Why is Elastic Beanstalk Traffic Splitting deploy strategy ignoring HTTP errors?

Question

I am using AWS Elastic Beanstalk. In there, I selected a Traffic Splitting deploy strategy, with a 100% split (so that 100% of new instances will have the new version and have their health evaluated).

Here's how (according to their documentation) that is supposed to work:

During a traffic-splitting deployment, Elastic Beanstalk creates a new set of instances in a separate temporary Auto Scaling group. Elastic Beanstalk then instructs the load balancer to direct a certain percentage of your environment's incoming traffic to the new instances. Then, for a configured amount of time, Elastic Beanstalk tracks the health of the new set of instances. If all is well, Elastic Beanstalk shifts remaining traffic to the new instances and attaches them to the environment's original Auto Scaling group, replacing the old instances. Then Elastic Beanstalk cleans up—terminates the old instances and removes the temporary Auto Scaling group.

And more specifically:

Rolling back the deployment to the previous application version is quick and doesn't impact service to client traffic. If the new instances don't pass health checks, or if you choose to abort the deployment, Elastic Beanstalk moves traffic back to the old instances and terminates the new ones.

However, it seems silly that it only looks at my internal /health health checks, and not the overall health status of the environment, from the HTTP status codes, that it already has information on.

I tried the following scenario:

1. Deploy a new version.
2. As soon as the "health evaluation period" begins, flood the server with error 500s (from an endpoint I made specifically for this purpose).
3. AWS then moves all my instances into "degraded" state, and "unhealthy", but then seems to ignore it, and goes on anyway.

See the following two log dump screenshots (they are oldest-first).

Is there any way that I can make AWS respect the HTTP status based health checks that it already performs, during a traffic split? Or am I bound to only rely on custom-developed health checks entirely?

Update 1: Even weirder, I tried making my own healthchecks fail always too, but it still decides to deploy the new version with the failed healthcheck!

Update 2: I noticed that the temporary auto scaling group that it creates while assessing health does only have an "EC2" type health check, and not "ELB". I think that might be the root cause. If I could only get it to use "ELB" instead.

Answer 1

That is interesting! I do not know if setting the health check type to "ELB" may do the job because we use CodeDeploy, which has far better rollback capabilities than AWS Elastic Beanstalk.

However, there is a well-document way in the docs [1] to apply the setting you are looking for:

[...] 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. [...]
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

It does not mention the new traffic splitting deployment feature, though. Thus, I cannot confirm this is the actual solution, but at least you can give it a shot.

[1] https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/environmentconfig-autoscaling-healthchecktype.html