1. In a computing environment, a system for testing a service comprising: one or more processors; a simulator, implemented on the one or more processors, configured to perform operations of multiple simulated users against the service, including by representing each simulated user as a state machine with context, the simulator configured to randomly select operations within a same state and operations within different states for each simulated user, and to log data indicative of the performance of the service with respect to handling at least some of the operations, wherein at least to two different operations within a state have different weights that bias how often each operation is randomly selected.