Skip to content

Generates a unified GraphQL schema from gRPC microservices and other Protobuf sources


Notifications You must be signed in to change notification settings



Build Status Coverage Status

Maven Central

  • Creates a uniform GraphQL schema from microservices
  • Allows the GraphQL schema to be flexibly defined and composed as shared components
  • Generates GraphQL types from Proto definitions
  • Populates request Proto based on GraphQL query parameters
  • Supplies a DSL to modify the generated schema
  • Joins data sources by annotating methods that fetch data
  • Creates Proto FieldMasks based on GraphQL selectors

Rejoiner Overview

Experimental Features

These features are actively being developed.

  • Expose any GraphQL schema as a gRPC service.
  • Lossless end to end proto scalar types when using gRPC.
  • Relay support [Example]
  • GraphQL Stream (based on gRPC streaming) [Example]

Schema Module

SchemaModule is a Guice module that is used to generate parts of a GraphQL schema. It finds methods and fields that have Rejoiner annotations when it's instantiated. It then looks at the parameters and return type of these methods in order to generate the appropriate GraphQL schema. Examples of queries, mutations, and schema modifications are presented below.

GraphQL Query

final class TodoQuerySchemaModule extends SchemaModule {
  ListenableFuture<ListTodoResponse> listTodo(ListTodoRequest request, TodoClient todoClient) {
    return todoClient.listTodo(request);

In this example request is of type ListTodoRequest (a protobuf message), so it's used as a parameter in the generated GraphQL query. todoService isn't a protobuf message, so it's provided by the Guice injector.

This is useful for providing rpc services or database access objects for fetching data. Authentication data can also be provided here.

Common implementations for these annotated methods:

  • Make gRPC calls to microservices which can be implemented in any language
  • Load protobuf messages directly from storage
  • Perform arbitrary logic to produce the result

GraphQL Mutation

final class TodoMutationSchemaModule extends SchemaModule {
  ListenableFuture<Todo> createTodo(
      CreateTodoRequest request, TodoService todoService, @AuthenticatedUser String email) {
    return todoService.createTodo(request, email);

Adding edges between GraphQL types

In this example we are adding a reference to the User type on the Todo type.

final class TodoToUserSchemaModule extends SchemaModule {
  @SchemaModification(addField = "creator", onType = Todo.class)
  ListenableFuture<User> todoCreatorToUser(UserService userService, Todo todo) {
    return userService.getUserByEmail(todo.getCreatorEmail());

In this case the Todo parameter is the parent object which can be referenced to get the creator's email.

This is how types are joined within and across APIs.

Rejoiner API Joining

Removing a field

final class TodoModificationsSchemaModule extends SchemaModule {
  TypeModification removePrivateTodoData =

Building the GraphQL schema


public final class TodoModule extends AbstractModule {
  protected void configure() {
    // Guice module that provides the generated GraphQLSchema instance
    install(new SchemaProviderModule());

    // Install schema modules
    install(new TodoQuerySchemaModule());
    install(new TodoMutationSchemaModule());
    install(new TodoModificationsSchemaModule());
    install(new TodoToUserSchemaModule());

Getting started

Dependency information

Apache Maven


Gradle/Grails compile ''

Scala SBT libraryDependencies += "" % "rejoiner" % "0.0.4"

Supported return types

All generated proto messages extend Message.

  • Any subclass of Message
  • ImmutableList<? extends Message>
  • ListenableFuture<? extends Message>
  • ListenableFuture<ImmutableList<? extends Message>>

Project information

  • Rejoiner is built on top of GraphQL-Java which provides the core GraphQL capabilities such as query parsing, validation, and execution.
  • Java code is formatted using google-java-format.
  • Note: This is not an official Google product.