Lab: Redis health checks, PING is not the whole story¶

Docker must be running. The script creates three redis:7-alpine containers: a healthy primary, a primary whose memory limit is set below current usage with noeviction, and a read-only replica. It then pauses and resumes the healthy primary.

Run from this directory in a checkout of the repository. The lab imports the article's functions and test doubles from the neighboring 2026-09-07-when-should-redis-fail-open directory, so keep both directories:

uv run --no-project --python 3.13 --with-requirements requirements.txt health_lab.py

Dependencies are pinned to redis-client-kit 0.3.0, redis-py 8.1.0 and testcontainers 4.15.0. The script prints the Redis server version (7.4.11 in the checked run).

It verifies that PING succeeds while writes fail on the read-only and memory-limited servers. The kit's write_key probe performs PING followed by SET with a 60-second TTL; it does not read the value back. The lab separately verifies the written value and TTL.

The same shop functions must return a database price despite a failed cache write, refuse login and payment, and make no charge while writes are rejected. Removing the memory limit and unpausing Redis must restore health with the same clients. Assertions check these outcomes; a successful run ends with PASS: Redis health scenarios.