Tactics for using PHP in a high-load site

Question

Before you answer this I have never developed anything popular enough to attain high server loads. Treat me as (sigh) an alien that has just landed on the planet, albeit one that knows PHP and a few optimisation techniques.

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I'm developing a tool in PHP that could attain quite a lot of users, if it works out right. However while I'm fully capable of developing the program I'm pretty much clueless when it comes to making something that can deal with huge traffic. So here's a few questions on it (feel free to turn this question into a resource thread as well).

Databases

At the moment I plan to use the MySQLi features in PHP5. However how should I setup the databases in relation to users and content? Do I actually need multiple databases? At the moment everything's jumbled into one database - although I've been considering spreading user data to one, actual content to another and finally core site content (template masters etc.) to another. My reasoning behind this is that sending queries to different databases will ease up the load on them as one database = 3 load sources. Also would this still be effective if they were all on the same server?

Caching

I have a template system that is used to build the pages and swap out variables. Master templates are stored in the database and each time a template is called it's cached copy (a html document) is called. At the moment I have two types of variable in these templates - a static var and a dynamic var. Static vars are usually things like page names, the name of the site - things that don't change often; dynamic vars are things that change on each page load.

My question on this:

Say I have comments on different articles. Which is a better solution: store the simple comment template and render comments (from a DB call) each time the page is loaded or store a cached copy of the comments page as a html page - each time a comment is added/edited/deleted the page is recached.

Finally

Does anyone have any tips/pointers for running a high load site on PHP. I'm pretty sure it's a workable language to use - Facebook and Yahoo! give it great precedence - but are there any experiences I should watch out for?

Answer 1

PDO is also very slow and its API is pretty complicated. No one in their sane mind should use it if portability is not a concern. And let's face it, in 99% of all webapps it is not. You just stick with MySQL or PostrgreSQL, or whatever it is you are working with.

As for the PHP question and what to take into account. I think premature optimization is the root of all evil. ;) Get your application done first, try to keep it clean when it comes to programming, do a little documentation and write unit tests. With all of the above you will have no issues refactoring code when the time comes. But first you want to be done and push it out to see how people react to it.

Answer 2

APC is an absolute must. Not only does it make for a great caching system, but the gain from the auto-cached PHP files is a godsend. As for the multiple database idea, I don't think you would get much out of having different databases on the same server. It may give you a bit of a gain in speed during query time, but I doubt the effort it would take to deploy and maintain the code for all three while making sure they are in sync would be worth it.

I also highly recommend running Xdebug to find bottlenecks in your program. It made optimization a breeze for me.

Answer 3

I don't see myself switching from MySQL anytime soon - so I guess I don't need the abstraction capabilities of PDO. Thanks for those articles DavidM, they've helped me a lot.

Answer 4

For what it's worth, caching is DIRT SIMPLE in PHP even without an extension/helper package like memcached.

All you need to do is create an output buffer using ob_start().

Create a global cache function. Call ob_start, pass the function as a callback. In the function, look for a cached version of the page. If exists, serve it and end.

If it doesn't exist, the script will continue processing. When it reaches the matching ob_end() it will call the function you specified. At that time, you just get the contents of the output buffer, drop them in a file, save the file, and end.

Add in some expiration/garbage collection.

And many people don't realize you can nest ob_start()/ob_end() calls. So if you're already using an output buffer to, say, parse in advertisements or do syntax highlighting or whatever, you can just nest another ob_start/ob_end call.

Answer 5

A lot of good answers were given already, but I would like to point you to an alternate opcode cache called XCache. It is created by a lighty contributor.

Also, if you may need load balancing your database server in future, MySQL Proxy could very well help you to achieve this.

Both of those tools should plug into an existing application quite easily, so this optimization can be done when you need it, without too much hassle.