# Apollo Federation

Apollo Federation is a GraphQL architecture for combining multiple GraphQL services, or subgraphs, into a single supergraph. You can read more in the [official documentation](https://www.apollographql.com/docs/apollo-server/federation/).

> To see a complete example of federation, check out the [federation example](https://github.com/async-graphql/examples/tree/master/federation). 

## Enabling federation support

`async-graphql` supports all the functionality of Apollo Federation v2. Support will be enabled automatically if any `#[graphql(entity)]` resolvers are found in the schema. To enable it manually, use the `enable_federation` method on the `SchemaBuilder`.

```rust
# extern crate async_graphql;
# use async_graphql::*;
# struct Query;
# #[Object]
# impl Query {
#    async fn hello(&self) -> String { "Hello".to_string() }
# }
fn main() {
  let schema = Schema::build(Query, EmptyMutation, EmptySubscription)
    .enable_federation()
    .finish();
  // ... Start your server of choice
}
```

This will define the [`@link` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#link) on your schema to enable Federation v2.

## Entities and `@key`

[Entities](https://www.apollographql.com/docs/federation/entities) are a core feature of federation, they allow multiple subgraphs to contribute fields to the same type. An entity is a GraphQL `type` with at least one [`@key` directive][`@key`]. To create a [`@key`] for a type, create a reference resolver using the `#[graphql(entity)]` attribute. This resolver should be defined on the `Query` struct, but will not appear as a field in the schema.

### Example

```rust
# extern crate async_graphql;
# use async_graphql::*;
# #[derive(SimpleObject)]
# struct User { id: ID }
struct Query;

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

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

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

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

- `find_user_by_id`: Use `id` to find a `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 (e.g., via `@external` and `@requires`).

- `find_user_by_id_and_username`: Use `id` and `username` to find an `User` object, the keys for `User` are `id` and `username`.

The resulting schema will look like this:

```graphql
type Query {
  # These fields will not be exposed to users, they are only used by the router to resolve entities
  _entities(representations: [_Any!]!): [_Entity]!
  _service: _Service!
}

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

### Defining a compound primary key

A single primary key can consist of multiple fields, and even nested fields, you can use `InputObject` to implements a nested primary key.

In the following example, the primary key of the `User` object is `key { a b }`.

```rust
# extern crate async_graphql;
# use async_graphql::*;
# #[derive(SimpleObject)]
# struct User { key: Key }
# #[derive(SimpleObject)]
# struct Key { a: i32, b: i32 }
#[derive(InputObject)]
struct NestedKey {
  a: i32,
  b: i32,
}

struct Query;

#[Object]
impl Query {
  #[graphql(entity)]
  async fn find_user_by_key(&self, key: NestedKey) -> User {
    let NestedKey { a, b } = key;
    User { key: Key{a, b} }
  }
}
```

The resulting schema will look like this:

```graphql
type Query {
  # These fields will not be exposed to users, they are only used by the router to resolve entities
  _entities(representations: [_Any!]!): [_Entity]!
  _service: _Service!
}

type User @key(fields: "key { a b }") {
  key: Key!
}

type Key {
  a: Int!
  b: Int!
}
```

## `@shareable`

Apply the [`@shareable` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#shareable) to a type or field to indicate that multiple subgraphs can resolve it.

### `@shareable` fields
```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
#[graphql(complex)]
struct Position {
  #[graphql(shareable)]
  x: u64,
}

#[ComplexObject]
impl Position {
  #[graphql(shareable)]
  async fn y(&self) -> u64 {
    0
  }
}
```

The resulting schema will look like this:

```graphql
type Position {
  x: Int! @shareable
  y: Int! @shareable
}
```


### `@shareable` type

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
#[graphql(shareable)]
struct Position {
  x: u64,
  y: u64,
}
```

The resulting schema will look like this:

```graphql
type Position @shareable {
  x: Int!
  y: Int!
}
```

## `@inaccessible`

The [`@inaccessible` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#inaccessible) is used to omit something from the supergraph schema (e.g., if it's not yet added to all subgraphs which share a `@shareable` type).

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
#[graphql(shareable)]
struct Position {
  x: u32,
  y: u32,
  #[graphql(inaccessible)]
  z: u32,
} 
```

Results in:

```graphql
type Position @shareable {
  x: Int!
  y: Int!
  z: Int! @inaccessible
}
```

## `@override`

The [`@override` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#override) is used to take ownership of a field from another subgraph. This is useful for migrating a field from one subgraph to another.

For example, if you add a new "Inventory" subgraph which should take over responsibility for the `inStock` field currently provided by the "Products" subgraph, you might have something like this:

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
struct Product {
  id: ID,
  #[graphql(override_from = "Products")]
  in_stock: bool,
}
```

Which results in:

```graphql
type Product @key(fields: "id") {
  id: ID!
  inStock: Boolean! @override(from: "Products")
}
```

## `@external`

The [`@external` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#external) is used to indicate that a field is usually provided by another subgraph, but is sometimes required by this subgraph (when combined with `@requires`) or provided by this subgraph (when combined with `@provides`).

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
struct Product {
  id: ID,
  #[graphql(external)]
  name: String,
  in_stock: bool,
}
```

Results in:

```graphql
type Product {
  id: ID!
  name: String! @external
  inStock: Boolean!
}
```

## `@provides`

The [`@provides` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#provides) is used to indicate that a field is provided by this subgraph, but only sometimes.

