Concurrent/distributed system bugs can be really finicky because they may depend on subtle timing conditions to manifest. So you might see a bug once, then try to re-run the test using the "same" inputs, and the bug doesn't appear a second time. This might be because e.g. threads aren't scheduled the same way as before, so some 1-microsecond-wide window of vulnerability for a race condition was missed. If you can't reliably reproduce the bug, it's much harder to study and fix.
Determinism lets you perfectly reproduce the bug as many times as you want. Perfectly as in, exactly the same thread+process scheduling, exact same memory and disk access times, exact same network packet transit times and orderings .. exact same everything. Then once you have returned to the bug, you can rewind time, to do things like explore counterfactual scenarios by varying the random seed from that moment on.
We do have randomness of course, otherwise it wouldn't be a very good fuzzer. But we save all the seeds, so it's a controlled, reproducible randomness.
Concurrent/distributed system bugs can be really finicky because they may depend on subtle timing conditions to manifest. So you might see a bug once, then try to re-run the test using the "same" inputs, and the bug doesn't appear a second time. This might be because e.g. threads aren't scheduled the same way as before, so some 1-microsecond-wide window of vulnerability for a race condition was missed. If you can't reliably reproduce the bug, it's much harder to study and fix.
Determinism lets you perfectly reproduce the bug as many times as you want. Perfectly as in, exactly the same thread+process scheduling, exact same memory and disk access times, exact same network packet transit times and orderings .. exact same everything. Then once you have returned to the bug, you can rewind time, to do things like explore counterfactual scenarios by varying the random seed from that moment on.
We do have randomness of course, otherwise it wouldn't be a very good fuzzer. But we save all the seeds, so it's a controlled, reproducible randomness.