EC2 Auto Scaling basics

Topic: Cloud aws core

Use an Auto Scaling group (ASG) to maintain a desired number of instances; scale on demand or on a schedule. Attach to a load balancer target group for traffic distribution. Use this when you need high availability or when scaling instance count based on load or schedule.

Intent: How-to

Quick answer

  • Create launch template (AMI, instance type, key, security group). Create ASG: min, max, desired capacity; attach launch template; select VPC and subnets (multiple AZs for HA). Optionally attach to target group so new instances get traffic.
  • Scaling: add scaling policy (target tracking on CPU or request count, or step scaling on CloudWatch alarm). Scale-out adds instances; scale-in removes (respect cooldown and scale-in protection). Use instance refresh to roll out new launch template version.
  • Health: ASG replaces instances that fail ELB health check or EC2 check. Set health check type to ELB so unhealthy instances are replaced. Protect scale-in for critical instances if needed.

Prerequisites

Steps

  1. Launch template

    Create launch template: choose AMI, instance type, key pair, security group, user data if needed. Use for ASG so every new instance is consistent.

  2. Create ASG

    Create ASG: min 1, max 10, desired 2 (example); attach launch template; select at least two subnets (two AZs). Attach to target group so LB sends traffic to new instances. Enable ELB health check.

  3. Scaling policy

    Add target tracking policy: e.g. target CPU 70%; or scale on request count per target. Scale-in: allow or disable; set cooldown. Test by triggering scale (increase load or lower desired to see scale-in).

  4. Instance refresh

    When updating launch template (new AMI or config), use instance refresh to roll out new instances gradually. Set min healthy percentage and instance warmup so traffic is not dropped.

Summary

Create a launch template and an ASG with min/max/desired; attach to a target group; add scaling policies. Use this for HA and to scale instance count with load.

Prerequisites

Steps

Step 1: Launch template

Create a launch template with the desired AMI, type, and config.

Step 2: Create ASG

Create ASG with capacity; attach launch template and target group; use multiple AZs and ELB health check.

Step 3: Scaling policy

Add target tracking or step scaling; configure scale-in and cooldown.

Step 4: Instance refresh

Use instance refresh to roll out launch template changes.

Verification

  • ASG maintains desired capacity; scaling policies add/remove instances; new instances pass health check and receive traffic.

Troubleshooting

Instances not receiving traffic — Ensure ASG is attached to target group; wait for health check to pass. Scale-in too aggressive — Increase cooldown; use scale-in protection for critical instances.

Next steps

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