Question

I have

1. 1-many units in the database
2. Each unit has it’s own update time in seconds
3. Webpage that has dynamic contents
4. Currently there is auto-refresh (2minutes) embedded see below every time it’s called

function refreshPage(){

minutes = 2;
seconds = 0;  
interval=120000;
getCheckedUnits(); //call to function that checks and updates stuff

}

function timedRefresh(timeoutPeriod) {
    //refreshPage();
    setInterval(refreshPage, timeoutPeriod);
    countdown('countdown');
}

var interval;
var minutes = 2;
var seconds = 0;

function countdown(element) {
    interval = setInterval(function() {
        var el = document.getElementById(element);
        if(seconds == 0) {
            if(minutes == 0) {
                el.innerHTML = "Waiting for refresh";
                clearInterval(interval);
                return;
            } else {
                minutes--;
                seconds = 60;
            }
        }
        if(minutes > 0) {
            var minute_text = minutes + (minutes > 1 ? ' minutes' : ' minute');
        } else {
            var minute_text = '';
        }
        var second_text = seconds > 1 ? 'seconds' : 'second';
        el.innerHTML = 'Page will auto-refresh in: ' + minute_text + ' ' + seconds + ' ' + second_text + ' remaining';
        seconds--;
    }, 1000);
}

What I need to do, it to make it transparent to user and call update based on the need (I have value that is set in the database).

As a solution (in JS-Ajax), I can use the same timer that I have above (set it to 5 seconds) and create an array with objects that store id, last update time and next update time. Then I can update each object only when current time is more that next update time.
Or I can approach this method via cron-job.

I have never done this before and I would like to hear opinions that based on experience. What would be the best option?

Answer 1

Your scheme of fetching last update times and comparing them before fetching the much larger data is a perfectly valid approach to this problem, but there are others you can consider. In particular, you are trying to achieve is called server push. Using the last-update timestamps can approximate this, but alternative techniques like long polling and (in the future/some browsers) WebSockest are some alternatives.

Here is a better writeup to a similar question.