Memcache;
    $mc->connect('127.0.0.1', 11211);

    $period = 30;   // longpoll reconnect
    $delay = 1;     // reconnect delay
    $uid = 1;

    if(!$last_active = $mc->get("last_active_{$uid}")){
        $mc->set("last_active_{$uid}", time());
        die();
    }

    $now = time();
    $online_time = $now - $last_active;
    $counter = $mc->get("online_counter_{$uid}") ?: 0;

    if($online_time < $period+$delay){
        $counter += $online_time;
    } else {
        $counter += $period;
    }
    $mc->set("online_counter_{$uid}", $counter);
    $mc->set("last_active_{$uid}", time());

    echo "User ({$uid}) online time: {$counter}";

Answer 3

Every minute you send a request to the server via AJAX / websockets from the client, and you keep track of it.