aws cloudformation for atomic stack-update

Question

I have a stack consisting of

1. ec2 instances as an application server
2. one ELB as a load balancer
3. two Elasticsearch nodes

If I update the stack for whatever reason, perhaps to increase the number of application servers, all new components are ready to handle my current stack and my current stack is replaced by the new one at the same time, like atomic operations in database systems?

Since this reduction involves the desharding of existing data in Elasticsearch, does the same procedure also apply if I reduce the number of Elasticsearch nodes?

Answer 1

When you perform a stack update, only the resources that you modify will be updated or replaced. In your use case, if you make changes in the number of nodes in your ES cluster, your EC2 and ELB instances will stay in place and will not change.

If you want to check which resources will affect the changes made by your template, you can use the  CloudFormation Change Sets to preview the changes before applying them.

If you just add the number of instances in your auto scaling group, the existing ones will remain there. However, if you reduce the number of instances in your ASG, the oldest ones will be deleted first. You can use the AutoScalingRollingUpdate policy to gain control over it.

In addition, when you upgrade, you can specify a replacement for your entire ASG with a new one, including all new instances of EC2. Using the AutoScalingReplacingUpdate policy, you can customize this behavior.