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Implementing federation spec (#280)
* Create new graphql-kotlin-federation module that extends graphql-kotlin-schema-generator to generate federated GraphQL schemas. Federated GraphQL architecture is an alternative to schema stitching approach that relies on declarative programming composition of schemas. graphql-kotlin-federation adds support for the following: new @key, @extends, @external, @requires and @provides directives new _FieldSet and _Any scalar types new _Entity union type of all federated types new _service query that returns _Service object with sdl field new _entity query that returns list of _Entity objects In order to generate federate schemas we have to annotate the underlying type with proper new directives and then use new toFederatedSchema function to build the schema (instead of toSchema function from graphql-kotlin-schema-generator). FederatedSchemaGeneratorHooks also require to specify FederatedTypeRegistry that holds a mapping between federated type names and their corresponding resolver. Those new FederatedTypeResolvers are used to instantiate the federated types based on the federated _entity query generated from the gateway. See: https://www.apollographql.com/docs/apollo-server/federation/introduction/ Resolves: #238
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‎README.md‎

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## Modules
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* [graphql-kotlin-federation](graphql-kotlin-federation/README.md) - schema generator extension to build federated GraphQL schemas
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* [graphql-kotlin-schema-generator](graphql-kotlin-schema-generator/README.md) - code only GraphQL schema generation for Kotlin
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* [graphql-kotlin-spring-example](graphql-kotlin-spring-example/README.md) - example SpringBoot app that uses GraphQL Kotlin schema generator
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‎graphql-kotlin-federation/README.md‎

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# GraphQL Kotlin Federated Schema Generator
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`graphql-kotlin-federation` extends the functionality of `graphql-kotlin-schema-generator` and allows you to easily generate federated GraphQL schemas directly from the code. Federated schemas rely on a number of directives to instrument the behavior of the underlying graph, see corresponding wiki pages to learn more about new directives. Once all the federated objects are annotated, you will also have to configure corresponding [FederatedTypeResolver]s that are used to instantiate federated objects and finally generate the schema using `toFederatedSchema` function ([link]).
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See more
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* [Federation Spec](https://www.apollographql.com/docs/apollo-server/federation/federation-spec/)
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## Installation
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Using a JVM dependency manager, simply link `graphql-kotlin-federation` to your project.
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With Maven:
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```xml
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<dependency>
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<groupId>com.expedia</groupId>
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<artifactId>graphql-kotlin-federation</artifactId>
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<version>${latestVersion}</version>
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</dependency>
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```
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With Gradle:
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```groovy
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compile(group: 'com.expedia', name: 'graphql-kotlin-federation', version: "$latestVersion")
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```
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## Usage
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In order to generate valid federated schemas, you will need to annotate both your base schemas and the one extending them. Federated Gateway (e.g. Apollo) will then combine the individual graphs to form single federated graph.
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#### Base Schema
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Base schema defines GraphQL types that will be extended by schemas exposed by other GraphQL services. In the example below, we define base `Product` type with `id` and `description` fields. `id` is the primary key that uniquely identifies the `Product` type object and is specified in `@key` directive.
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```kotlin
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@KeyDirective(fields = FieldSet("id"))
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data class Product(val id: Int, val description: String)
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class ProductQuery {
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fun product(id: Int): Product? {
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// grabs product from a data source, might return null
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}
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}
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// Generate the schema
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val federatedTypeRegistry = FederatedTypeRegistry(emptyMap())
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val config = FederatedSchemaGeneratorConfig(supportedPackages = listOf("org.example"), hooks = FederatedSchemaGeneratorHooks(federatedTypeRegistry))
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val queries = listOf(TopLevelObject(ProductQuery()))
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toFederatedSchema(config, queries)
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```
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Generates the following schema with additional federated types
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```graphql
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schema {
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query: Query
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}
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union _Entity = Product
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type Product @key(fields : "id") {
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description: Int!
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id: String!
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}
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type Query {
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_entities(representations: [_Any!]!): [_Entity]!
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_service: _Service
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product(id: Int!): Product!
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}
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type _Service {
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sdl: String!
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}
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```
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#### Extended Schema
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Extended federated GraphQL schemas provide additional functionality to the types already exposed by other GraphQL services. In the example below, `Product` type is extended to add new `reviews` field to it. Primary key needed to instantiate the `Product` type (i.e. `id`) has to match the `@key` definition on the base type. Since primary keys are defined on the base type and are only referenced from the extended type, all of the fields that are part of the field set specified in `@key` directive have to be marked as `@external`.
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```kotlin
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@KeyDirective(fields = FieldSet("id"))
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@ExtendsDirective
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data class Product(@property:ExternalDirective val id: Int) {
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fun reviews(): List<Review> {
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// returns list of product reviews
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}
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}
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data class Review(val reviewId: String, val text: String)
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// Generate the schema
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val productResolver = object: FederatedTypeResolver<Product> {
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override fun resolve(keys: Map<String, Any>): Product {
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val id = keys["id"]?.toString()?.toIntOrNull()
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// instantiate product using id
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}
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}
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val federatedTypeRegistry = FederatedTypeRegistry(mapOf("Product" to productResolver))
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val config = FederatedSchemaGeneratorConfig(supportedPackages = listOf("org.example"), hooks = FederatedSchemaGeneratorHooks(federatedTypeRegistry))
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toFederatedSchema(config)
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```
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Generates the following federated schema
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```graphql
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schema {
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query: Query
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}
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union _Entity = Product
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type Product @extends @key(fields : "id") {
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id: Int! @external
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reviews: [Review!]!
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}
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type Query {
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_entities(representations: [_Any!]!): [_Entity]!
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_service: _Service
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}
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type Review {
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reviewId: String!
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text: String!
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}
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type _Service {
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sdl: String!
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}
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```
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Federated Gateway will then combine the schemas from the individual services to generate single schema.
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#### Federated GraphQL schema
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```graphql
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schema {
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query: Query
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}
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type Product {
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description: String!
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id: String!
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reviews: [Review!]!
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}
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type Review {
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reviewId: String!
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text: String!
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}
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type Query {
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product(id: String!): Product!
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}
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```
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## Documentation
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There are more examples and documentation in our [Wiki](https://github.com/ExpediaDotCom/graphql-kotlin/wiki) or you can view the [javadocs](https://www.javadoc.io/doc/com.expedia/graphql-kotlin) for all published versions.
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If you have a question about something you can not find in our wiki or javadocs, feel free to [create an issue](https://github.com/ExpediaDotCom/graphql-kotlin/issues) and tag it with the question label.

