Not specifying query-frontend downstream URL doesn't log any error

[adityacs]

Describe the bug
If we don't specify frontend-downstream URL, there is no error logged. When we try to send any request to query-frontend, the request hangs and there is no error returned to client.

To Reproduce
Steps to reproduce the behavior:

1. Start Loki with target as query-fronted without specifying frontend.downstream-url

Expected behavior
Error returned to client

/cc @cyriltovena

[adityacs]

If there is no downstream URL, cortex by default configures GRPC Roundtrip. I am not sure how that flow works with Cortex. Is loki expected to behave same way?

Am I missing something?

[cyriltovena]

Yes by default it uses GRPC unless you provide that downstream URL.

For sure we could warn the users if there is no connected GRPC client ready to process item in the frontend queue. But that's more work for Cortex.

[adityacs]

Thanks @cyriltovena . Yeah, it would be good to warn user. So, he will be aware of what is not configured

[owen-d]

More explicitly, the query frontend operates in one of two fashions:

1. with frontend.downstream-url. This just proxies requests over http to said url.
2. without (1). This way it defaults to grpc as a pull service. This requires the as-of-yet undocumented Document frontend worker #1867. In this form, the frontend instantiates per-tenant queues that downstream queriers pull queries from via grpc. In your example configuration, it's enqueueing queries, but nothing is dequeueing them :)

[adityacs]

I was trying to figure out how GRPC thing works without providing any address to connect to 😉. I couldn't get any thing. Now it's clear to me

Thanks @owen-d