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
struct Product {
    id: ID,
    #[graphql(external)]
    human_name: String,
    in_stock: bool,
}

struct Query;

#[Object]
impl Query {
    /// This operation will provide the `humanName` field on `Product
    #[graphql(provides = "humanName")]
    async fn out_of_stock_products(&self) -> Vec<Product> {
      vec![Product {
        id: "1".into(),
        human_name: "My Product".to_string(),
        in_stock: false,
      }]
    }
    async fn discontinued_products(&self) -> Vec<Product> {
        vec![Product {
            id: "2".into(),
            human_name: String::new(),  // This is ignored by the router
            in_stock: false,
        }]
    }
    #[graphql(entity)]
    async fn find_product_by_id(&self, id: ID) -> Product {
        Product {
            id,
            human_name: String::new(),  // This is ignored by the router
            in_stock: true,
        }
    }
}
```

Note that the `#[graphql(provides)]` attribute takes the field name as it appears in the schema, not the Rust field name.

The resulting schema will look like this:

```graphql
type Product @key(fields: "id") {
    id: ID!
    humanName: String! @external
    inStock: Boolean!
}

type Query {
    outOfStockProducts: [Product!]! @provides(fields: "humanName")
    discontinuedProducts: [Product!]!
}
```

## `@requires`

The [`@requires` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#requires) is used to indicate that an `@external` field is required for this subgraph to resolve some other field(s). If our `shippingEstimate` field requires the `size` and `weightInPounts` fields, then we might want a subgraph entity which looks like this:

```graphql
type Product @key(fields: "id") {
  id: ID!
  size: Int! @external
  weightInPounds: Int! @external
  shippingEstimate: String! @requires(fields: "size weightInPounds")
}
```

In order to implement this in Rust, we can use the `#[graphql(requires)]` attribute:

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
#[graphql(complex)]
struct Product {
  id: ID,
  #[graphql(external)]
  size: u32,
  #[graphql(external)]
  weight_in_pounds: u32,
}

#[ComplexObject]
impl Product {
  #[graphql(requires = "size weightInPounds")]
  async fn shipping_estimate(&self) -> String {
    let price = self.size * self.weight_in_pounds;
    format!("${}", price)
  }
}
```

Note that we use the GraphQL field name `weightInPounds`, not the Rust field name `weight_in_pounds` in `requires`. To populate those external fields, we add them as arguments in the entity resolver:

```rust
# extern crate async_graphql;
# use async_graphql::*;
# #[derive(SimpleObject)]
# struct Product {
#     id: ID,
#     #[graphql(external)]
#     size: u32,
#     #[graphql(external)]
#     weight_in_pounds: u32,
# }
# struct Query;
#[Object]
impl Query {
  #[graphql(entity)]
  async fn find_product_by_id(
    &self, 
    #[graphql(key)] id: ID, 
    size: Option<u32>, 
    weight_in_pounds: Option<u32>
  ) -> Product {
    Product {
      id,
      size: size.unwrap_or_default(),
      weight_in_pounds: weight_in_pounds.unwrap_or_default(),
    }
  }
}
```

The inputs are `Option<>` even though the fields are required. This is because the external fields are _only_ passed to the subgraph when the field(s) that require them are being selected. If the `shippingEstimate` field is not selected, then the `size` and `weightInPounds` fields will not be passed to the subgraph. **Always use optional types for external fields.**

We have to put _something_ in place for `size` and `weight_in_pounds` as they are still required fields on the type, so we use `unwrap_or_default()` to provide a default value. This looks a little funny, as we're populating the fields with nonsense values, but we have confidence that they will not be needed if they were not provided.  **Make sure to use `@requires` if you are consuming `@external` fields, or your code will be wrong.**

### Nested `@requires`

A case where the `@requires` directive can be confusing is when there are nested entities. For example, if we had an `Order` type which contained a `Product`, then we would need an entity resolver like this:

```rust
# extern crate async_graphql;
# use async_graphql::*;
# #[derive(SimpleObject)]
# pub struct Order { id: ID }
# struct Query;
#[Object]
impl Query {
  #[graphql(entity)]
  async fn find_order_by_id(&self, id: ID) -> Option<Order> {
      Some(Order { id })
  }
}
```

There are no inputs on this entity resolver, so how do we populate the `size` and `weight_in_pounds` fields on `Product` if a user has a query like `order { product { shippingEstimate } }`? The supergraph implementation will solve this for us by calling the `find_product_by_id` separately for any fields which have a `@requires` directive, so the subgraph code does not need to worry about how entities relate.

## `@tag`

The [`@tag` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#tag) is used to add metadata to a schema location for features like [contracts](https://www.apollographql.com/docs/studio/contracts/). To add a tag like this:

```graphql
type User @tag(name: "team-accounts") {
  id: String!
  name: String!
}
```

You can write code like this:

```rust
# extern crate async_graphql;
# use async_graphql::*;
#[derive(SimpleObject)]
#[graphql(tag = "team-accounts")]
struct User {
  id: ID,
  name: String,
}
```

[`@key`]: https://www.apollographql.com/docs/federation/entities#1-define-a-key