Show HN: Retrace – reverse debugging for production CPython applications

14 points

25 days ago

Nathan here, one of the people who built Retrace. Happy to answer technical questions. Retrace records a CPython application's interactions with the nondeterministic outside world i.e. network, DB, filesystem, time, randomness, subprocesses, and lets you replay that execution locally and deterministically. The goal is to take a production failure and open the same execution in VS Code, with the ability to step forwards and backwards through the replay.

The core idea is to record boundary crossings rather than tracing every Python line in production. External calls are recorded as calls/results/errors, and replay stubs return the recorded results so the original application code runs again deterministically.

The preview today covers Python 3.11/3.12 on macOS and Linux, with Flask, Django, requests, psycopg2, and threading/forking covered. There is a compatibility table in the README.

This is a preview, not a finished product. Things we know are missing: async support is partial, FastAPI is not in the table yet, Windows is not supported, and free-threaded 3.13 is detected and refused.

Happy to go deep on: - how we get determinism on real Python stacks (threads, async, third-party libraries, C extensions) - recording overhead, what it depends on, and what we have actually benchmarked vs claimed - what works and what does not yet - how this differs from rr, Replay.io, pdb time-travel forks, and APM tools

Blog post (longer write-up): https://retracesoftware.com/blog/introducing-retrace/

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