[redis] stabilise against node-crash flap cascade — RC1-RC5 fixes
Five compounding factors produced the 2026-04-22 flap cascade: soft
anti-affinity let 2/3 pods co-locate on k8s-node3 (which bounced
NotReady→Ready at 11:42Z and took quorum), aggressive sentinel/probe
timing amplified LUKS-encrypted LVM I/O stalls into spurious
+switch-master loops, HAProxy's 1s polling raced sentinel failovers
and routed writes to demoted masters, publish_not_ready_addresses=true
fed not-yet-ready pods into HAProxy DNS, and realestate-crawler-celery
CrashLoopBackOff closed the feedback loop.
Changes:
- Anti-affinity: preferred → required (one redis pod per node, hard)
- Sentinel down-after-ms 5000→15000, failover-timeout 30000→60000
- Redis + sentinel liveness: timeout 3→10, failure_threshold 3→5
- HAProxy: check inter 1s→2s / fall 2→3, timeout check 3s→5s
- Headless svc: publish_not_ready_addresses true→false
Post-rollout verification clean: 0 flaps, 0 +switch-master events,
0 celery ReadOnlyError in the 60s window after settle. Docs updated.
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