Announcing Connect-Kotlin: Connect is now fully supported on mobile!

Alan Chiu and Michael Rebello on Feb 22, 2023/4 min read

When writing software, engineers often make mistakes implementing an agreed upon API by incorrectly defining the data structure, misnaming a key, or even specifying the wrong endpoint path. These errors make collaborating across iOS, Android, frontend, and backend difficult as they necessitate back-and-forth coordination and exhaustive testing. Connect’s goal is to eliminate these hardships by using a single Protobuf schema to generate idiomatic and deterministic code for all platforms, replacing handwritten boilerplate. So far, Connect-Swift has made it incredibly easy to build iOS APIs, and Connect-Web and Connect-Go have done the same for the frontend and backend.

Today we are releasing Connect-Kotlin which brings this experience to Kotlin and enables Android engineers to collaborate with their Swift, Go, and TypeScript counterparts to build mobile applications completely end-to-end using Protobuf, Connect, gRPC, and gRPC-Web. With Connect-Kotlin, the family of Connect libraries now has full first class support on mobile!

  • Idiomatic, typed APIs. No more hand-writing REST/JSON endpoints and JSON data models. Connect-Kotlin generates idiomatic APIs and eliminates the need to worry about serialization.
  • Easy-to-use tooling. Connect-Kotlin integrates with the Buf CLI, enabling remote code generation without having to install and configure local dependencies.
  • Flexibility. Connect-Kotlin uses OkHttp, making it easy to integrate existing OkHttp configurations and interceptors. The library also provides extension points for using custom compression algorithms, adding interceptors, and swapping out the networking library.
  • Simple and small. The Connect-Kotlin library itself is very small with only a few familiar dependencies (e.g., OkHttp, Okio, and Coroutines).

If you want to go right to a hands-on demo, we created a getting started guide for building a Connect-enabled Kotlin chat app in ~10 minutes.

Easy-to-use typed APIs

The following .proto file defines an ElizaService containing a Say RPC (Remote Procedure Call, essentially an HTTP endpoint) that accepts a SayRequest and returns a SayResponse, each containing a sentence string field:

package connectrpc.eliza.v1;

message SayRequest {
  string sentence = 1;

message SayResponse {
  string sentence = 1;

service ElizaService {
  rpc Say(SayRequest) returns (SayResponse) {}

Connect-Kotlin’s protoc-gen-connect-kotlin generator plugin then uses this file to generate a simple client interface (ElizaServiceClientInterface.kt) and implementation class (ElizaServiceClient.kt) to communicate with the defined ElizaService:

// ElizaServiceClientInterface.kt
public interface ElizaServiceClientInterface {
  public suspend fun say(request: SayRequest, headers: Headers = emptyMap()):

// ElizaServiceClient.kt
public class ElizaServiceClient(
	private val client: ProtocolClientInterface,
) : ElizaServiceClientInterface {

  public override suspend fun say(request: SayRequest, headers: Headers):
      ResponseMessage<SayResponse> = client.unary(

Code hints are provided within the IDE to indicate how to use these generated classes for making requests, and they’re typically consumed within an Android Activity:

fun suspend talkToEliza(sentence: String) {
  // Make a request to Eliza.
  val sayRequest = SayRequest.newBuilder()
  val response = elizaServiceClient.say(sayRequest)
  val elizaSentence = response.success { success ->
    // Get Eliza's reply from the response.

Supported message generator plugins

Although the SayRequest and SayResponse models above reference outputs from Google’s Java Protobuf generator plugin (protoc-gen-java), Connect-Kotlin supports several Protobuf model generators, including:

  • Google’s Java Protobuf generator (protoc-gen-java), along with the javalite option.
  • Google’s Kotlin Protobuf generator (protoc-gen-kotlin), along with the kotlinlite option.
  • Coming soon: We are also building support for the square/wire generator.

Supported protocols

Connect-Kotlin supports three protocols out of the box:

  • The Connect protocol: A simple, protocol that works over HTTP/1.1 or HTTP/2. It takes the best parts of gRPC/gRPC-Web, including streaming, and packages them into a protocol that works well on all platforms, including mobile. By default, JSON- and binary-encoded Protobuf is supported.
  • The gRPC protocol: Allows clients to communicate with existing gRPC services.
  • The gRPC-Web protocol: Allows clients to communicate with existing gRPC-Web services.

Get started with Connect-Kotlin

We’re excited for you to try out Connect-Kotlin! Here are some resources to help you dive in:

Connect-Kotlin is in beta, so we’re all ears for feedback - you can reach us through the Buf Slack or by filing a GitHub issue and we’d be more than happy to chat!

Get started for free with the Buf Schema Registry