async-graphql/docs/en/src/apollo_federation.md

Apollo Federation

Apollo Federation is a GraphQL API gateway which can combine multiple GraphQL services, allowing each service to implement the subset of the API it is responsible for. You can read more in the official documentation.

Async-graphql supports all the functionality of Apollo Federation, but some modifications to your Schema are required.

• You can use the extends property declaration on async_graphql::Object and async_graphql::Interface to extend a type offered by another implementing service.

• The external property declares that a field comes from another service。

• The provides property indicates the fields provided by a service.

Entity lookup function

struct Query;

#[Object]
impl Query {
    #[entity]
    async fn find_user_by_id(&self, id: ID) -> User {
        User { ... }
    }

    #[entity]
    async fn find_user_by_id_with_username(&self, #[arg(key)] id: ID, username: String) -> User {
        User { ... }
    }

    #[entity]
    async fn find_user_by_id_and_username(&self, id: ID, username: String) -> User {
        User { ... }
    }
}

Notice the difference between these three lookup functions, which are all looking for the User object.

• find_user_by_id

  Use id to find an User object, the key for User is id.

• find_user_by_id_with_username

  Use id to find an User object, the key for User is id, and the username field value of the User object is requested.

• find_user_by_id_and_username

  Use id and username to find an User object, the keys for User are id and username.

For a complete example, refer to: https://github.com/async-graphql/examples/tree/master/federation.