Value types in Apollo Federation

In a federated graph, it's common to want to reuse a GraphQL type across multiple subgraphs.

For example, suppose you want to define and reuse a generic Position type in different subgraphs:

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type Position {
2
  x: Int!
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  y: Int!
4
}

Types like this are called value types. This article describes how to share value types and their fields in federated graph, enabling multiple subgraphs to define and resolve them.

Sharing object types

By default in Federation 2 subgraphs, a single object field can't be defined or resolved by more than one subgraph schema.

Consider the following Position example:

❌

type Position {
  x: Int!
  y: Int!
}

type Position {
  x: Int!
  y: Int!
}

Attempting to compose these two subgraph schemas together will break composition. The gateway doesn't know which subgraph is responsible for resolving Position.x and Position.y. To enable multiple subgraphs to resolve these fields, you must first mark that field as @shareable.

Using @shareable

The @shareable directive enables multiple subgraphs to resolve a particular object field (or set of object fields).

To use @shareable in a subgraph schema, you first need to add the following snippet to that schema to opt in to Federation 2:

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extend schema
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  @link(url: "https://specs.apollo.dev/federation/v2.0",
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        import: ["@key", "@shareable"])

Then you can apply the @shareable directive to an object type, or to individual fields of that type:

✅

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

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

Both subgraphs A and B can now resolve the x and y fields for the Position type, and our subgraph schemas will successfully compose into a supergraph schema.

⚠️ Important considerations for @shareable

• If a type or field is marked @shareable in any subgraph, it must be marked @shareable or @external in every subgraph that defines it. Otherwise, composition fails.
• If multiple subgraphs can resolve a field, make sure each subgraph's resolver for that field behaves identically. Otherwise, queries might return inconsistent results depending on which subgraph resolves the field.

Differing shared fields

Shared fields can differ between subgraphs in specific ways:

• The return type of a shared field can vary in nullability (String / String!).
  • A shared field's return type can't vary in its core type (Int vs. String) or in whether it returns a list (Int vs. [Int]).
• A field can be omitted from a subgraph, IF that field is always resolvable.

For example, take a look at the shared Food type below:

type Food @shareable {
  name: String!
  price: Int!
}

type Food @shareable {
  name: String!
  price: Int # Nullable
  inStock: Boolean! # Not in A
}

The above Food types differ in the nullability of their fields and the fields included in each type.

Return types

Let's say two subgraphs both define an Event object type with a timestamp field:

❌

type Event @shareable {
  timestamp: Int!
}

type Event @shareable {
  timestamp: String!
}

Subgraph A's timestamp returns an Int, and Subgraph B's returns a String. This is invalid. When composition attempts to generate an Event type for the supergraph schema, it fails due to an unresolvable conflict between the two timestamp field definitions.

Next, look at these varying definitions for the Position object type:

✅

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

type Position @shareable {
  x: Int
  y: Int
}

The x and y fields are non-nullable in Subgraph A, but they're nullable in Subgraph B. This is valid! Composition recognizes that it can use the following definition for Position in the supergraph schema:

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type Position {
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  x: Int
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  y: Int
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}

This definition works for querying Subgraph A, because Subgraph A's definition is more restrictive than this (a non-nullable value is always valid for a nullable field). In this case, composition coerces Subgraph A's Position fields to satisfy the reduced restrictiveness of Subgraph B.

Arguments

Arguments for a shared field can differ between subgraphs in certain ways:

• If an argument is required in at least one subgraph, it can be optional in other subgraphs. It cannot be omitted.
• If an argument is optional in every subgraph where it's defined, it can be omitted in other subgraphs. However:
  • ⚠️ If a field argument is omitted from any subgraph, that argument is omitted from the supergraph schema entirely! This means that clients can't provide the argument for that field.

