Apollo Federation

In many cases, exposing single GraphQL API that exposes unified view of all the available data provides tremendous value to their clients. As the underlying graph scales, managing single monolithic GraphQL server might become less and less feasible making it much harder to manage and leading to unnecessary bottlenecks. Migrating towards federated model with an API gateway and a number of smaller GraphQL services behind it alleviates some of those problems and allows teams to scale their graphs more easily.

Apollo Federation is an architecture for composing multiple GraphQL services into a single graph. Federated schemas rely on a number of custom directives to instrument the behavior of the underlying graph and convey the relationships between different schema types. Each individual GraphQL server generates a valid GraphQL schema and can be run independently. This is in contrast with traditional schema stitching approach where relationships between individual services, i.e. linking configuration, is configured at the GraphQL Gateway level.

Install

Using a JVM dependency manager, link graphql-kotlin-federation to your project.

With Maven:

<dependency>
  <groupId>com.expediagroup</groupId>
  <artifactId>graphql-kotlin-federation</artifactId>
  <version>${latestVersion}</version>
</dependency>

With Gradle:

implementation("com.expediagroup", "graphql-kotlin-federation", latestVersion)

Usage

graphql-kotlin-federation build on top of graphql-kotlin-schema-generator and adds a few extra methods and class to use to generate federation compliant schemas.

toFederatedSchema

Just like the basic toSchema, toFederatedSchema accepts four parameters: config, queries, mutations and subscriptions. The difference is that the config class is of type FederatedSchemaGeneratorConfig. This class extends the base configuration class and adds some default logic. You can override the logic if needed, but do so with caution as you may no longer generate a spec compliant schema.

You can see the definition for toFederatedSchema in the source

Example

@KeyDirective(fields = FieldSet("id"))
data class User(
  val id: ID,
  val name: String
)

class Query {
  fun getUsers(): List<User> = getUsersFromDB()
}

val config = FederatedSchemaGeneratorConfig(
  supportedPackages = "com.example"
)

toFederatedSchema(
  config = config,
  queries = listOf(TopLevelObject(Query()))
)

will generate

type Query {
 getUsers: [User!]!
}

type User @key(fields : "id") {
 id: ID!
 name: String!
}