Get the serviceName in Service discovery

[mengxiangyue]

What are you trying to achieve?

When I am creating the client, I want to use service discovery to get the service name. But I need to create Client then I can get the service name, if I want to create Client I need to use host/port to create a channel, at here I need to use service name to get host. it become a cycle. could you give a suggestion?

// Configure the channel, we're not using TLS so the connection is `insecure`.
let channel = ClientConnection.insecure(group: group)
  .connect(host: "localhost", port: self.port) // I need to use service name to get the host/port

// Provide the connection to the generated client.
let greeter = Helloworld_GreeterClient(channel: channel)

[glbrntt]

I suspect the correct way of resolving this is to generate some static metadata for each service and integrate with swift-service-discovery. However, that'll take a little bit of time.

---

As a workaround for the meantime you can create a no-op client to get the service name by creating something which conforms to GRPCChannel:

struct NoOpChannel: GRPCChannel {
  func makeCall<Request: SwiftProtobuf.Message, Response: SwiftProtobuf.Message>(
    path: String,
    type: GRPCCallType,
    callOptions: CallOptions,
    interceptors: [ClientInterceptor<Request, Response>]
  ) -> Call<Request, Response> { 
    fatalError()
  }

  func makeCall<Request: GRPCPayload, Response: GRPCPayload>(
    path: String,
    type: GRPCCallType,
    callOptions: CallOptions,
    interceptors: [ClientInterceptor<Request, Response>]
  ) -> Call<Request, Response> {
    fatalError()
  }

  func close() -> EventLoopFuture<Void> { 
    fatalError()
  }
}

Then you can extract the service name for service discovery:

let noOpGreeter = Helloworld_GreeterClient(channel: NoOpChannel())
let serviceName = noOpGreeter.serviceName

I appreciate that it's not the nicest solution but it should at least unblock you!

[mengxiangyue]

@glbrntt thanks your response so fast, what do you think if we change the serviceName into static property?

[glbrntt]

@glbrntt thanks your response so fast, what do you think if we change the serviceName into static property?

Unfortunately we can't just make it static since we'd effectively be breaking the API.

We'd most likely want to generate some static service metadata alongside the protocol and then add a static property with a default implementation to the generated protocol, something like:

protocol Echo_EchoClientProtocol: GRPCClient {
  // existing
  var serviceName: String { get }
  
  // ... 
  // RPC definitions
  // ...

  // new:
  static var metadata: GRPCServiceMetadata { get }
}

extension Echo_EchoClientProtocol {
  // new, default impl.
  static let metadata = GRPCServiceMetadata(name: "echo.Echo")
}

[weissi]

@glbrntt couldn't we just add a static one?

[mengxiangyue]

agree with you we can't break the api. I also have another question, I don't find the GRPCServiceMetadata in current code, will it like this?

struct GRPCServiceMetadata {
  let name: String
  let version: String
  ......
}

[glbrntt]

@glbrntt couldn't we just add a static one?

We could, but it might just be more useful to have a more complete set of metadata (i.e. include RPCs as well).

[glbrntt]

agree with you we can't break the api. I also have another question, I don't find the GRPCServiceMetadata in current code, will it like this?

struct GRPCServiceMetadata {
  let name: String
  let version: String
  ......
}

It doesn't exist yet! It could look something like that, it could also include other info like RPC metadata (i.e. name, request type, response type, whether the RPC streams requests or responses etc.). I'm just thinking out loud at this point though.

[glbrntt]

The 1.8.0 release adds support for this. See

grpc-swift/Sources/Examples/Echo/Model/echo.grpc.swift

Lines 387 to 424 in 703d351

	public enum Echo_EchoClientMetadata {
	public static let serviceDescriptor = GRPCServiceDescriptor(
	name: "Echo",
	fullName: "echo.Echo",
	methods: [
	Echo_EchoClientMetadata.Methods.get,
	Echo_EchoClientMetadata.Methods.expand,
	Echo_EchoClientMetadata.Methods.collect,
	Echo_EchoClientMetadata.Methods.update,
	]
	)
	public enum Methods {
	public static let get = GRPCMethodDescriptor(
	name: "Get",
	path: "/echo.Echo/Get",
	type: GRPCCallType.unary
	)
	public static let expand = GRPCMethodDescriptor(
	name: "Expand",
	path: "/echo.Echo/Expand",
	type: GRPCCallType.serverStreaming
	)
	public static let collect = GRPCMethodDescriptor(
	name: "Collect",
	path: "/echo.Echo/Collect",
	type: GRPCCallType.clientStreaming
	)
	public static let update = GRPCMethodDescriptor(
	name: "Update",
	path: "/echo.Echo/Update",
	type: GRPCCallType.bidirectionalStreaming
	)
	}
	}

as an example.