It has come to my needs to gather more information about load balancing process and stuff related. So far it's clear that users are distributed to multiple servers depending un their current load and they all run the same app. My question is - how these multiple servers share the same app? Like - the data must be synchronized all the time. The app files can be uploaded manually when needed, but what about the database? Also what I should keep in mind when writing PHP code for a load balanced app?