Recommended usage for GraphQL interfaces

GraphQL interfaces enable schema fields to return one of multiple object types, all of which must implement that interface. To implement an interface, an object type must define all fields that are included in that interface (otherwise, the schema is invalid):

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interface Media {
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  title: String!
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}
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type Book implements Media {
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  title: String!
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  author: String!
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}

Because interfaces can enforce this requirement on implementing object types, it can be tempting to create interfaces purely to enforce field definitions. The following schema requires a name field on all of its object types by declaring that those types implement the Nameable interface:

❌

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interface Nameable {
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  name: String
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}
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type Cat implements Nameable {
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  name: String
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  meowVolume: Int
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}
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type Dog implements Nameable {
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  name: String
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  barkVolume: Int
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}
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type Owner implements Nameable {
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  name: String
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  cats: [Cat]
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  dogs: [Dog]
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}
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type Query {
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  owners: [Owner]
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}

This is not a recommended use for interfaces! Note that in this schema, Nameable is never used as the return type for a field. This means that clients have no way to take advantage of this polymorphic relationship in their operations.

The lack of valid client use cases indicates a code smell. The Nameable interface is unnecessary, and it might cause problems when making changes to your schema in the future. And because distributing an interface across subgraphs presents its own challenges, removing unnecessary interfaces usually makes life simpler.

In the next example, we specify that both the Cat and Dog types implement the Pet interface so that we can return a polymorphic list of both Cats and Dogs in the Owner.pets field. Now that there's a valid client use case for polymorphism, using an interface is justified.

✅

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interface Pet {
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  name: String
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}
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type Cat implements Pet {
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  name: String
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  meowVolume: Int
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}
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type Dog implements Pet {
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  name: String
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  barkVolume: Int
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}
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type Owner {
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  name: String
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  pets: [Pet]
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}
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type Query {
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  owners: [Owner]
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}

Implications of adding new implementing types

When a client includes a field returning an abstract type in an operation, they usually enumerate which fields from the concrete types they're interested in with "conditional fragments", like so:

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query OwnersAndPets {
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  owners {
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    name
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    pets {
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      name
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      ... on Cat {
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        meowVolume
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      }
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      ... on Dog {
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        barkVolume
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      }
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    }
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  }
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}

If your API adds additional possible types to the abstract type, clients must change their operations to select data from those new types. Although this isn't a breaking change, this situation warrants some defensive programming.

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// Trigger a TypeScript error if a new type is introduced but
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// we haven't added a switch case for it.
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const assertUnknown = (x: never) =>
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  console.log(`Unknown Pet type: ${(x as any).__typename}`);
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switch (pet.__typename) {
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  case 'Cat':
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    console.log(`Cat meows ${pet.meowVolume}`);
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    break;
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  case 'Dog':
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    console.log(`Dog barks ${pet.barkVolume}`);
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    break;
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  default:
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    assertUnknown(pet);
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    console.log('Fallback behavior');
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}

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// `when` must be used as an expression to require exhaustivity
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val result = when (pet.__typename) {
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  "Cat" -> println("Cat meows ${pet.onCat?.meowVolume}")
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  "Dog" -> println("Dog barks ${pet.onDog?.barkVolume}")
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  else -> println("Unknown animal ${pet.__typename}")
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}

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switch pet.__typename {
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case "Cat":
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    print("Cat meows \(pet.asCat?.meowVolume ?? 0)")
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case "Dog":
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    print("Dog barks \(pet.asDog?.barkVolume ?? 0)")
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default:
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    print("Unknown animal \(pet.__typename)")
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}