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Termination Policy & Life Cycle Hooks

Links: 101 AWS SAA Index

Termination Policy

  • Termination policy decides which instance will be deleted in case of a scale in.

  • Default termination policy steps:

    • Find the AZ with the most number of instances.
    • Find instances that are not protected from scale in.
    • If there are multiple instances delete the one with the oldest launch template or configuration.
    • If still there are multiple instances delete the one closest to the next billing hour.
    • If still there are multiple instances then delete one at random.

Life cycle hooks

  • We have one hook at the time of launching and other at the time of termination.

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  • One use case of launch hook would be to do some installations and checks before the machine is provisioned.

    • Here installations and checks are the important terms.
  • One use case of termination hook would be storing the logs or uploading it somewhere.
  • By default as soon as an instance is launched in ASG it is InService.
  • Lifecycle hooks are integrated with EventBridge, SNS and SQS.
The lifecycle hook puts the instance into a wait state (Pending: Wait or Terminating: Wait). The instance is paused until you continue or the timeout period ends.

Example use case: Use the Auto Scaling group lifecycle hook to put the instance in a wait state and launch a custom script that installs the proprietary forensic tools and performs a pre-activation status check

A company has an ASG where random EC2 instances suddenly crashed in the past month. They can't troubleshoot why the EC2 instances crash as the ASG terminates the unhealthy EC2 instances and replaces them with new EC2 instances. What will you do to troubleshoot the issue and prevent unhealthy EC2 instances from being terminated by the ASG?

Use ASG Lifecycle Hooks to pause the EC2 instance in the Terminating state for troubleshooting.

Last updated: 2023-03-03