Kgen

Kgen creates idiomatic coroutine-based gRPC Kotlin client libraries from a protocol buffer description of an API.

It supports full-stack Kotlin development on the server and in Android applications.

Note This project is a preview. Please try it out and let us know what you think, but there are currently no guarantees of any form of stability or support.

Why Kgen?

Protocol buffers and gRPC have great tool chains, but they do not have first class support for Kotlin and they do not provide many configuration options for generated code. Kgen generates idiomatic Kotlin clients for protobuf APIs and introduces new configuration options to make the code even more enjoyable to use.

Clients generated using Kgen can also take advantage of the Kotlin API extension library that simplifies common operations like customizing request and response metadata, handling paged responses, and using client-side gRPC interceptors with with your API.

Quick Start

Kgen can be used with docker, gradle, or as a protoc plugin.

Docker

To get started with docker, clone the project and run the following to generate a client for the example service.

$ mkdir my-output 
$ docker run --rm \
             --mount type=bind,source="$(pwd)"/example-server/src/main/proto,target=/proto \
             --mount type=bind,source="$(pwd)"/my-output,target=/generated \
         gcr.io/gapic-images/gapic-generator-kotlin

Gradle

To use gradle put all of your .proto files in app/src/main/proto (Android) or src/main/proto (non-Android) and let the Protobuf plugin for gradle take care of the rest. For example, add the following sections to your application's build.gradle:

repositories {
   // jitpack releases are required until we start publishing to maven
   maven { url 'https://jitpack.io' }
}

plugins {
    id "com.google.protobuf" version "0.8.8"
}

dependencies {
    // pick the ONE dependency that is appropriate for your platform (server or Android) 
    implementation 'com.github.googleapis.gax-kotlin:kgax-grpc:master-SNAPSHOT'
    //implementation 'com.github.googleapis.gax-kotlin:kgax-grpc-android:master-SNAPSHOT'
}

protobuf {
    protoc {
        artifact = 'com.google.protobuf:protoc:3.7.1'
    }
    plugins {
        // For android projects, uncomment the lines below
        //javalite {
        //    artifact = 'com.google.protobuf:protoc-gen-javalite:3.0.0'
        //}
        client {
            artifact = 'com.github.googleapis:gapic-generator-kotlin:master-SNAPSHOT:core@jar'
        }
    }
    generateProtoTasks {
        all().each { task ->
            // For android projects, uncomment the lines below
            //task.builtins {
            //    remove java
            //}
            task.plugins {
                // For android projects, uncomment the lines below
                //javalite {}

                // this generates your client library and helper Kotlin builders!
                client {}
            }
        }
    }
}

Enjoy your new client library! The generated source code will available on the classpath for your application to use, and you can find it at app/build/generated/source/proto (Android) or build/generated/source/proto (standalone application).

See the RUNNING.md for more details, configuration, and command line options.

Example

A simple "hello world" style example is in the example-server and example-client directories. Here's how it works:

First, describe the API like this (complete proto file):

service HelloService {
  rpc HiThere (HiRequest) returns (HiResponse);
}

message HiRequest {
    string query = 1;
}

message HiResponse {
    string result = 1;
}

Next, run Kgen on the proto files and it will produce Kotlin code that you can use to call the API, like this (complete example):

fun main() = runBlocking<Unit> {
    // create a client with an insecure channel
    val client = HelloServiceClient.create(
        channel = ManagedChannelBuilder.forAddress("localhost", 8080)
            .usePlaintext()
            .build()
    )

    // call the API
    val response = client.hiThere(hiRequest {
        query = "Hello!"
    })

    // print the result
    println("The response was: ${response.result}")

    // shutdown
    client.shutdownChannel()
}

The generator creates three things from the proto files:

1. A client for each service declared
2. A type-safe builder for each message declared
3. Unit tests for each generated client

Finally, you can add annotations to the proto to customize the way Kgen generates code. For example:

rpc HiThere (HiRequest) returns (HiResponse) {
  option (google.api.method_signature) = {
    fields: ["result"]
  };
}

will change the client so that you can call the example API like this instead:

// call the API
val response = client.hiThere("Hello!")

Of course, don't forget to implement the API (example implementation).

You can run this example locally by using gradle:

$ cd example-server && ./gradlew run
$ cd example-client && ./gradlew run

More complex examples, using Google Cloud APIs, can be found in the examples directories:

• Server side examples.
• Android examples.

Configuration

Kgen can be configured to produce Kotlin code that's easy to use in various flavors. See the CONFIGURATION.md to learn about these additional features.

Contributing

Contributions to this library are always welcome and highly encouraged.

See the CONTRIBUTING documentation for more information on how to get started.

Versioning

This library is currently a preview with no guarantees of stability or support. Please get involved and let us know if you find it useful and we'll work towards a stable version.

Disclaimer

This is not an official Google product.