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.