Why New Instances in Elastic Beanstalk Don't Receive Immediate Traffic

Answer - B.

Before the EC2 Instances can start receiving traffic, they will be checked via the health checks of the ELB.

Once the health checks are successful, the EC2 Instances will change the state to InService and then the EC2 Instances can start receiving traffic.

For more information on ELB health checks, please refer to the below link:

When Elastic Beanstalk deploys a new version of an application, it typically deploys new instances, waits for them to pass health checks, and then redirects traffic to them. However, in some cases, Elastic Beanstalk detaches the current instances and deploys new instances before reattaching them. This approach is used to minimize downtime during deployments.

In this scenario, the new instances are deployed and reattached to the Elastic Load Balancer (ELB). However, the new instances are not receiving any traffic immediately, even though they have passed their health checks.

The reason for this is that it takes time for the ELB to register the instances. The ELB needs to update its routing tables to include the new instances before it can start directing traffic to them. This process can take several minutes, depending on the size of the ELB and the number of instances attached to it.

Therefore, it is normal for the new instances not to receive traffic immediately after being reattached to the ELB. It is important to allow time for the ELB to update its routing tables before expecting the new instances to receive traffic.

Option A is incorrect because if the instances were of the wrong AMI, they would not be able to pass the health checks and would not be reattached to the ELB.

Option C is incorrect because it is possible to detach and reattach instances to an ELB within the same Elastic Beanstalk application.

Option D is incorrect because the instances are reattached before the new application version is deployed, and the issue is with the ELB's routing tables, not the application version.