Question

Let’s say your writing a PHP application that will be hosted in a load-balanced/multi-server setup. What are the things you need to know in order to ensure smooth operation? Right now the only thing I think will be an issue is PHP sessions (i.e., you must use a custom database handler for it). Anything else?

Answer 1

Let’s turn this into an answer:

In my experience, the overwhelming majority of PHP applications is not or not only constrained by PHP horsepower on the webserver, but at least as much by backing store, i.e. Database and/or files.

So load balancing a PHP application without carefull analysis bears the potential to make things worse: Hit the weakest link in the chain with more and more load.

So the first – and IMHO most important “thing to know when writing a web app hosted in a load-balanced server” is the load pattern, and its potential for balancing. If your app performs bad, you load-balance it on more servers, then find out you now have more servers waiting for the DB, you are in trouble.

Here is an out-of-the blue checklist, please reagrd it as a brainstorm (or a brainfart) only:

• First: Are you really CPU-bound?
• Which pages are hit most (see your log)
• For the top N of these (with a suitable N) check the processing pattern: Where do the CPU cycles go?
• What would be the side effects of making sessions, uploads, file storage (add whatever you use) shared and would it be offset by the load balancing?

Comments welcome, I am very sure to have not even scratched the surface!

Edit

Just thought of something that bit me once in this context: Resource locking. Brace yourself for a higher degree of concurrency, if you go multi-server