The paper is devoted to a virtual laboratory system, which in particular can be used to test knowledge through research. The participant can prefer which tools to operate and what actions should be taken. For the most of the tasks, there are copious ways to obtain the correct solution. One of the most important features of the system that distinguish this one among other simulation packages and educational systems is the pseudo-random physical parameter generation technique. The technique supports constraints and relationships between variables. As a result, it provides correctness and equal complexity of the generated task. The system can be very complex and is highly customizable by internal script system executed on server-side. The system is used as a part of distolymp Learning Management System with about 40 thousand participants per year.