How to Reinstall an AMI on an Amazon EC2 Instance

Overview

Reinstalling an AMI (Amazon Machine Image) on an Amazon EC2 instance typically involves terminating the old instance and launching a new one with the desired AMI. This is a common operation to reset the instance state, apply a new base image, or fix system issues. Important: This process will destroy the original instance and any associated instance store volume data.

Step-by-Step Procedure

Step 1: Terminate the Old Instance

1. Log into the AWS Management Console and navigate to the EC2 service.
2. Under 'Instances' in the left navigation pane, click Instances.
3. In the instances list, select the target instance you want to reinstall.
4. From the top 'Actions' menu, select Instance state, then click Terminate instance.

Critical Warnings:

• Terminating an instance will cause permanent loss of all data on its instance store volumes (if used).
• For EBS volumes (the default root volume type), deletion depends on the 'Delete on Termination' attribute. Before terminating:
  • Check the root volume's properties on the instance details page under the 'Storage' tab.
  • If set to 'Yes' (default), the EBS volume will be deleted automatically when the instance terminates.
  • If the volume contains important data and 'Delete on Termination' is 'Yes', you must create a snapshot or detach the volume before termination to preserve the data.
• Ensure you have backed up all necessary data.

Step 2: Launch a New Instance with the New AMI

1. Wait for the target instance's status to change to terminated in the console.
2. In the EC2 console left navigation pane, click Instances, then click the Launch instance button at the top.
3. In the 'Launch an instance' wizard:
   1. Name and tags: Name your new instance.
   2. Application and OS Images (Amazon Machine Image): Select the new AMI you want to install. Choose from Amazon-provided, community, or your own AMIs.
   3. Instance type: Select an instance type based on your needs (e.g., t2.micro, t3.medium).
   4. Key pair (login): Select or create a key pair for SSH access.
   5. Network settings: Configure VPC, subnet, security groups, etc. Using settings similar to the old instance is recommended for service continuity.
   6. Configure storage: Set the size and type for the root volume and any additional EBS volumes.
   7. Advanced details: Configure IAM roles, user data scripts, etc., as needed.
4. Review all configurations and click Launch instance.

Alternatives and Best Practices

Alternative: Create a New Instance from AMI, Then Switch Traffic

For production environments, a better approach is:

1. Directly launch a new instance using the target AMI, without terminating the old one.
2. Deploy and validate your application on the new instance.
3. Gradually switch traffic to the new instance by updating the load balancer target group, DNS records, or Elastic IP address.
4. After confirming the new instance is stable, decide whether to terminate the old instance.

This method enables zero-downtime deployment or reset.

Best Practices Summary

• Always back up: Ensure important data is backed up via EBS snapshots or other means before terminating any instance.
• Understand deletion behavior: Clearly distinguish between instance store (ephemeral) and EBS storage (persistent), and know the 'Delete on Termination' attribute for EBS volumes.
• Use Infrastructure as Code (IaC): For repetitive operations, use tools like AWS CloudFormation or Terraform to define and launch instances, ensuring environment consistency.
• Consider using launch templates: AWS Launch Templates save instance configurations for quick, consistent launches.