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 during a holiday.

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

Whenever the lambda is triggered it will potentially terminate one instance per Auto Scaling Group in the region. By default the probability of terminating an ASG's instance is 1 in 6. This probability can be overridden by setting a chaos-lambda-termination tag on the ASG with a value between 0.0 and 1.0, where 0.0 means never terminate and 1.0 means always terminate.

Run make zip to create a chaos-lambda.zip file containing the lambda function. Upload it to a S3 bucket in your account, taking note of the bucket name (eg my-bucket) and the path (eg lambdas/chaos-lambda.zip).

Create the lambda function via CloudFormation using the cloudformation/templates/lambda.json template, entering the bucket name and path.

In the AWS console go to the Lambda service, select the newly created lambda function, go into the Event sources tab, and click Add event source. Choose Scheduled Event for the Event source type, then fill in the fields as follows (enter the Name first as changing it resets the other fields):

• Name: business-hours
• Description: Assume things fail
• Schedule expression: cron(0 10-16 ? * MON-FRI *)

Ensure Enable now is selected and click the Submit button. You can disable or re-enable the hourly trigger at any time by clicking the Enabled/Disabled link in the State column.

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.

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>

Example:

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>

Example:

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>

Example:

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>

Example:

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

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