# graphql-composition
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An implementation of [GraphQL federated schema composition](https://www.apollographql.com/docs/federation/federated-types/composition/).
## Example
```rust
use graphql_composition::{Subgraphs, compose};
fn main() {
let user_subgraph = r#"
extend schema
@link(url: "https://specs.apollo.dev/federation/v2.3",
import: ["@key"])
type Query {
findUserByEmail(email: String!): User
}
type User @key(fields: "id") {
id: ID!
name: String!
}
"#;
let cart_subgraph = r#"
extend schema
@link(url: "https://specs.apollo.dev/federation/v2.3",
import: ["@key", "@shareable"])
type User @key(fields: "id") {
id: ID!
cart: Cart
}
type Cart @shareable {
items: [String!]!
}
"#;
let [user_subgraph, cart_subgraph] = [user_subgraph, cart_subgraph]
.map(|sdl| async_graphql_parser::parse_schema(&sdl).unwrap());
let mut subgraphs = Subgraphs::default();
subgraphs.ingest(&user_subgraph, "users-service", "http://users.example.com");
subgraphs.ingest(&cart_subgraph, "carts-service", "http://carts.example.com");
let composed = compose(&subgraphs).into_result().unwrap().to_sdl().unwrap();
let expected = r#"
directive @core(feature: String!) repeatable on SCHEMA
directive @join__owner(graph: join__Graph!) on OBJECT
directive @join__type(
graph: join__Graph!
key: String!
resolvable: Boolean = true
directive @join__field(
graph: join__Graph
requires: String
provides: String
) on FIELD_DEFINITION
directive @join__graph(name: String!, url: String!) on ENUM_VALUE
enum join__Graph {
USERS_SERVICE @join__graph(name: "users-service", url: "http://users.example.com")
CARTS_SERVICE @join__graph(name: "carts-service", url: "http://carts.example.com")
}
type User
@join__type(graph: USERS_SERVICE, key: "id")
@join__type(graph: CARTS_SERVICE, key: "id")
{
cart: Cart @join__field(graph: CARTS_SERVICE)
id: ID!
name: String! @join__field(graph: USERS_SERVICE)
}
type Cart {
items: [String!]!
}
type Query {
findUserByEmail(email: String!): User @join__field(graph: USERS_SERVICE)
}
"#;
assert_eq!(expected.trim(), composed.trim());
}
```
## Features
- The output is a structured, serializable format with all relevant relationships (FederatedGraph).
- FederatedGraph can be rendered to GraphQL SDL (see the example above).
- Good composition error messages.
## Status
The crate is being actively developed and maintained.