Periodic refresh or polling

Answer 1

This is quite unreliable and probably far too taxing on the server in most cases.

Perhaps you should have a look into a push interface: http://en.wikipedia.org/wiki/Push%5Ftechnology

I've heard Comet is the most scalable solution.

Answer 2

I suspect Facebook uses a Flash movie (they always download one called SoundPlayerHater.swf) which they use to do some comms with their servers. This does not get caught by Firebug (might be by Fiddler though).

Answer 3

You can check for new data every 10 seconds, but instead of checking the db, you need to do a lower impact check.

What I would do is modify the db update process so that when it makes a change to some data, it also updates the timestamp on a file to show that there is a recent change.

If you want better granularity than "something changed somewhere in the db" you can break it down by username (or some other identifier). The file(s) to be updated would then be the username for each user who might be interested in the update.

So, when you script asks the server if there is any information for user X newer than time t, instead of making a DB querry, the server side script can just compare the timestamp of a file with the time parameter and see if there is anything new in the database.

In the process that is updating the DB, add code that (roughly) does:

   foreach username interested in this update
   {
     touch the file \updates\username
   }

Then your function to see if there is new data looks something like:

   function NewDataForUser (string username, time t)
   {
     timestamp ts = GetLastUpdateTime("\updates\username");
     return (ts > t);
   }

Once you find that there is new data, you can then do a full blown DB query and get whatever information you need.

Answer 4

I left facebook open with firebug running and I'm seeing requests about once a minute, which seems like plenty to me.

The other approach, used by Comet, is to make a request and leave it open, with the server dribbling out data to the client without completing the response. This is a hack, and violates every principle of what HTTP is all about :). But it does work.

Answer 5

This is not a better approach. Because you ended up querying your server in every 10 seconds even there is no real updates.

Instead of this polling approach, you can simulate the server push (reverrse AJAX or COMET) approach. This will compeletly reduce the server workload and only the client is updated if there is an update in server side.

As per wikipedia

Reverse Ajax refers to an Ajax design pattern that uses long-lived HTTP connections to enable low-latency communication between a web server and a browser. Basically it is a way of sending data from client to server and a mechanism for pushing server data back to the browser.

For more info, check out my other response to the similar question

Cheers

Ramesh Vel