# Dead man’s switch with AWS CloudWatch: Freshness-Alerting for Backups and Co

- **期号**: SRE Weekly Issue #82（2017-07-23）
- **作者**: —
- **链接**: https://ruempler.eu/2017/06/26/dead-mans-switch-backup-alerting-aws-cloudwatch/

## 简介

Here’s a nice little how-to:

> A recent challenge for one of the teams I am currently involved was to find a way in AWS CloudWatch:To alert if the metric breaches a specified threshold.To alert if a particular metric has not been sent to CloudWatch within a specified interval.

## 正文

A recent challenge for one of the teams I am currently involved was to find a way in AWS CloudWatch:

1. To alert if the metric breaches a specified threshold.
2. To alert if a particular metric has **not** been sent to CloudWatch within a specified interval.

While the first one is pretty much standard CloudWatch functionality, the latter is a bit more tricky. In the Nagios/Icinga world it’s called “freshness”. You could also call it special case of a “[Dead man’s switch](https://en.wikipedia.org/wiki/Dead_man%27s_switch)“ for periodic tasks / cronjobs.

So for example in our case we wanted to have monitored and alerted whether a backup job runs once per day.

So here is what we did (CloudFormation snippet below):

- Set the check period to the interval during which the metric is supposed to be sent. E.g. `86400` if the metric should is supposed to be sent every day. This instructs CloudWatch to check once per day.
- Set evaluation periods to `1` : We want to get alerted immediately when there is no data written or the threshold has been breached.
- And now the important one: We have to treat missing data as `breaching` , so that,if there has been no entry within the evaluation period then the alarm gets triggered.

Example in CloudFormation syntax:

| 1 2 3 4 5 6 7 | HealthCheckAlarm: Type: AWS::CloudWatch::Alarm Properties: Period: 86400 EvaluationPeriods: 1 TreatMissingData: breaching ... | 

## 

      
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