How can I create an entity fetcher for an interface? Also using existing batch data loader if possible.

[SaranSundar]

For our location service our schema looks like this

interface Location {
    id: Int!
    type: LocationEnum
    name: String
    shortName: String
    longName: String
    seoAlias: String
}

type City implements Location @key(fields: "id")  {
    id: Int!
    type: LocationEnum
    name: String
    shortName: String
    longName: String
    state: State
}

type State implements Location @key(fields: "id")  {
    id: Int!
    type: LocationEnum
    name: String
    shortName: String
    longName: String
}

Dgs class like so

@DgsComponent
public class AtlasFederatedDataFetchersImpl implements AtlasFederatedDataFetchers {

  @Override
  @DgsEntityFetcher(name = "City")
  public CompletableFuture<City> findCityById(
      Map<String, Object> parameters, DataFetchingEnvironment dfe) {
    DataLoader<Integer, City> dataLoader = dfe.getDataLoader("city");
    Integer id = (Integer) parameters.get("id");

    return dataLoader.load(id);
  }
  
    @Override
  @DgsEntityFetcher(name = "State")
  public CompletableFuture<State> findStateById(
      Map<String, Object> parameters, DataFetchingEnvironment dfe) {
    DataLoader<Integer, State> dataLoader = dfe.getDataLoader("state");
    Integer id = (Integer) parameters.get("id");

    return dataLoader.load(id);
  }
  }
  }

In a different subgraph we have a type that has a property

queryLocation: Location

Our old original graph built on bare bones graphql java does this to federate the Location interface

    @Override
    public RuntimeWiring.Builder wire(RuntimeWiring.Builder builder) {
        GQLRegUtil.with(builder)
                .query(query)
                .addFieldDataFetchers(City.class, cityResolver)
                .addFieldDataFetchers(Metro.class, metroResolver)
                .addFieldDataFetchers(State.class, stateResolver)
                .addFieldDataFetchers(Country.class, countryResolver);

        builder.type(newTypeWiring("Location", this::LocationTypeResolver));

        return builder;
    }

    private TypeRuntimeWiring.Builder LocationTypeResolver(TypeRuntimeWiring.Builder builder) {
        return builder.typeResolver(env -> {
            final Object obj = env.getObject();
            if (obj instanceof City) {
                return env.getSchema().getObjectType("City");
            } else if (obj instanceof Metro) {
                return env.getSchema().getObjectType("Metro");
            } else if (obj instanceof State) {
                return env.getSchema().getObjectType("State");
            } else if (obj instanceof Country) {
                return env.getSchema().getObjectType("Country");
            } else {
                throw new IllegalStateException("Unknown Location Type: " + obj.getClass().getName());
            }
        });
    }

    @Override
    public DataLoaderRegistry registerDataLoaders(DataLoaderRegistry registry) {
        return atlasDataLoaders.initialize(registry);
    }

How can I federate the Location interface in dgs? Preferably with a batch data loader if possible that will hook into the existing city and state batch data loader?

[coleturner]

Hey @SaranSundar what you have so far is looking like a great start and matches what is mentioned in the Federation documentation. You need an entity fetcher for every concrete type that implements Location, and DGS will resolve the type using the default type resolver.

If you try the "Testing a Federated query", writing a federated query against your Location interface, does it succeed?

[SaranSundar]

Ah thanks i found out i don't to create a seperate entity fetcher for Location, apollo graph gateway first resolves interfaces into a type before resolving the type