Allow returning metadata alongside success values

[NavidJalali]

In the current API grpc metadata is only allowed to be read from (using the power API) or written to in the case of an error using the rich error model. Would it be possible to add metadata alongside the success values?

[pjfanning]

There is already:

• client-side: https://pekko.apache.org/docs/pekko-grpc/current/client/details.html#request-metadata
• server-side: https://pekko.apache.org/docs/pekko-grpc/current/server/details.html#accessing-request-metadata

[NavidJalali]

Hey @pjfanning thank you for the prompt response. So client side is okay for me. On the server side, I can quite easily read the metadata as well. The problem lies however with writing the metadata on the server side when the response is NOT an error. If the response IS an error then using the GrpcServiceException lets me provide the Metadata but for the successful channel I only get to succeed with a Future[Result] whereas I would like to be able to also write the metadata with a Future[(Result, Metadata)].

[pjfanning]

This looks like akka/akka-grpc#872 - so far, there seems to be no support for this.

[NavidJalali]

Do you have ideas on how we could go about adding such support? I could perhaps give it a shot

[pjfanning]

There are a few problems to solve if you want the Server to support Future[(Result, Metadata].

• you will need a codegen enhancement - maybe support a new codegen config that works alongside the existing server_power_apis option
• you will also need to then be able to take the Future[(Result, Metadata] and to generate a Pekko HTTP response from that.

Another option is to try to build something like akka/akka-grpc#887