Auto-discovery of services

[v0idpwn]

Problem statement

Currently, whenever adding a new service, one must add it the reflection server module:

defmodule MyApp.Reflection.Server do
  use GrpcReflection.Server,
    version: :v1,
    services: [
      MyApp.ServiceA,
      MyApp.ServiceB,
      #...
    ]
end

This is error prone (you must remember to add every service here, and it's generally not essential for the application to work, so it's easy to forget), and in a lot of cases, it is just duplication that doesn't add much value.

Solution

The GRPC library provides reflection primitives that allow us to read all services from a given endpoint. For example:

endpoint.__meta__(:servers)
|> Enum.map(fn server ->
  service_module = server.__meta__(:service)
  service_name = service_module.__meta__(:name)
  %{server: server, service_module: service, service_name: service_name}
end)

This information can be used to optionally set a reflection server to use all services from an endpoint. Some example APIs would be:

defmodule MyApp.Reflection.Server do
  use GrpcReflection.Server,
    version: :v1,
    services: {:all, MyApp.GrpcEndpoint}
end

# Alternative without specifying the endpoint, it could read the endpoint when
# receiving the request. Might not be viable to implement in the current architecture.
defmodule MyApp.Reflection.Server do
  use GrpcReflection.Server, version: :v1
end

I'd be happy to PR if you agree.

[mjheilmann]

It sounds useful, though I would specify using {:all, MyApp.GrpcEndpoint} as my preferred approach. It still requires users to specify that they want all services from that endpoint to be reflected.

This reflection implementation is meant to be opt-in and not very automatic by design to reduce the risk of our users reflecting schemas that they did not mean to. We can't protect users from themselves, but we can steer them by how we expose the configuration options. This is the same approach that could result in a Grpc endpoint containing a mix of services where only some should be reflected.

[v0idpwn]

Agreed on your concern. Will PR soon, and make sure to warn in the documentation about how it's potentially not a good idea.