Thinking in entities

Entities are the core building blocks of a federated graph, so the adoption of any schema design best practice must be approached with the unique role of entities in mind. While there's no requirement for subgraphs to define any entities at all with Apollo Federation, the federated schema design process often begins by thinking about what the initial entity types will be and how they will be referenced and extended throughout the graph to help preserve the separation of concerns between subgraphs both today and as the graph evolves in the future.

Define, reference, and extend entities as needed

The Apollo Federation specification indicates that an Object or Interface type can be made into an entity by adding the @key directive to its definition in a subgraph schema. The @key directive defines a unique key for the entity and its fields argument will contain one or more of the type's fields. In the following example, the Product entity's primary key would be its upc field:

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

Setting the upc field as the key means that other subgraphs that want to use this entity will need to know at least that value for any product. The keys we define should be values that uniquely identify a resource. This is because we want to avoid scenarios where they are used to arbitrarily pass dynamic field data around query execution between subgraphs.

After defining an entity in a schema, other subgraphs can reference that entity in their schemas. In order for the referencing subgraph's schema to be valid, it must define a stub of the entity in its schema. For example, we can reference a Product type defined in the products subgraph as the return type corresponding to a product field on a Review type defined in reviews subgraph:

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type Review {
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  rating: Int
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  product: Product
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}
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type Product @key(fields: "upc") {
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  upc: String!
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}

The @key directive indicates that the reviews subgraph will be able to identify a product by its UPC value and therefore be able to connect to a product based on its upc primary key field, but the reviews subgraph does not need to be aware of any other details about a given product.

Referencing entities is a key feature of federation, but it's only half of the story. While an entity will be owned by a single subgraph, other subgraphs might wish to add additional fields to the entity's type to provide a more holistic representation of the entity in the graph. Doing so is a simple as adding the additional field to the extended type in a non-originating subgraph. For example, a reviews subgraph's schema might add a reviews field to the extended Product type that was originally defined in the products subgraph:

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type Review {
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  rating: Int
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  product: Product
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}
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type Product @key(fields: "upc") {
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  upc: String!
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  reviews: [Review]
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}

When extending entities, it's important to keep in mind that the entity's originating subgraph will not be aware of the added fields. Additionally, each field in an entity must only be defined once or the gateway will encounter schema composition errors.

Work from the entities outward

When migrating from a client-only or monolithic GraphQL pattern, that work begins by identifying what entities will be exposed in the first subgraph extracted from the larger schema. When migrating from an architecture consisting of BFF-based GraphQL APIs or any other architecture of multiple overlapping graphs, the work of identifying entities (and determining new subgraph boundaries, in general) might be a bit more complex and involve some degree of negotiation with respect to type ownership, as well as a migration process to help account for any breaking changes that might result for clients.

Whatever your architectural starting point, Apollo Federation was designed to allow the work of identifying entities and defining subgraph boundaries to be done in an incremental, non-disruptive fashion. Beginning to identify these entities is also the essential prerequisite for adopting the other schema design best practices that will follow.

For more information on entity usage and capabilities, please see the Entities documentation page.

@defer and entities

Entities aren't just useful for connecting data across subgraphs. You can also use entities to enable the new @defer directive for client-controlled prioritization of response data. The Apollo Router can defer resolution of fields in entities (and root fields) and handle sending data back to the client in prioritized chunks. By defining types as entities within your graph, clients can improve perceived user experience by adding a directive to their operations.