What are Auto Scaling Hooks?

Lifecycle hooks in AWS Auto Scaling Groups (ASG) allow you to pause instance transitions (launching or terminating) and perform custom actions before they complete.

✅ Use Case Example:

• Run custom scripts before an EC2 instance becomes available.

• Notify a monitoring system before terminating an instance.

• Attach additional storage or perform security checks before starting an instance.

🔹 Types of Lifecycle Hooks

AWS Auto Scaling supports two lifecycle hooks:

🔹 How Lifecycle Hooks Work

1️⃣ An instance enters the pending state (when launching) or terminating state (when shutting down).
2️⃣ The lifecycle hook pauses the process and waits for a response.
3️⃣ You perform custom actions (run scripts, trigger Lambda, notify SNS/SQS).
4️⃣ The instance remains in the paused state until the hook times out or you signal it to continue.

🔹 How to Create a Lifecycle Hook

🛠 Create Hook Using AWS CLI

aws autoscaling put-lifecycle-hook \
  --lifecycle-hook-name "MyLaunchHook" \
  --auto-scaling-group-name "MyAutoScalingGroup" \
  --lifecycle-transition "autoscaling:EC2_INSTANCE_LAUNCHING" \
  --heartbeat-timeout 300 \
  --default-result CONTINUE

🔹 heartbeat-timeout 300 → The hook waits 300 seconds (5 minutes) before proceeding.
🔹 default-result CONTINUE → If no response, the instance continues automatically.

🔹 Responding to Lifecycle Hooks

You must tell AWS when your custom action is complete.

✅ Notify Hook Completion

aws autoscaling complete-lifecycle-action \
  --lifecycle-hook-name "MyLaunchHook" \
  --auto-scaling-group-name "MyAutoScalingGroup" \
  --lifecycle-action-result CONTINUE \
  --instance-id i-0123456789abcdef

• Use ABANDON instead of CONTINUE if the instance should not proceed.

🔹 Automate Hook Actions (Lambda Example)

AWS Lifecycle Hooks work well with AWS Lambda, SNS, or SQS.
Example: Trigger Lambda when an instance launches.

1️⃣ Create an SNS Topic for Notifications

aws sns create-topic --name MyScalingTopic
aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyScalingTopic \
  --protocol lambda --notification-endpoint arn:aws:lambda:us-east-1:123456789012:function:MyLambdaFunction

2️⃣ Add SNS Notification to the Lifecycle Hook

aws autoscaling put-lifecycle-hook \
  --lifecycle-hook-name "MyLaunchHook" \
  --auto-scaling-group-name "MyAutoScalingGroup" \
  --lifecycle-transition "autoscaling:EC2_INSTANCE_LAUNCHING" \
  --notification-target-arn "arn:aws:sns:us-east-1:123456789012:MyScalingTopic" \
  --role-arn "arn:aws:iam::123456789012:role/MyAutoScalingRole"

Now, when an instance launches, it pauses, triggers SNS, which invokes a Lambda function to install software or configure the instance before continuing.

🔹 Summary

✔ Lifecycle Hooks pause Auto Scaling events to allow custom tasks.
✔ Two types: Launch (before instance starts) & Terminate (before shutdown).
✔ Hooks wait for a response (from CLI, Lambda, SNS, etc.).
✔ Use hooks for security checks, log backups, app installations, etc.