✅

type Building @shareable {
  # Argument is required
  height(units: String!): Int!
}

type Building @shareable {
  # Argument can be optional
  height(units: String): Int!
}

❌

type Building @shareable {
  # Argument is required
  height(units: String!): Int!
}

type Building @shareable {
  # ⚠️ Argument can't be omitted! ⚠️
  height: Int!
}

⚠️

type Building @shareable {
  # Argument is optional
  height(units: String): Int!
}

type Building @shareable {
  # Argument can be omitted, BUT
  # it doesn't appear in the
  # supergraph schema!
  height: Int!
}

For more information, see Input types and field arguments.

Omitting fields

Look at these two definitions of a Position object type:

⚠️

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

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

Subgraph B defines a z field, but Subgraph A doesn't. In this case, when composition generates the Position type for the supergraph schema, it includes all three fields:

type Position {
  x: Int!
  y: Int!
  z: Int!
}

This definition works for Subgraph B, but it presents a problem for Subgraph A. Let's say Subgraph A defines the following Query type:

type Query {
  currentPosition: Position!
}

According to the hypothetical supergraph schema, the following query is valid against the supergraph:

❌

query GetCurrentPosition {
  currentPosition {
    x
    y
    z # ⚠️ Unresolvable! ⚠️
  }
}

And here's the problem: if Subgraph B doesn't define Query.currentPosition, this query must be executed on Subgraph A. But Subgraph A is missing the Position.z field, so that field is unresolvable!

Composition recognizes this potential problem, and it fails with an error. So how do we fix it? Check out Solutions for unresolvable fields.

Adding new shared fields

Adding a new field to a value type can cause composition issues, because it's challenging to add the field to all defining subgraphs at the same time.

Let's say we're adding a z field to our Position value type, and we start with Subgraph A:

⚠️

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

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

It's likely that when we attempt to compose these two schemas, composition will fail, because Subgraph B can't resolve Position.z.

To incrementally add the field to all of our subgraphs without breaking composition, we can use the @inaccessible directive.

Using @inaccessible

If you apply the @inaccessible directive to a field, composition omits that field from your gateway's API schema. This helps you incrementally add a field to multiple subgraphs without breaking composition.

To use @inaccessible in a subgraph, first make sure you include it in the import array of your Federation 2 opt-in declaration:

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extend schema
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  @link(url: "https://specs.apollo.dev/federation/v2.0",
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        import: ["@key", "@shareable", "@inaccessible"])

Then, whenever you add a new field to a value type, apply @inaccessible to that field if it isn't yet present in every subgraph that defines the value type:

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

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

With the syntax above, composition omits Position.z from the generated API schema, and the resulting Position type includes only x and y fields.

Whenever you're ready, you can now add Position.z to Subgraph B:

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

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

At this point, Position.z is still @inaccessible, so composition continues to ignore it.

Finally, when you've added Position.z to every subgraph that defines Position, you can remove @inaccessible from Subgraph A:

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

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

Composition now successfully includes Position.z in the supergraph schema!

Unions, interfaces, and inputs

In Federation 2, interface and union type definitions can be shared between subgraphs by default, and those definitions can differ:

union Media = Book | Movie

interface User {
  name: String!
}

union Media = Book | Podcast

interface User {
  name: String!
  age: Int!
}

Compositional logic merges these definitions in your supergraph schema:

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union Media = Book | Movie | Podcast
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# The object types that implement this interface are
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# responsible for resolving these fields.
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interface User {
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  name: String!
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  age: Int!
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}

This can be useful when different subgraphs are responsible for different subsets of a particular set of related types or values.

Subgraphs can share input type definitions, but composition merges them differently. When input types are composed across multiple subgraphs, only their mutual fields are carried over into the supergraph schema:

input UserInput {
  name: String!
  age: Int
}

input UserInput {
  name: String!
  email: String
}

Compositional logic merges only the fields that all input types have in common. To learn more, see Merging input types and field arguments.

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input UserInput {
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  name: String!
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}

To learn more about how composition merges different schema types under the hood, see Merging types during composition.