Thanos Query Frontend gRPC port

[vincent-olivert-riera]

I'm planning to setup an architecture using Thanos Querier distributed execution mode.

The idea is to have one central Thanos Querier where users will connect to perform queries.
That central Thanos Querier will be connected to multiple local Thanos Querier components.
Those local Thanos Querier servers will be connected to Thanos Sidecar and Thanos Store components.

Now, I want to add a Thanos Query Frontend in front of each local Thanos Querier.

As far as I can see, the definition of the endpoints in Thanos Querier uses the gRPC port.
So, in theory, my central Thanos Querier should connect to the local Thanos Query Frontend via the gRPC port. Like this:

---
endpoints:
  - address: local-thanos-query-frontend-001.mydomain.com:10901
  - address: local-thanos-query-frontend-002.mydomain.com:10901
  ...
  - address: local-thanos-query-frontend-XYZ.mydomain.com:10901

However, I have noticed that Thanos Query Frontend does not have a --grpc-address flag.
So, is this even possible? I would like to avoid placing the Thanos Query Frontend in front of the central Thanos Querier, because I would prefer to have one on each local Thanos Querier in order to use different response cache servers.

I have read this from Thanos Querier distributed execution mode documentation:

A Querier running in the distributed mode will only talk to Queriers, or other components which implement the Query API.

Does that mean the endpoints defined for the central Thanos Querier should use the HTTP port instead of the gRPC one?

If I'm not mistaken, this PR implemented a gRPC query API for Thanos Querier, but not for Thanos Query Frontend (CC @fpetkovski).

If I use the HTTP port for the endpoins of my central Thanos Querier (those endpoints are Thanos Query Frontend components), will that work? Or does Thanos Querier always use the gRPC protocol to connect to the endpoints regardless of the port number?