Replaying distributed systems involves playing a distributed system in a simulator using data from a real-world operation. In an example embodiment, a simulation process is to simulate a distributed system that has a first instance and a second instance. The simulation process includes a first simulation object corresponding to the first instance and a second simulation object corresponding to the second instance. In another example embodiment, communications between the first instance and the second instance are simulated within the simulation process using function calls between the first simulation object and the second simulation object.