Change the number of replicas on my cluster without recreate it using CloudFormation

Question

I have a CloudFormation stack that creates a [`AWS::ElastiCache::ReplicationGroup`](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticache-replicationgroup.html) wiht 1 shard (`NumNodeGroups`) which contains 1 primary nodes and 2 replicas. It have cluster mode enabled and uses multi az.

Every time I need to update the parameter `ReplicasPerNodeGroup` CloudFormation recreates the entire cluster instead of only change shards adding or deleting replicas. The [documentation says that the update requires a replacement](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticache-replicationgroup.html#cfn-elasticache-replicationgroup-replicaspernodegroup).

But adding or deleting replicas from console only modify the current cluster without recreate it, keeping the data.

Is there a way to create a Redis Cluster with cluster mode enabled and multi az, using cloudformation, that allows me to change the number of replicas without recreating the entire cluster ? The current cloudformation behaviour does not make sense at all.

Accepted Answer

Hello,

Currently when CloudFormation requires an REPLACEMENT of a resource, there is no workaround for getting around the replacement of the resource.

For the specific  AWS::ElastiCache::ReplicationGroup resource, this resource in the past did not support  updating  the parameter ReplicasPerNodeGroup via console as well. Upon which  the cloudformation resource was designed initially.

That being said the Cloudformation service team is aware of this behavior change for the ReplicasPerNodeGroup  parameter via the console and are working towards releasing a fix so that you can update ReplicasPerNodeGroup via Cloudformation without a replacement of the cluster in the same way  you are able to do it via the console.

I do not have an ETA when this would be released however as soon as it gets released, it should be publicly announced in either one of the following links:

AWS Blog - https://aws.amazon.com/blogs/aws/
AWS New - https://aws.amazon.com/new/

Change the number of replicas on my cluster without recreate it using CloudFormation

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