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Merge pull request #1080 from dbanty/update-federation-docs 

docs: Update federation docs with examples of each directive
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docs/en/src/apollo_federation.md
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@ -1,26 +1,36 @@
# 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).
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/).
`Async-graphql` supports all the functionality of `Apollo Federation v2`, but some modifications to your `Schema` are required.
> To see a complete example of federation, check out the [federation example](https://github.com/async-graphql/examples/tree/master/federation).
- You can use the `extends` property declaration on `async_graphql::Object` and `async_graphql::Interface` to extend a type offered by another implementing service.
## Enabling federation support
- The `external` property declares that a field comes from another service。
`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`.
- The `provides` directive is used to annotate the expected returned fieldset from a field on a base type that is guaranteed to be selectable by the gateway.
```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
}
```
- The `requires` directive is used to annotate the required input fieldset from a base type for a resolver. It is used to develop a query plan where the required fields may not be needed by the client, but the service may need additional information from other services.
This will define the [`@link` directive](https://www.apollographql.com/docs/federation/federated-types/federated-directives#link) on your schema to enable Federation v2.
- The `shareable` directive is used to indicate that an object type's field is allowed to be resolved by multiple subgraphs (by default, each field can be resolved by only one subgraph).
## Entities and `@key`
- The `inaccessible` directive is used to indicate that a location in the schema cannot be queried at the supergraph level, but can still be queried at the subgraph level.
[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.
- The `tag` directive is used to provide a mechanism for applying arbitrary string metadata to the fields and types of a schema. Tags will be propagated up into composed supergraphs.
- The `override` directive is used to indicate that a field is now to be resolved by the current subgraph instead of the named subgraph.
## Entity lookup function
### Example
```rust
# extern crate async_graphql;
@ -50,21 +60,27 @@ impl Query {
**Notice the difference between these three lookup functions, which are all looking for the `User` object.**
- `find_user_by_id`
- `find_user_by_id`: Use `id` to find a `User` object, the key for `User` is `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 (e.g., via `@external` and `@requires`).
- `find_user_by_id_with_username`
- `find_user_by_id_and_username`: Use `id` and `username` to find an `User` object, the keys for `User` are `id` and `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.
The resulting schema will look like this:
- `find_user_by_id_and_username`
```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!
}
Use `id` and `username` to find an `User` object, the keys for `User` are `id` and `username`.
type User @key(fields: "id") @key(fields: "id username") {
id: ID!
}
```
For a complete example, refer to: <https://github.com/async-graphql/examples/tree/master/federation>.
## Defining a compound primary key
### 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.
@ -74,7 +90,9 @@ In the following example, the primary key of the `User` object is `key { a b }`.
# extern crate async_graphql;
# use async_graphql::*;
# #[derive(SimpleObject)]
# struct User { id: i32 }
# struct User { key: Key }
# #[derive(SimpleObject)]
# struct Key { a: i32, b: i32 }
#[derive(InputObject)]
struct NestedKey {
a: i32,
@ -87,7 +105,346 @@ struct Query;
impl Query {
#[graphql(entity)]
async fn find_user_by_key(&self, key: NestedKey) -> User {
User { id: key.a }
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