Pass LookbackDelta as query hint to remote read

[mtanda]

Proposal

Related to #4122 .

I'm implementing read adapter.
The read adapter response size could be big, so I reduce size by calculating expected step, and response each datapoints correspond to the step.

Prometheus only accept datapoints in refTime - lookbackDelta.
https://github.com/prometheus/prometheus/blob/v2.2.1/promql/engine.go#L796

refTime is calculated like refTime = startTime x n x interval.
https://github.com/prometheus/prometheus/blob/v2.2.1/promql/engine.go#L402

read adapter need response datapoitns which fit in each time bucket.

But, the start time of query range is affected by lookbackDelta.
https://github.com/prometheus/prometheus/blob/v2.2.1/promql/engine.go#L492
https://github.com/prometheus/prometheus/blob/v2.2.1/promql/engine.go#L476

It is difficult to align data.

If I may misunderstand something, please tell me how I can align the data.

[brian-brazil]

It's fine to add this, however you don't need to.

For instant vector evaluation all you need to do is return the first sample at or before each step. Watch out for 'offset' though.

[mtanda]

Now I understand more detail.
My read adapter inject staleNan to datapoint, so I confuse the behavior related to staleness.
Don't need to pass LookbackDelta, it seems to work without it.
So I close this issue.

Thanks for your help!

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.