Switch Cortex Ruler to query Query Frontend

[yeya24]

Is your feature request related to a problem? Please describe.
As we can see from the Cortex architecture https://cortexmetrics.io/docs/architecture/, Ruler queries ingesters (and store gateways) to get data for rules evaluation. This works fine currently but it hard to reuse some features supported in query frontend like vertical query sharding to make rule evaluation faster.

By switching Ruler to query Query Frontend, it simplifies the current architecture to make Query Frontend the entrypoint for both external and internal queries. Ruler components can use less resources as it doesn't need to embed a querier anymore.

Describe the solution you'd like
This feature should be behind a feature flag. If the flag is specified, Ruler will query Query Frontend instead. The implementation should be similar to how Thanos does https://github.com/thanos-io/thanos/blob/main/cmd/thanos/rule.go#L795, by having a custom query func and send HTTP requests to query frontend.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[xuyangwang0825]

Hi @yeya24!
I'm interested in this issue and would like to work on it under the LFX Mentorship. I've started studying the Cortex architecture and how Thanos do the query.

[yeya24]

Hi @xuyangwang0825, thanks for expressing interest in this project. Feel free to apply through the LFX website!

[mallikpramod]

Hello @yeya24
it's Pramod Kumar Mallik, i get to know about cortex quite some day ago and i started learning and searching about it, and i found it quite interesting mainly about partitioning compactor and vertical query sharding.
So i would like to join with this project and being guided by you.

[yeya24]

Hi @mallikpramod, please apply in the LFX mentorship website but I don't think I am mentoring this project. We have @alvinlin123 and @qinxx108 mentoring it.

[xuyangwang0825]

Hi @yeya24
May I ask if not receiving any notifications means that I'm not selected for the project? My status on the LFX portal still shows as pending.

[yeya24]

@xuyangwang0825 Yes, selected mentee name should be in the LFX project page.

[xuyangwang0825]

🥹

[xuyangwang0825]

@yeya24

I remain deeply interested in open-source projects in cloud native. However, I recognize that there may be areas in which I need to improve in order to become a better candidate.

Could you please provide me with some feedback on my application? I would appreciate any insights you can offer on which aspects of my background or experience, or skills I should focus on improving.

Thank you so much!

[SungJin1212]

@yeya24
I interested in this issue. Can I address this issue?
If then, I was thinking I define FrontendAddress in the ruler config. If it is set, the ruler can query to the query frontend. Is it reasonable?

[yeya24]

Hi @SungJin1212, it would be great if you could start a proposal outlining possible approaches and alternatives.

[SungJin1212]

@yeya24
I was thinking below approaches. Could you please take a look?

1. Define queryFrontendQueryable implementing Select func querying to the query frontend using prom client like the thanos.
2. In ruler init part on modules.go, initiate the manager using queryFrontendQueryable

Sincerely

[SungJin1212]

@yeya24
I make a draft pr. (https://github.com/cortexproject/cortex/pull/6151/files) I would appreciate some guidance.