AWS Elastic Beanstalk
Developer Guide

Auto Scaling Health Check Setting

Amazon EC2 Auto Scaling monitors the health of each Amazon Elastic Compute Cloud (Amazon EC2) instance that it launches. If any instance terminates unexpectedly, Auto Scaling detects the termination and launches a replacement instance. By default, the Auto Scaling group created for your environment uses Amazon EC2 status checks. If an instance in your environment fails an Amazon EC2 status check, Auto Scaling takes it down and replaces it.

Amazon EC2 status checks only cover an instance's health, not the health of your application, server, or any Docker containers running on the instance. If your application crashes, but the instance that it runs on is still healthy, it may be kicked out of the load balancer, but Auto Scaling won't replace it automatically. The default behavior is good for troubleshooting. If Auto Scaling replaced the instance as soon as the application crashed, you might not realize that anything went wrong, even if it crashed quickly after starting up.

If you want Auto Scaling to replace instances whose application has stopped responding, you can use a configuration file to configure the Auto Scaling group to use Elastic Load Balancing health checks. The following example sets the group to use the load balancer's health checks, in addition to the Amazon EC2 status check, to determine an instance's health.

Example .ebextensions/autoscaling.config

Resources: AWSEBAutoScalingGroup: Type: "AWS::AutoScaling::AutoScalingGroup" Properties: HealthCheckType: ELB HealthCheckGracePeriod: 300

For more information about the HealthCheckType and HealthCheckGracePeriod properties, see AWS::AutoScaling::AutoScalingGroup in the AWS CloudFormation User Guide and Health Checks for Auto Scaling Instances in the Amazon EC2 Auto Scaling User Guide.

By default, the Elastic Load Balancing health check is configured to attempt a TCP connection to your instance over port 80. This confirms that the web server running on the instance is accepting connections. However, you might want to customize the load balancer health check to ensure that your application, and not just the web server, is in a good state. The grace period setting sets the number of seconds that an instance can fail the health check without being terminated and replaced. Instances can recover after being kicked out of the load balancer, so give the instance an amount of time that is appropriate for your application.