‎graphql-kotlin-federation/pom.xml‎

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<parent>
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<groupId>com.expedia</groupId>
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<artifactId>graphql-kotlin</artifactId>
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<version>1.0.0-RC4-SNAPSHOT</version>
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<relativePath>..</relativePath>
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</parent>
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<artifactId>graphql-kotlin-federation</artifactId>
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<name>graphql-kotlin-federation</name>
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<packaging>jar</packaging>
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<build>
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<sourceDirectory>${project.basedir}/src/main/kotlin</sourceDirectory>
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<testSourceDirectory>${project.basedir}/src/test/kotlin</testSourceDirectory>
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<plugins>
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<plugin>
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<groupId>org.apache.maven.plugins</groupId>
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<artifactId>maven-antrun-plugin</artifactId>
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</plugin>
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</plugins>
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</build>
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<dependencies>
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<dependency>
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<groupId>com.expedia</groupId>
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<artifactId>graphql-kotlin-schema-generator</artifactId>
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<version>${project.parent.version}</version>
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</dependency>
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<dependency>
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<groupId>org.junit.jupiter</groupId>
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<artifactId>junit-jupiter-params</artifactId>
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<version>${junit-jupiter.version}</version>
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</dependency>
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</dependencies>
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</project>

‎graphql-kotlin-federation/src/main/kotlin/com/expedia/graphql/federation/FederatedSchemaGenerator.kt‎

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package com.expedia.graphql.federation
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import com.expedia.graphql.TopLevelObject
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import com.expedia.graphql.federation.directives.ExtendsDirective
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import com.expedia.graphql.generator.SchemaGenerator
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import graphql.schema.GraphQLSchema
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import org.reflections.Reflections
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import kotlin.reflect.full.createType
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/**
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* Generates federated GraphQL schemas based on the specified configuration.
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*/
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class FederatedSchemaGenerator(generatorConfig: FederatedSchemaGeneratorConfig) : SchemaGenerator(generatorConfig) {
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override fun generate(
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queries: List<TopLevelObject>,
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mutations: List<TopLevelObject>,
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subscriptions: List<TopLevelObject>,
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builder: GraphQLSchema.Builder
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): GraphQLSchema {
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builder.federation(config.supportedPackages)
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return super.generate(queries, mutations, subscriptions, builder)
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}
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/**
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* Scans specified packages for all the federated (extended) types and adds them to the target schema.
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*/
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fun GraphQLSchema.Builder.federation(supportedPackages: List<String>): GraphQLSchema.Builder {
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supportedPackages
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.map { pkg -> Reflections(pkg).getTypesAnnotatedWith(ExtendsDirective::class.java).map { it.kotlin } }
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.flatten()
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.map {
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val graphQLType = if (it.isAbstract) {
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interfaceType(it)
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} else {
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objectType(it)
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}
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// workaround to explicitly apply validation
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config.hooks.didGenerateGraphQLType(it.createType(), graphQLType)
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}
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.forEach {
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this.additionalType(it)
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}
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return this
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}
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}

‎graphql-kotlin-federation/src/main/kotlin/com/expedia/graphql/federation/FederatedSchemaGeneratorConfig.kt‎

