I like to run simulated test-clients that act as if e.g. 1000 clients execute tasks at the same time on a mongodb and see how the server stands. Together with profiling tools I like to be able to identify bottlenecks and fix them before they become a problem.
What tool would you to suggest to run simulated tests on an API Level? What profiling tool would you suggest to identify bottlenecks, free tools are currently preferred.