graphql-federation-example

An example repository using GraphQL Federation of multiple REST API's

Must read

https://www.apollographql.com/docs/federation/subgraphs/

Cross referencing resolvers

Although its not in this example a resolver may need to get data from a different sub graph. For example, if you wanted to query match history and as part of that you needed to resolve a user type

query GetMatchHistory ($userId: ID!) {
  matchHistory (id: $userId) {
    matchID {
      players: {
        id
        username
      }
    }
  }
}

The match subgraph wouldn't know how to resolve those users, to do this you need to connect the subgraphs by each having a resolving function that lets other subgraphs resolve data from them across types, also known as __resolveReference.

This means in the user subgraph you setup a __resolveReference resolver like:

// User subgraph
const resolvers = {
  User: {
    __resolveReference(reference) {
      return fetch(`/user/${reference.id}`);
    }
  },
  // ...
}

Then inside the resolver for the match history you tell it the typename and required key:

// Match subgraph
const resolvers = {
  Match: {
    user(match) {
      return {
        __typename: "User",
        id: match.playerId
      };
    }
  },
  // ...
}

This way the supergraph thats called the match graphql subgraph knows where to go next for its data and the keys are already present to fetch it.

What about the elixir GraphQL API...

Well I haven't found out how todo this nicely with apollo in my 1 day timebox, but did come across this interesting article which is a service that allows even easier stitching of graphql services by using directives for rest and graphql services instead of having apollo subgraphs everywhere, its a worthwhile read. https://stepzen.com/blog/schema-stitching-and-federation-not-the-best-solutions-for-graphql-microservices