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package com.expedia.graphql.federation
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import com.expedia.graphql.SchemaGeneratorConfig
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import com.expedia.graphql.TopLevelNames
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import com.expedia.graphql.execution.KotlinDataFetcherFactoryProvider
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/**
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* Settings for generating the federated schema.
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*/
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class FederatedSchemaGeneratorConfig(
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override val supportedPackages: List<String>,
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override val topLevelNames: TopLevelNames = TopLevelNames(),
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override val hooks: FederatedSchemaGeneratorHooks,
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override val dataFetcherFactoryProvider: KotlinDataFetcherFactoryProvider = KotlinDataFetcherFactoryProvider(hooks)
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) : SchemaGeneratorConfig(supportedPackages, topLevelNames, hooks, dataFetcherFactoryProvider)

‎graphql-kotlin-federation/src/main/kotlin/com/expedia/graphql/federation/FederatedSchemaGeneratorHooks.kt‎

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package com.expedia.graphql.federation
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import com.expedia.graphql.annotations.GraphQLName
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import com.expedia.graphql.extensions.print
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import com.expedia.graphql.federation.directives.FieldSet
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import com.expedia.graphql.federation.types.ANY_SCALAR_TYPE
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import com.expedia.graphql.federation.types.FIELD_SET_SCALAR_TYPE
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import com.expedia.graphql.federation.types.SERVICE_FIELD_DEFINITION
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import com.expedia.graphql.federation.types._Service
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import com.expedia.graphql.federation.types.generateEntityFieldDefinition
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import com.expedia.graphql.hooks.SchemaGeneratorHooks
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import graphql.TypeResolutionEnvironment
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import graphql.schema.DataFetcher
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import graphql.schema.FieldCoordinates
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import graphql.schema.GraphQLCodeRegistry
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import graphql.schema.GraphQLObjectType
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import graphql.schema.GraphQLSchema
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import graphql.schema.GraphQLType
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import kotlin.reflect.KType
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import kotlin.reflect.full.findAnnotation
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/**
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* Hooks for generating federated GraphQL schema.
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*/
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open class FederatedSchemaGeneratorHooks(private val federatedTypeRegistry: FederatedTypeRegistry) : SchemaGeneratorHooks {
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private val validator = FederatedSchemaValidator()
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override fun willGenerateGraphQLType(type: KType): GraphQLType? = when (type.classifier) {
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FieldSet::class -> FIELD_SET_SCALAR_TYPE
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else -> null
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}
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override fun didGenerateGraphQLType(type: KType, generatedType: GraphQLType): GraphQLType {
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validator.validateGraphQLType(generatedType)
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return super.didGenerateGraphQLType(type, generatedType)
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}
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override fun willBuildSchema(builder: GraphQLSchema.Builder): GraphQLSchema.Builder {
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val originalSchema = builder.build()
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val originalQuery = originalSchema.queryType
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val federatedSchema = GraphQLSchema.newSchema(originalSchema)
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val federatedQuery = GraphQLObjectType.newObject(originalQuery)
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.field(SERVICE_FIELD_DEFINITION)
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val federatedCodeRegistry = GraphQLCodeRegistry.newCodeRegistry(originalSchema.codeRegistry)
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val entityTypeNames = originalSchema.allTypesAsList
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.asSequence()
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.filterIsInstance<GraphQLObjectType>()
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.filter { type -> type.getDirective("key") != null }
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.map { it.name }
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.toSet()
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// register new federated queries
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if (entityTypeNames.isNotEmpty()) {
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val entityField = generateEntityFieldDefinition(entityTypeNames)
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federatedQuery.field(entityField)
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val sdl = originalSchema.print()
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federatedCodeRegistry.dataFetcher(FieldCoordinates.coordinates(originalQuery.name, SERVICE_FIELD_DEFINITION.name), DataFetcher { _Service(sdl) })
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federatedCodeRegistry.dataFetcher(FieldCoordinates.coordinates(originalQuery.name, entityField.name), DataFetcher {
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val representations: List<Map<String, Any>> = it.getArgument("representations")
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representations.map { representation ->
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val type = representation["__typename"]?.toString() ?: throw FederationException("invalid _entity query - missing __typename in the representation, representation=$representation")
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val resolver = federatedTypeRegistry.getFederatedResolver(type) ?: throw FederationException("Federation exception - cannot find resolver for $type")
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resolver.resolve(representation)
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}.toList()
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})
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federatedCodeRegistry.typeResolver("_Entity") { env: TypeResolutionEnvironment -> env.schema.getObjectType(env.getObjectName()) }
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federatedSchema.additionalType(ANY_SCALAR_TYPE)
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}
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return federatedSchema.query(federatedQuery.build())
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.codeRegistry(federatedCodeRegistry.build())
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}
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}
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private fun TypeResolutionEnvironment.getObjectName(): String? {
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val kClass = this.getObject<Any>().javaClass.kotlin
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return kClass.findAnnotation<GraphQLName>()?.value
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?: kClass.simpleName
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}

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