Create a PullController for metrics

[dyladan]

Currently we only implement the PushController, but we would also like a PullController for cases like prometheus where the exporter is called by the backend.

[Bhargavamacha]

I would like to take this issue up if that's fine

[vmarchaud]

@Bhargavamacha It is ! Don't hesitate to ping us on gitter or here if you have any questions or need help :)

[Bhargavamacha]

Sure!

[Bhargavamacha]

I am trying to understand how exactly the PullController should work and this is how I think the flow goes:
The PushController controls the flow of metrics to the exporter. Now, these metrics will be polled by Prometheus and PullController should control the flow to Prometheus.
Prometheus has a scrape_interval that lets it collect metrics at certain time intervals. My question is, what role does PullController really play? I found this to be a good explanation, but I am not really sure how it can exactly be implemented.

[dyladan]

@Bhargavamacha any update here?

[Bhargavamacha]

I am still trying to understand how it should be implemented, but I couldn't really solve the question in the previous comment. Any help would be appreciated.

[dyladan]

Controllers are constructed with a meter and an exporter. What I would do is add an optional function to the exporter interface called registerPullController or similar. When a Meter is constructed, if this function exists then a PullController is created, otherwise a PushController is created. registerPullController would be a function that receives a PullController, and calls its public collect method when it wants metrics to be exported.

[dyladan]

@Bhargavamacha does that make sense to you?

[Bhargavamacha]

Yup. Thanks!

[dyladan]

@Bhargavamacha any update here?

[legendecas]

With #1017, I believe PullController can be left to users (who do the actual pulling) to implement it since it will be very straightforward to do it. There is no meaningful reason to add a default PullController since the actual pulling strategies vary across use cases.

[dyladan]

The reason to add a pullcontroller is that prometheus polls an endpoint, and it would be ideal for that endpoint to be updated on each poll rather than on a schedule

[dyladan]

Previous SDK. No longer applies