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Federated schemas

Overview


A federated uses multiple "types" of :

(Composition succeeds)
(Remove routing machinery)
Subgraph
schema
A
Subgraph
schema
B
Subgraph
schema
C
🛠
Composition
Supergraph schema
(A + B + C + routing machinery)
API schema
(A + B + C)

  • Subgraph schemas. Each subgraph has a distinct schema that indicates which types and of your composed supergraph it can resolve.
    • These are the only schemas that your teams define manually.
  • Supergraph schema. This schema combines all of the types and from your , plus some federation-specific information that tells your which can resolve which .
    • This schema is the result of performing composition on your collection of .
  • API schema. This schema is similar to the , but it omits federation-specific types, , and that are considered "machinery" and are not part of your public API.
    • This is the schema that your exposes to clients, which don't need to know internal implementation details about your .

Let's look at an example!

Subgraph schemas

Below are example schemas for three in an e-commerce company's . Each subgraph is implemented as a separate API:

Users
type Query {
me: User
}
type User @key(fields: "id") {
id: ID!
username: String! @shareable
}
# (Subgraph schemas include
# this to opt in to
# Federation 2 features.)
extend schema
@link(url: "https://specs.apollo.dev/federation/v2.3",
import: ["@key", "@shareable"])
Products
type Query {
topProducts(first: Int = 5): [Product]
}
type Product @key(fields: "upc") {
upc: String!
name: String!
price: Int
}
extend schema
@link(url: "https://specs.apollo.dev/federation/v2.3",
import: ["@key", "@shareable"])
Reviews
type Review {
body: String
author: User @provides(fields: "username")
product: Product
}
type User @key(fields: "id") {
id: ID!
username: String! @external
reviews: [Review]
}
type Product @key(fields: "upc") {
upc: String!
reviews: [Review]
}
# (This subgraph uses additional
# federated directives)
extend schema
@link(url: "https://specs.apollo.dev/federation/v2.3",
import: ["@key", "@shareable", "@provides", "@external"])

As these schemas show, multiple can contribute unique to a single type. For example, the Products subgraph and the Reviews subgraph both contribute fields to the Product type.

Supergraph schema

The is the output of schema composition. It serves the following purposes:

  • It provides your with the name and endpoint URL for each of your .
  • It includes all types and defined by all of your .
  • It tells your which of your can resolve which .

Here's the composed with the subgraph schemas above:

As you can see, the includes a lot of Federation-specific additions! These additions are used only by the , and you'll never need to add them manually.

API schema

The uses its supergraph schema to produce an API schema, which it exposes to clients as your actual API. This schema cleanly and logically represents the combination of your subgraph schemas:

type Product {
name: String!
price: Int
reviews: [Review]
upc: String!
}
type Query {
me: User
topProducts(first: Int = 5): [Product]
}
type Review {
author: User
body: String
product: Product
}
type User {
id: ID!
reviews: [Review]
username: String!
}

Unlike the , this schema hides the fact that your API is composed of multiple distinct APIs.

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