AJAX Applications vs Server Load?

Squink asks: "I've got the fun job of having to recode a medium sized (500-1000 users) community site from the ground up. For this project, gratuitous use of XMLHttpRequest appears to be in order. However - with the all of the hyperbole surrounding AJAX, I've not been able to find any useful information regarding server load [Apache + MySQL] when using some of the more useful AJAX applications, such as autocomplete. Is this really a non-issue, or are people neglecting to discuss this for fear of popping the Web2.0 bubble?"

Comments

Cache, cache and CACHE

[Anonymous Coward]

After doing quite a bit of AJAX type work for my employer, that's the best advice I can give you. The most common things will be queried the most often, so caching is the key. If you're using PHP and MySQL, use something like eAccelerator for PHP (less important) and MySQL's query cache (most important!) properly tuned. And remember, not everything AJAX has to query a database.

Decide what you really need

[b4k3d b34nz]

It's going to be tempting to use a lot of AJAX, especially if sounds fun. In reality though, you should be considering user experience, since this is a community site. Don't use an AJAX call where someone might expect a page refresh.

With that said, it's best to try to cache frequently accessed items in memory (regardless of whether you're doing AJAX calls). ASP.NET does a good job of this--I don't know what you're programming in, but definitely find out how to cache so that you don't have to read the database all the time. This reduced our database server load from 55% to 45% upon implementation (it's separate from the web server).

To specifically answer your question, the thing that's fast about AJAX is mostly perceived. Yes, you'll reduce calls, but at the sacrifice of having to code things twice: once for users with JS, once for those without. Use it in places where it's senseless to reload an entire page. For example, opening a nested menu. Searches that aren't done by keyword are good as well. Like has been said above, delay a server request until the user is done typing so that you can reduce calls. Remember, it's still a hit on your server, it just doesn't have to get all the rest of the crap on the page.

To reduce bandwidth, use JSON instead of XML, and only pass the headers that you need to into the AJAX call. To reduce server strain, cache frequently accessed database calls/results. Also, other non-AJAX javascript can help reduce calls, such as switching between "tabs" with some display:none action instead of reloading a page.

The answer is not gratuitous AJAX, the answer is thinking through how people will most commonly use your site, and making those parts easiest (so users don't have to redo things, therefore wasting your server capacity/bandwidth). Take things that shouldn't have to refresh the page and make them work using javascript, AJAX or not. Depending on how crappily things are coded now, you should see between a 15 and 35% reduction in server load and database calls.