Randomly terminate ASG instances during business hours
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EC2 instances are volatile and can be recycled at any time without warning. Amazon recommends running them under Auto Scaling Groups to ensure overall service availability, but it's easy to forget that instances can suddenly fail until it happens in the early hours of the morning when everyone is on holiday.

Chaos Lambda increases the rate at which these failures occur during business hours, helping teams to build services that handle them gracefully.

Quick setup

Create the lambda function in the region you want it to target using the cloudformation/templates/lambda_standalone.json CloudFormation template. There are two parameters you may want to change:

  • Schedule: change if the default run times don't suit you (once per hour between 10am UTC and 4pm UTC, Monday to Friday); see http://docs.aws.amazon.com/AmazonCloudWatch/latest/events/ScheduledEvents.html for documentation on the syntax.
  • DefaultProbability: by default all Auto Scaling Groups in the region are targets; set this to 0.0 and only ASGs with a chaos-lambda-termination tag (see below) will be affected.


To receive notifications if the lambda function fails for any reason, create another stack using the cloudformation/templates/alarms.json template. This takes the lambda function name (something similar to chaos-lambda-ChaosLambdaFunction-EM2XNWWNZTPW) and the email address to send the alerts to.

Probability of termination

Every time the lambda triggers it examines all the Auto Scaling Groups in the region and potentially terminates one instance in each. The probability of termination can be changed at the ASG level with a tag, and at a global level with the DefaultProbability stack parameter.

At the ASG level the probability can be controlled by adding a chaos-lambda-termination tag with a value between 0.0 (never terminate) and 1.0 (always terminate). Typically this would be used to opt out a legacy system (0.0).

The DefaultProbability parameter sets the probability of termination for any ASG without a valid chaos-lambda-termination tag. If set to 0.0 the system becomes "opt-in", where any ASG without this tag is ignored. The default is 0.166 (or 1 in 6).


The lambda is triggered by a CloudWatch Events rule, the name of which can be found from the ChaosLambdaFunctionOutput output of the lambda stack. Locate this rule in the AWS console under the Rules section of the CloudWatch service, and you can disable or enable it via the Actions button.


By default the lambda will target ASGs running in the same region. It's generally a good idea to avoid cross-region actions, but if necessary an alternative list of one or more region names can be specified in the Regions stack parameter.

The value is a comma separated list of region names with optional whitespace, so the following are all valid and equivalent:

  • ap-south-1,eu-west-1,us-east-1
  • ap-south-1, eu-west-1, us-east-1
  • ap-south-1 , eu-west-1 , us-east-1

Log messages

Chaos Lambda log lines always start with a timestamp and a word specifying the event type. The timestamp is of the form YYYY-MM-DDThh:mm:ssZ, eg 2015-12-11T14:00:37Z, and the timezone will always be Z. The different event types are described below.


<timestamp> bad-probability [<value>] in <asg name>


2015-12-11T14:07:21Z bad-probability [not often] in test-app-ASG-7LJI5SY4VX6T

If the value of the chaos-lambda-termination tag isn't a number between 0.0 and 1.0 inclusive then it will be logged in one of these lines. The square brackets around the value allow CloudWatch Logs to find the full value even if it contains spaces.


<timestamp> result <instance id> is <state>


2015-12-11T14:00:40Z result i-fe705d77 is shutting-down

After asking EC2 to terminate each of the targeted instances the new state of each is logged with a result line. The <state> value is taken from the code property of the InstanceState AWS type described at http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_InstanceState.html


<timestamp> targeting <instance id> in <asg name>


2015-12-11T14:00:38Z targeting i-168f9eaf in test-app-ASG-1LOMEKEVBXXXS

The targeting lines list all of the instances that are about to be terminated, before the TerminateInstances call occurs.


<timestamp> triggered <region>


2015-12-11T14:00:37Z triggered eu-west-1

Generated when the lambda is triggered, indicating the region that will be affected.