How to asynchronously scale aurora serverless v2 replicas

Question

I have an aurora postgresql serverless v2 cluster with 2 instances, one writer and another reader and would like them to scale independently.
According to [documentation](https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/aurora-serverless-v2-administration.html#aurora-serverless-v2-choosing-promotion-tier:~:text=Aurora%20Serverless%20v2%20reader%20DB%20instances%20in%20tiers%202%E2%80%9315%20don%27t%20have%20the%20same%20constraint%20on%20their%20minimum%20capacity.%20When%20they%20are%20idle%2C%20they%20can%20scale%20down%20to%20the%20minimum%20Aurora%20capacity%20unit%20(ACU)%20value%20specified%20in%20the%20cluster%27s%20capacity%20range.), if the reader instance is in a Failover priority other than 0 or 1. They SHOULD scale independently but still no matter what I do, they always scale synchronously.
I have a workload that runs twice a day and demands the higher acu count but the reader instance has very low usage, so I would like them to scale independently to save on costs. In my use case it is not a problem if there is a longer downtime for the reader instance to scale and take over in case of failure.

Thanks

Michel Galle · Answer

I checked and they are not busy as the writer on the heavy workloads. as an experiment I created a provisioned aurora server (with more resources than the serverless ones had), failed over to it and removed one of the serverless instances. and now the serverless instance hasn't gone past 8 acu a single time. if I get the metrics of serverless capacity of the past 24 hours we can clearly see the difference on acu allocation after the changes.
And about replication, I might be wrong but my understanding is that with aurora, replication is done at the storage layer and wouldn't put any pressure on reader instances.

Steve Kinsman · Answer

The documentation says "**When they are idle**, they can scale down to the minimum" - have you confirmed in CloudWatch that they really are idle?  Handling replication from a busy writer will put some load on a reader but you would hope not too much.  Still it's worth looking how busy it is.

How to asynchronously scale aurora serverless v2 replicas

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