fix(ttl-reaper): add alarm for the reaper not being invoked at all - #504
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) The four alarms from #469 all correctly use TreatMissingData: notBreaching -- no sentinel datapoint genuinely is good news. But that means a reaper that never runs (disabled EventBridge rule, deleted schedule, reserved concurrency 0, or the function deleted) produces zero breaching datapoints across all four, so nothing pages while nothing is being enforced. The existing invocation-errors alarm doesn't cover it either: AWS/Lambda Errors requires an invocation to produce a datapoint at all. New NotInvokedAlarm on AWS/Lambda Invocations, 30-minute window (~3 missed ticks at rate(10 minutes)), the one alarm in this stack that intentionally uses TreatMissingData: breaching, since absence of data is exactly the failure being detected here. Fixes #475
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Summary
The four alarms added by #469 (
…-reached-no-accounts,…-account-unreachable,…-fsx-unreachable,…-invocation-errors) all correctly useTreatMissingData: notBreaching— no sentinel datapoint genuinely is good news. But that means a reaper that never runs at all — the EventBridge rule disabled, its schedule deleted or target detached, reserved concurrency set to 0, or the function deleted outright — produces zero breaching datapoints across all four, so they all sit contentedly inOKwhile nothing is being enforced.…-invocation-errorsdoesn't cover it either:AWS/LambdaErrorsrequires an invocation to produce a datapoint at all — no invocation, no error, no alarm.This is #469's own thesis one level up, and #65's two levels up:
sporedcan die silently → the reaper could fail silently (#469) → the reaper can be absent silently (this).Fix: new
NotInvokedAlarmonAWS/LambdaInvocations,Sum,Period: 1800(~3 missed ticks at the defaultrate(10 minutes)),ComparisonOperator: LessThanThreshold,Threshold: 1, and — the one alarm in this template where it's correct —TreatMissingData: breaching, since absence of data is exactly the failure this alarm exists to detect. Documented in the README's sentinel/alarm table alongside the other four, with a comment explaining why this alarm'sTreatMissingDatadiffers from its neighbors (per the issue's own request, so nobody "fixes" the inconsistency later).Fixes #475.
Test plan
sam validate --template-file template.yaml --lint— cleango build ./...,go vet ./...,go test ./...inlambda/ttl-reaper— cleanNotInvokedAlarmresource is present via a Python YAML-with-CFN-tags parseCHANGELOG.mdupdated under## [Unreleased]Acceptance from the issue (deploy-time verification, not done in this PR):
OK