A Scaler interface for push based events

[ahmelsayed]

This is a RFC for adding a push based scaler interface that looks like this

type PushScaler interface {
	Scaler

	Start(ctx context.Context, active chan<- struct{})
}

scaler.go

Use-Case

For event sources that support a streaming client. This allows KEDA to connect as a client to an endpoint that can stream "isActive" requests per scaledObject.

Specification

• active updates from the push scaler reset the cooldownPeriod. They just skip the pollingInterval.

• provide a generic GRPC implementation

service ExternalPushScaler {
    rpc StreamScaleRequests(ScaledObjectRef) returns (stream activeResponse) {}
}

this can be used in the scaled object with

...
spec:
  triggers:
    - type: external-push
      metadata:
        address: grpc-server-address:port

Few remarks:

I have been back and forth on the signature or Start().

	Start(ctx context.Context, active chan<- struct{})
// vs
	Start(ctx context.Context, active func())

it seems that a callback is simpler for this scenario, but I haven't seen a lot of go code with callbacks like that, so I'm not sure if it's something that's avoided.

[zroubalik]

Looking good! I like this idea.

As for the signature, I'd prefer to use "the standard" go way, ie. with chan. But maybe you'll find other (better) ways during the implementation.

[tomkerkhove]

Do I still have some time to review this or am I too late?

(out on holiday)

[ahmelsayed]

sure, I still need to update the docs as well. Will wait for your review

[tomkerkhove]

It looks good to me but we’ll need good documentation that cover the use-case as it might not be clear what to expect or when to use it.

Maybe even document an example scenario.

[zroubalik]

Merged

[tomkerkhove]

Reopening as we don't have docs merged for it yet kedacore/keda-docs#193