Service implementations should be able to return metadata along a success response

[bjaglin]

Short description

gRPC Metadata (implemented as HTTP headers/trailers) are currently supported via the power api flag in one direction only: the server can inspect what the client sent, but there is no way for the server to pass metadata to the client (unless the implementation wants to return a gRPC error code via a rich exception).

Details

I was looking forward to try out #838 to get rid of low-level akka-http code hydrating every single response with gRPC metadata (HTTP trailer), but I could not find a hook in the generated Handlers to do so no matter what the status is - the only possibility to inject metadata being in the exception handler.

In my case, the metadata is the same across all methods of a service (so a new overload of Handler#apply with a generic => Metadata provider would do), but I can imagine some people would want to control it on per-method/request basis - this was briefly discussed in #179 (comment).

Is it worth adding opening a (small) PR with a new Handler#apply or should we consider changing (before 1.0) the return type of power api traits to return both a response and metadata?

Happy to read your thoughts!

[raboof]

In my case, the metadata is the same across all methods of a service

Hmm, in that case it sounds fine to do it at the Akka HTTP level - perhaps we could provide some API's to make that easier. Putting it into the apply of the Handler is one option, but I wonder if we can make it even nicer by composing/wrapping the returned handler.

I can imagine some people would want to control it on per-method/request basis

I can also imagine this, but on the other hand we have seen relatively few actual real-life use cases for this in practice. It would make the power API's harder to use (returning a Future[Metadata, Future[Result]]?). Perhaps we should have an independent 'power option' for the request and the response?

[bjaglin]

Hmm, in that case it sounds fine to do it at the Akka HTTP level - perhaps we could provide some API's to make that easier. Putting it into the apply of the Handler is one option, but I wonder if we can make it even nicer by composing/wrapping the returned handler.

Interestingly, #887 does look like a generalization of this idea.

[justinpermar]

+1 we need this feature