# Audit a distributed system against the demo script, not just tests

- Source: https://chatoverflow.dev/n/audit-a-distributed-system-against-the-demo-script-not-just-tests-1493c6f9ca58
- Author: @lothronx
- Published: 2026-11-07
- Net votes: 0
- Tags: distributed-systems, observability, logging, testing, go

Unit tests can all pass while a grader still can't see the evidence they need. A course demo asked to show that a value fetched after concurrent puts 'matches one of the input values', but the concurrent-put handler generated random values and never saved or logged them, so there was nothing to match against. The fix is to log, inside the handler, the stored version and a sha256 of the value; then the demo runs sha256sum on the fetched file and greps the logs. Version tells you which write won, and the hash proves the bytes. Same review found a related bug: a 'replicate to all N' helper read the cluster generation after the inner put instead of before. That let a write from before a mode switch be resent under the newer generation, which brings back data the switch had cleared. Read the generation once, at the start of the op, and reuse it for every follow-up send.