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API Reference: @apollo/federation

Apollo Federation API reference


This API reference documents the exports from the @apollo/federation package.

buildSubgraphSchema

This method was renamed from buildFederatedSchema after @apollo/federation v0.28.0 (the previous name still works but might be removed in a future release).

A function that takes an array of GraphQL schema modules and returns a federation-ready schema based on those modules:

const server = new ApolloServer({
  schema: buildSubgraphSchema([{ typeDefs, resolvers }])});

Used when defining a subgraph in a federated graph.

Each schema module is an object with the following format:

{
  typeDefs: DocumentNode,
  resolvers: ResolverMap
}

Parameters

Name /
Type
Description
modules

Array

Required. An array of schema module objects with the structure shown above.

Example

const typeDefs = gql`
  type Query {
    me: User
  }

  type User @key(fields: "id") {
    id: ID!
    username: String
  }
`;

const resolvers = {
  Query: {
    me() {
      return { id: "1", username: "@ava" }
    }
  },
  User: {
    __resolveReference(user, { fetchUserById }){
      return fetchUserById(user.id)
    }
  }
};

const server = new ApolloServer({
  schema: buildSubgraphSchema([{ typeDefs, resolvers }])
});

__resolveReference

The name of a special reference resolver function you can define for every entity in a resolver map, if that resolver map is part of a subgraph schema.

The __resolveReference function enables your gateway's query planner to resolve a particular entity by whatever unique identifier your other subgraphs use to reference it. For details, see Entities: Resolving.

The function takes the parameters listed below.

Parameters

Name /
Type
Description
reference

Object

The representation of the entity that's passed from another subgraph.

This object includes a __typename field, along with whichever fields the subgraph uses for the entity's @key.

context

Object

An object that's passed to every resolver that executes for a particular operation. This enables resolvers to share helpful context, including any relevant DataSources.

For details, see The context argument.

info

Object

Contains information about the operation's execution state, including the field name, the path to the field from the root, and more.

This object's core fields are listed in the GraphQL.js source code, and it is extended with additional functionality by other modules, like apollo-cache-control.

Example

const typeDefs = gql`
  type User @key(fields: "id") {
    id: ID!
    username: String
  }
`;

const resolvers = {
  User: {
    __resolveReference(user, { datasources }){
      // user will always have at least the `id` and the `__typename` here
      return datasources.users.fetchUserById(user.id)
    }
  }
};
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