proposal: Added proposal for new Thanos component: Thanos Frontend.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

docs/proposals/202004_embedd_cortex_frontend.md

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+---
+title: Adding a New Thanos Component that Embeds Cortex Query Frontend
+type: proposal
+menu: proposals
+status: in-review
+owner: bwplotka
+---
+
+### Related Tickets
+
+* Response caching: https://github.com/thanos-io/thanos/issues/1651
+* Moving query frontend to separate repo: https://github.com/cortexproject/cortex/issues/1672
+
+## Summary
+
+This proposal describes addition of a new Thanos command (component) into `cmd/thanos` called `frontend`.
+This component will literally import a certain version of Cortex [frontend package](https://github.com/cortexproject/cortex/tree/4410bed704e7d8f63418b02b328ddb93d99fad0b/pkg/querier/frontend).
+
+We will go through rationales, and potential alternatives.
+
+## Motivation
+
+[Cortex Frontend](https://www.youtube.com/watch?v=eyBbImSDOrI&t=2s) was introduced by Tom in August 2019. It was designed 
+to be deployed in front of Prometheus Query API in order to ensure:
+
+* Query split by time.
+* Query step alignment.
+* Query retry logic
+* Query limit logic
+* Query response cache in memory, Memcached or Redis.
+
+Since the nature of Cortex backend is really similar to Thanos, with exactly the same PromQL API, and long term capabilities, the caching 
+work done for Cortex fits to Thanos. Given also our good collaboration in the past, it feels natural to reuse Cortex's code. 
+We even started discussion to move it to separate repo, but there was no motivation towards this, for a good reason.
+At the end we were advertising to use cortex query frontend on production on top of Thanos and this works considerably well, with some 
+problems on edge cases and for downsampled data as mentioned [here]()
+
+However, we realized recently that asking users to install suddenly Cortex component on top of Thanos system is extremely confusing:
+
+* Cortex has totally different way of configuring services. It requires to decide what module you have in single YAML file. Thanos in opposite
+have flags and different subcommand for each component.
+* Cortex has bit different way of configuring memcached, which is inconsistent with what we have in Thanos Stote Gateway.
+* There are many Cortex specific configuration items which can confuse Thanos user and increase complexity overall (e.g. )
+* We have many ideas how to improve Cortex Query Frontend on top of Thanos, but adding Thanos specific configuration options will increase 
+complexity on Cortex side as well.
+
+All of this were causing confusion and questions like [this](https://cloud-native.slack.com/archives/CK5RSSC10/p1586504362400300?thread_ts=1586492170.387900&cid=CK5RSSC10).
+
+At the end we decided with Thanos and Cortex maintainers that the ultimately it would be awesome to create a new Thanos subcommand called `frontend`. 
+
+## Use Cases
+
+* User can cache responses for query range
+* User can use the same configuration patterns as rest of Thanos components.
+* Thanos Developer does not need to write and maintain custom response cache logic. 
+
+## Goals of this design
+
+* Enable response caching that will easy to use for Thanos users.
+* Keep it extensible and scalable for future improvements like advanced query planning, queuing, rate limiting etc.
+* Reuse as much as possible between projects, contribute.
+
+## No Goals
+
+* Create Thanos specific response caching from scratch.
+
+## Proposal
+
+The idea is to create `thanos frontend` component that allows specifying following options:
+
+* `--query-range.split-interval`, `time.Duration`
+* `--query-range.max-retries-per-request`, `int`, default = `5`
+* `--query-range.disable-step-align`, `bool`
+* `--query-range.response-cache-ttl` `time.Duration` default = `1m` 
+* `--query-range.response-cache-config(-file)` `pathorcontent` + [CacheConfig](https://github.com/thanos-io/thanos/blob/55cb8ca38b3539381dc6a781e637df15c694e50a/pkg/store/cache/factory.go#L32) 
+
+We plan to have in-mem, fifo and memcached support for now. Cache config will be exactly the same as the one used for Store Gateway.
+
+### Open Questions
+
+* Is `thanos frontend` a valid name? I feel it's short and verbose enough.
+
+### Alternatives
+
+#### Don't add anything, document Cortex query frontend and add examples of usage
+
+Unfortunately we tried this path already without success. Reasons were mentioned in [Motivation](202004_embedd_cortex_frontend.md#Motivation)
+
+#### Add response caching to Querier itself, in the same binary.
+
+This will definitely simplify deployment if Querier would allow caching directly. However, this way is not really scalable.
+
+Furthermore, eventually frontend will be responsible for more than just caching. It is meant to do query planning like splitting or even
+advanced query paralization (query sharding). This might mean future improvements in terms of query scheduling, queuing and retrying.
+This means that at some point we would need an ability to scale query part and caching/query planner totally separately.
+
+NOTE: We can still consider just simple response caching inside Querier if user will request so.
+
+#### Write response caching from scratch.
+
+I think this does not need to be explained. Response caching has proven to be not trivial. It's really amazing that we
+have opportunity to work towards something that works with experts in the field like @tomwilkie and others from Loki and Cortex Team.
+
+Overall, [Reusing is caring](https://www.bwplotka.dev/2020/how-to-became-oss-maintainer/#5-want-more-help-give-back-help-others).
+
+## Work Plan
+
+1. Refactor [IndexCacheConfig](https://github.com/thanos-io/thanos/blob/55cb8ca38b3539381dc6a781e637df15c694e50a/pkg/store/cache/factory.go#L32) to generic cache config so we can reuse. 
+1. Add necessary changes to Cortex frontend
+ * Metric generalization (they are globals now).
+1. Add `thanos frontend` subcommand.
+1. Add proper e2e test using cache.
+1. Document new subcommand
+1. Add to [kube-thanos](https://github.com/thanos-io/kube-thanos0) 
+
+## Future Work
+
+Improvements to Cortex query frontend, so Thanos `frontend` as described [here](https://github.com/thanos-io/thanos/issues/1651)

docs/proposals/_index.md

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 ---
 title: "Proposals:"
 ---
+
+List of current proposals.
+
+Proposals can have 5 Statuses (`.Params.Status`):
+
+* accepted
+* complete
+* rejected
+* in-review
+* draft