HEALTH_CHECK_INTERVAL_MS was 1 h, but ROTATION_GRACE_PERIOD_MS is 15 min.
Phase skew between two peers' health-check ticks could stretch rotation
finalization desync up to ~1 h, during which signatures from the already-
finalized side verify against the other side's primary-only secret (grace
has expired; verifyPeerSignature stops trying the pending secret). With
AUTH_FAILURE_THRESHOLD = 5 and the existing backoff schedule, this
occasionally tripped legitimate rotations into needs_attention.
Setting the interval to 15 min (= ROTATION_GRACE_PERIOD_MS) guarantees a
finalization tick fires within one grace window on each side, so the
cross-verification window where one peer signs with NEW while the other
still treats NEW as pending cannot outlast the grace period.
Per-tick cost is negligible for the worker's steady state: the only
network fetches are per-active-peer /peer/rotate calls when the 90-day
rotation interval hits (rare) and per-unreachable-peer /instance/info
health pings (bounded by outage count). Going lower than 15 min would
reduce the residual desync but increase tick overhead with diminishing
returns; 15 min is the grace-period-aligned value that the original spec
("runs hourly") deviated from without justification.
Follow-up #20 from S2S DM unification backlog; reduces #19 false-positive
rate (outbox auth-failure transition) on legitimate rotations.
Jannis Braun
0d74d1d112