Entities

In Apollo Federation, an entity is a type that you define canonically in one implementing service and can then reference and extend in other implementing services. Entities are the core building block of a federated graph.

Defining

In a GraphQL schema, you can designate any object type as an entity by adding a @key directive to its definition, like so:

type Product @key(fields: "upc") {  upc: String!
  name: String!
  price: Int
}

The @key directive defines the entity's primary key, which consists of one or more of the type's fields. In the example above, the Product entity's primary key is its upc field. The gateway's query planner uses an entity's primary key to identify a given instance of the type.

An entity's @key cannot include fields that hold unions or interfaces.

Defining multiple primary keys

If an entity can be uniquely identified by more than one combination of fields, you can define more than one primary key for that entity.

In the following example, a Product entity can be uniquely identified by either its upc or its sku:

type Product @key(fields: "upc") @key(fields: "sku") {  upc: String!
  sku: String!
  price: String
}

This pattern is helpful when different services interact with different fields of an entity. For example, a reviews service might refer to products by their UPC, whereas an inventory service might use SKUs.

Defining a compound primary key

A single primary key can consist of multiple fields, and even nested fields.

The following example shows a primary key that consists of both a user's id and the id of that user's associated organization:

type User @key(fields: "id organization { id }") {  id: ID!
  organization: Organization!
}

type Organization {
  id: ID!
}

Referencing

After you define an entity in one implementing service, other implementing services can then reference that entity. If a products service defines the Product entity above, a reviews service can then add a field of type Product to its Review type, like so:

type Review {
  product: Product
}

# This is a "stub" of the Product entity (see below)
extend type Product @key(fields: "upc") {
  upc: String! @external
}

Because the Product entity is defined in another service, the reviews service needs to define a stub of it to make its own schema valid. The stub includes just enough information for the service to know how to interact with a Product:

• The extend keyword indicates that Product is an entity that is defined in another implementing service (in this case, the products service).
• The @key directive indicates that Product uses the upc field as its primary key. This value must match the value of exactly one @key defined in the entity's originating service, even if the entity defines multiple primary keys.
• The upc field must be included in the stub because it is part of the specified @key. It also must be annotated with the @external directive to indicate that the field originates in another service.

This explicit syntax has several benefits:

• It is standard GraphQL grammar.
• It enables you to run the reviews service standalone with a valid schema, including a Product type with a single upc field.
• It provides strong typing information that lets you catch mistakes at schema composition time.

Resolving

In our example, the reviews service needs to define its own resolver for the Product entity. The reviews service doesn't know much about Products, but fortunately, it doesn't need to. All it needs to do is return enough information to uniquely identify a given Product, like so:

{
  Review: {
    product(review) {
      return { __typename: "Product", upc: review.upc };
    }
  }
}

This return value is a representation of a Product entity. Services use representations to reference entities from other services. A representation requires only an explicit __typename definition and values for the entity's primary key fields.

The gateway provides this representation to the entity's originating service to fetch the full object. For this to work, the originating service (in this case, products) must define a reference resolver for the Product entity:

{
  Product: {
    __resolveReference(reference) {      return fetchProductByUPC(reference.upc);    }  }
}

Reference resolvers are a special addition to Apollo Server that enable entities to be referenced by other services. They are called whenever a query references an entity across service boundaries. To learn more about __resolveReference, see the API docs.

With this model, each implementing service's schema represents a true subset of the complete data graph. This prevents the need for defining foreign-key fields in individual schemas, and enables clients to transparently execute a query like the following, which hits both the products and reviews services:

{
  reviews {
    product {
      name
      price
    }
  }
}

Extending

An implementing service can add fields to an entity that's defined in another service. This is called extending the entity.

When a service extends an entity, the entity's originating service is not aware of the added fields. Only the extending service (along with the gateway) knows about these fields.

Each field of an entity should be defined in exactly one service. Otherwise, a schema composition error will occur.

Example #1

Let's say we want to add a reviews field to the Product entity. This field will hold a list of reviews for the product. The Product entity originates in the products service, but it makes much more sense for the reviews service to resolve this particular field.

To handle this case, we can extend the Product entity in the reviews service, like so:

extend type Product @key(fields: "upc") {
  upc: String! @external
  reviews: [Review]}

This definition is nearly identical to the stub we defined for the Product type in Referencing. All we've added is the reviews field. We don't include an @external directive, because this field does originate in the reviews service.

