Why Shared types across subgraphs in GraphQL? - Purpose & Use Cases

The Big Idea

What if your teams could speak the same data language without endless confusion?

The Scenario

Imagine you have several teams building different parts of a big app, each with its own data. They try to keep their data types separate, but some types are actually the same across teams. Without sharing, each team copies and changes these types on their own.

The Problem

This copying leads to confusion and mistakes. If one team updates a type, others might not know and keep using old versions. It becomes hard to keep data consistent, and fixing bugs takes a lot of time.

The Solution

Shared types across subgraphs let teams define common data types once and use them everywhere. This keeps data consistent, reduces errors, and makes collaboration smooth because everyone agrees on the same type definitions.

Before vs After

✗ Before

type User { id: ID! name: String! }
type Product { id: ID! ownerName: String! }

✓ After

type User @shared { id: ID! name: String! }
extend type User @key(fields: "id") { id: ID! }

What It Enables

It enables seamless data sharing and consistency across multiple teams and services working together.

Real Life Example

In a large online store, the User type is shared across the shopping, payment, and review services, so all parts of the app recognize the same user data without conflicts.

Key Takeaways

Manual copying of types causes errors and confusion.

Shared types keep data consistent across subgraphs.

Teams collaborate better with a single source of truth for common types.

Practice

1. What is the main purpose of using @key in shared types across GraphQL subgraphs?

easy

A. To mark fields that uniquely identify an entity across subgraphs

B. To define a field as optional in the schema

C. To specify the data type of a field

D. To mark a field as deprecated

Solution

1. Step 1: Understand the role of @key

   The @key directive marks fields that uniquely identify an entity across subgraphs, enabling them to share the same type.

2. Step 2: Differentiate from other directives

   Other directives like @external or @deprecated serve different purposes, not unique identification.

3. Final Answer:

   To mark fields that uniquely identify an entity across subgraphs -> Option A

4. Quick Check:

   @key marks unique identifiers [OK]

Hint: Remember: @key means unique ID for shared types [OK]

Common Mistakes:

• Confusing @key with @external
• Thinking @key marks optional fields
• Assuming @key defines data types

2. Which of the following is the correct way to mark a field as coming from another subgraph in a shared type?

easy

A. Use @key directive on the field

B. Use @provides directive on the field

C. Use @requires directive on the field

D. Use @external directive on the field

Solution

1. Step 1: Identify the directive for external fields

   The @external directive marks fields that are owned by another subgraph but referenced in the current one.

2. Step 2: Differentiate from other directives

   @key marks unique identifiers, @requires and @provides relate to field dependencies, not external ownership.

3. Final Answer:

   Use @external directive on the field -> Option D

4. Quick Check:

   @external marks fields from other subgraphs [OK]

Hint: External fields use @external directive [OK]

Common Mistakes:

• Using @key instead of @external
• Confusing @requires with @external
• Not marking external fields at all

3. Given the following subgraph schema snippet:

type Product @key(fields: "id") {
  id: ID!
  name: String
  price: Float @external
}

Which statement is true about the price field?

medium

A. It is defined and owned by this subgraph

B. It is a unique identifier for Product

C. It is defined in another subgraph and referenced here

D. It is deprecated and should not be used

Solution

1. Step 1: Analyze the @external directive on price

   The @external directive means price is not owned here but comes from another subgraph.

2. Step 2: Understand the role of @key on id

   The id field is the unique identifier, so price is not an ID.

3. Final Answer:

   It is defined in another subgraph and referenced here -> Option C

4. Quick Check:

   @external means field is from another subgraph [OK]

Hint: Fields with @external come from other subgraphs [OK]

Common Mistakes:

• Thinking @external means field is owned here
• Confusing @key with @external
• Assuming @external means deprecated

4. Consider this subgraph type definition:

type User @key(fields: "userId") {
  userId: ID!
  email: String @external
  name: String
}

Which statement is true about the email field?

medium

A. It is defined in another subgraph and referenced here

B. The @key directive must include email field

C. The userId field cannot be used as a key

D. The name field must be marked @external

Solution

1. Step 1: Analyze the @external directive on email

   The @external directive means email is defined in another subgraph and referenced here.

2. Step 2: Differentiate from other fields

   userId is the @key field provided locally, name is owned locally (no directive).

3. Final Answer:

   It is defined in another subgraph and referenced here -> Option A

4. Quick Check:

   @external means field from another subgraph [OK]

Hint: @external means field from another subgraph [OK]

Common Mistakes:

• Thinking @external means owned locally
• Believing @key must include all fields
• Assuming all fields need @external

5. You have two subgraphs sharing a Book type. Subgraph A defines:

type Book @key(fields: "isbn") {
  isbn: ID!
  title: String
}

Subgraph B defines:

extend type Book @key(fields: "isbn") {
  isbn: ID! @external
  author: String
}

Which statement best describes how these shared types work together?

hard

A. Both subgraphs own isbn, causing a conflict

B. Subgraph A owns isbn and title, Subgraph B extends Book using isbn as key and adds author

C. Subgraph B owns isbn and author, Subgraph A only references isbn

D. Subgraph B cannot extend Book without redefining title

Solution

1. Step 1: Identify ownership of fields

   Subgraph A defines Book with isbn and title, so it owns these fields.

2. Step 2: Understand extension in Subgraph B

   Subgraph B extends Book, marking isbn as @external (owned by A) and adds author.

3. Step 3: Confirm no conflicts

   Using @key with the same field isbn allows both subgraphs to share the type without conflict.

4. Final Answer:

   Subgraph A owns isbn and title, Subgraph B extends Book using isbn as key and adds author -> Option B

5. Quick Check:

   Extension uses @external keys to share types [OK]

Hint: Extension uses @external keys to share types [OK]

Common Mistakes:

• Thinking both subgraphs own the same key field
• Believing extension requires redefining all fields
• Assuming conflicts occur with shared keys