[jcranmer]

Time travel debugging generally works on record-and-replay, with there being a lot of research into figuring out what you need to record to get deterministic replay. Recording the results of system calls is a necessary step [1] to get deterministic replay, and that lets you replay even things that rely on modifying external state like database connections.

[1] Necessary, but not sufficient. Multithreaded applications require a lot more care, and rr relies on very accurate hardware counters to get multithreaded executions working correctly (and not all hardware supports these hardware counters!).