Please add Expression field to AWS CloudWatch scaler

[jeffjen]

Proposal

We'd like to add simple math aggregation to our metric data.

As it stands, we need to write a Lambda Function to poll and compute new metric data from a set of metrics, and then publish them back to cloudwatch.

Would be nice to be able to perform this kind of operation without need to provision Lambda

Use-Case

According to https://forums.aws.amazon.com/thread.jspa?threadID=309159

We can get a rough estimate on how many active connections are there by performing math expression.

[tomkerkhove]

Interesting request as this could be interesting for multiple scalers. What would the expression look like?

[coderanger]

I would imagine they mean a CloudWatch expression. We hardwire things to MetricStat here https://github.com/kedacore/keda/blob/main/pkg/scalers/aws_cloudwatch_scaler.go#L278 but CloudWatch allows asking for either a single metric or an expression https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/API_MetricDataQuery.html

[tomkerkhove]

Makes sense to have this as a field indeed! Are you willing to contribute this @jeffjen ?

[jeffjen]

Makes sense to have this as a field indeed! Are you willing to contribute this @jeffjen ?

I can work on this

My proposed change will be to not mess with multiple metrics in a query since it will change the struct to include an array of dimensions / namespace

The first step will be to just provide a way to add Math Expression for a single metric on designated dimension and namespace.

[tomkerkhove]

Thanks!

I don't know how the SDK works but I'd just add a new optional expression field which just passes through to AWS CloudWatch and let it handle everything?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[tomkerkhove]

/keep-alive

[dekelev]

I've opened this PR to add alternative expression field and also changed the scaler to use int64 instead of float64.

[JorTurFer]

I'd say that this is done by kedacore/keda-docs#755. Feel free to reopen the issue if it's not done