Whenever a service extends an entity with a new field, it is also responsible for resolving the field. The gateway is automatically aware of this responsibility. In our example, the generated query plan will fetch the upc field for each Product from the products service and pass those to the reviews service, where you can then access these fields on the object passed into your reviews resolver:

{
  Product: {
    reviews(product) {
      return fetchReviewsForProduct(product.upc);
    }
  }
}

Example #2

Let's say we want to be able to query for the inStock status of a product. That information lives in an inventory service, so we'll add the type extension there:

extend type Product @key(fields: "upc") {
  upc: ID! @external
  inStock: Boolean}

{
  Product: {
    inStock(product): {
      return fetchInStockStatusForProduct(product.upc);
    }
  }
}

Similar to the reviews relationship example above, the gateway fetches the required upc field from the products service and passes it to the inventory service, even if the query didn't ask for the upc:

query GetTopProductAvailability {
  topProducts {
    inStock
  }
}

The Query and Mutation types

In Apollo Federation, the Query and Mutation base types originate in the graph composition itself and all of your implementing services are automatically treated as extending these types to add the operations they support without explicitly adding the extends keyword.

For example, the products service might extend the root Query type to add a topProducts query, like so:

type Query {
  topProducts(first: Int = 5): [Product]
}

Migrating a field to another service (advanced)

As your federated graph grows, you might decide that you want a particular field of an entity to originate in a different service.

For example, let's say your products service defines a Product entity that includes an inStock boolean field. Then, you add an inventory service to your federated graph. It now makes sense for the inStock field to originate in the inventory service instead.

Apollo Gateway helps you perform this migration much like you perform a database migration, with the following steps:

1. In the inventory service's schema, extend the Product entity to add the inStock field.

   Note that this is technically a composition error, because inStock is already defined in the products service. However, this error is handled gracefully, as described below.

2. In the inventory service, add a resolver for the inStock field. This service should resolve the field with the exact same outcome as the resolver in the products service.

3. Push the updated inventory service to your environment.

   Again, this technically deploys a composition error. However, this error is handled gracefully in one of two ways, depending on whether you are using managed federation:

   • If you are using managed federation, Apollo Studio does not push an updated schema to your gateway, and the gateway continues to resolve the inStock field in the products service.
   • If you are not using managed federation, your gateway starts resolving the inStock field in whichever service is listed last in your gateway's serviceList.

4. In the products service's schema, remove the inStock field and push the updated service to your environment.

   This takes care of the composition error, regardless of whether you are using managed federation. The gateway will begin resolving the inStock field in the inventory service.

5. Remove the resolver for inStock from the products service and push the updated service to your environment.

   By removing the inStock field from products before removing its associated resolver, you guarantee that the gateway never attempts to resolve the field in a service that lacks a resolver for it.

Extending an entity with computed fields (advanced)

When you extend an entity, you can define fields that depend on fields in the entity's originating service. For example, a shipping service might extend the Product entity with a shippingEstimate field, which is calculated based on the product's size and weight:

extend type Product @key(fields: "sku") {
  sku: ID! @external
  size: Int @external
  weight: Int @external
  shippingEstimate: String @requires(fields: "size weight")}

As shown, you use the @requires directive to indicate which fields (and subfields) from the entity's originating service are required.

You cannot require fields that are defined in a service besides the entity's originating service.

In the above example, if a client requests a product's shippingEstimate, the gateway will first obtain the product's size and weight from the products service, then pass those values to the shipping service. This enables you to access those values directly from your resolver:

{
  Product: {
    shippingEstimate(product) {
      return computeShippingEstimate(product.sku, product.size, product.weight);    }
  }
}

Resolving another service's field (advanced)

Sometimes, multiple implementing services are capable of resolving a particular field for an entity, because all of those services have access to a particular data store. For example, an inventory service and a products service might both have access to the database that stores all product-related data.

When you extend an entity in this case, you can specify that the extending service @provides the field, like so:

type InStockCount {
  product: Product! @provides(fields: "name price")  quantity: Int!
}

extend type Product @key(fields: "sku") {
  sku: String! @external
  name: String @external  price: Int @external}

This is a completely optional optimization. When the gateway plans a query's execution, it looks at which fields are available from each implementing service. It can then attempt to optimize performance by executing the query across the fewest services needed to access all required fields.

Keep the following in mind when using the @provides directive:

• Each service that @provides a field must also define a resolver for that field. That resolver's behavior must match the behavior of the resolver in the field's originating service.
• When an entity's field can be fetched from multiple services, there is no guarantee as to which service will resolve that field for a particular query.
• If a service @provides a field, it must still list that field as @external, because the field originates in another service.