# 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](https://www.apollographql.com/docs/apollo-server/federation/introduction).
`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
```rust
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, #[graphql(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>.