I am about to begin developing a logging system for future implementation in a current PHP application to get load and usage statistics from a MYSQL database.
The statistic will later on be used to get info about database calls per second, query times etc.
Of course, this will only be used when the app is in testing stage, since It will most certainly cause a bit of additional load itself.
However, my biggest questionmark right now is if i should use MYSQL to log the queries, or go for a file-based system. I'll guess that it would be a bit of a headache to create something that would allow writings from multiple locations when using a file based system to handle the logs?
How would you do it?
MySQL already had logging built in- Chapter 5.2 of the manual describes these. You'll probably be interested in The General Query Log (all queries), the Binary Query Log (queries that change data) and the Slow log (queries that take too long, or don't use indexes).
If you insist on using your own solution, you will want to write a database middle layer that all your DB calls go through, which can handle the timing aspects. As to where you write them, if you're in devel, it doesn't matter too much, but the idea of using a second db isn't bad. You don't need to use an entirely separate DB, just as far as using a different instance of MySQL (on a different machine, or just a different instance using a different port). I'd go for using a second MySQL instance instead of the filesystem- you'll get all your good SQL functions like SUM and AVG to parse your data.