Why is HTTP-Polling so laggy?
What I have is a button, and whenever a user clicks it a MySQL database field gets updated and the value is displayed to the user. I'm polling every 800 milliseconds and it's very laggy/glitchy. Sometimes when clicking the button it doesn't register it. And I actually need to be polling quite a bit more frequent than every 800 milliseconds.
This is also with just 1 user on the website at a time... When in the end there is going to be many at once.
When you need real-time information you should avoid polling(frequently). Below I would try to explain why this is wrong. You could compare it to a child in the back of your car screaming every second "are we there yet" while you are replying "we are not there yet" all the time.
Instead you would like to have something like long-polling/HTTP-streaming or websockets. You could compare this to a child in the back of your car telling you to let him know when "we are there" instead of asking us every second. You could imagine this is way more efficient then the previous example.
To be honest I don't think PHP is the right tool for this kind of applications(yet). Some options you have available are:
hosted solutions:
Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
Our free Sandbox plan includes up to 20 connections and 100,000 messages per day. Simply upgrade to a paid plan when you're ready.
Beaconpush is a push service for creating real-time web apps using HTML5 WebSockets and Comet.
host yourself:
Socket.IO aims to make realtime apps possible in every browser and mobile device, blurring the differences between the different transport mechanisms
When becoming very big the "host yourself" solution is going to less expensive, but on the other hand using something like pusherapp will get you started easier(friendly API) and also is not that expensive. For example pusherapp's "Bootstrap" can have 100 concurrent connections and 200,000 messages per day for $19 per month(but when small beaconpush is cheaper => do the math :)). As a side-note this plan does not include SSL so can not be used for sensitive data. I guess having a dedicated machine(VPS) will cost you about the same amount of money(for a simple website) and you will also have to manage the streaming solution yourself, but when getting bigger this is probably way more attractive.
whenever a user clicks it a MySQL database field gets updated and the value is displayed to the user
When comparing disc I/O(MySQL in standard mode) to memory it is extremely slow. You should be using an in-memory database like for example redis(also has persistent snapshots) or memcached(completely in memory) to speed up the process. I myself really like redis for it's insane speed, simplicity and persistent snapshots. http://redistogo.com/ offers a free plan with 5MB of memory which will probably cover your needs. If not the mini plan of $5 a month will probably cover you, but when getting even bigger a VPS will be cheaper and in my opinion the prefered solution.
The best solution(especially if you are getting big) is to host socket.io/redis yourself using a VPS(cost money). If really small I would use redistogo, if not I would host it myself. I would also start using something like beaconpush/pusherapp because of it's simplicity(getting started immediately). Hosting socket.io(advice to play with it on your own machine for when getting big) is pretty simple, but in my opinion more difficult than beaconpush/pusherapp.
Laggy/glitchy? Sounds like a client-side problem. As does the button thing. I'd get your JavaScript in order first.
As for polling, 0.8 sounds a bit time-critical. I don't know about most countries, but here in the third world simple network packets may get delayed for as long a few seconds. (Not to mention connection drops, packet losses and the speed of light.) Is your application ready to cope with all that?
As for an alternate approach, I agree with @Vern in that an interrupt-driven one would be much better. In HTTP terms, it translates to a long-standing HTTP request that does not receive a response until the server has some actual data to send, minimizing delay and bandwidth. (AFAIK) it's an older technique than AJAX, though has been named more recently. Search for "COMET" and you'll end up with both client- and server-side libraries.
there are many things that might cause the lag that you are experiencing. Your server might be able to process the requests fast enough, but if the connection between your client and the server is slow, then you'll see the obvious lag.
The first thing you should try is to ping the server and see what response time you're getting.
Secondly, rather than poll, you might want to consider an interrupt driven approach. This means that only when your server replies, will you send out your next request. This makes sense, so that many clients won't be flooding the server with requests till the point the server cannot cope. This is especially true, then the RTT (Round-Trip-Time) of your request is pretty long.
Hope it helps. Cheers!
A good place to start would be to use a tool like Firebug in Mozilla Firefox that will allow you to watch the requests being sent to the server and look for bottlenecks.
Firebug will break down each part of the request, so you can see if you are having trouble talking to the server or if it is simply taking a long time to come up with a response.
Along with @Vern's answer I would also say that if at all possible I would have the server cache the data ahead of time and then all of the clients will pull from that same cache and not need separate MySQL calls to reach the same data for every update. Then you just have your PHP update the cache whenever the actual DB data changes.
By cache I mean having php write to a file on the sever side, and then clients will simply look at the contents of that one file to see the most updated info. There might be better ways of caching, but being that I have never done this personally before, this is the first solution that popped into my mind.
