Schema evolution strategies in GraphQL - Mini Project: Build & Apply

Schema Evolution Strategies in GraphQL

📖 Scenario: You are working on a GraphQL API for a book store. Over time, the schema needs to evolve without breaking existing clients. You will practice adding new fields, deprecating old fields, and introducing new types safely.

🎯 Goal: Build a GraphQL schema that demonstrates schema evolution strategies: adding new fields, marking fields as deprecated, and adding new types while keeping backward compatibility.

📋 What You'll Learn

Create an initial GraphQL schema with a Book type containing id, title, and author fields.

Add a new optional field publishedYear to the Book type.

Deprecate the author field with a reason.

Add a new type Author with id and name fields and update Book to reference Author instead of a string.

💡 Why This Matters

🌍 Real World

APIs evolve over time. Schema evolution strategies help keep APIs stable while adding new features or improving data models.

💼 Career

Understanding schema evolution is essential for backend developers and API designers to maintain backward compatibility and improve client experience.

1

Create initial Book type

Create a GraphQL type called Book with fields id of type ID!, title of type String!, and author of type String!.

GraphQL

# Define the Book type with id, title, and author fields
# Your code here

Hint

Use type Book { ... } and define the fields with their types exactly as specified.

2

Add publishedYear field

Add a new optional field called publishedYear of type Int to the existing Book type.

GraphQL

type Book {
  id: ID!
  title: String!
  author: String!
  # Add publishedYear field here
}

Hint

Remember that optional fields do not have an exclamation mark !.

3

Deprecate author field

Mark the author field in the Book type as deprecated with the reason Use the author field with type Author instead.

GraphQL

type Book {
  id: ID!
  title: String!
  # Deprecate author field here
  author: String! @deprecated(reason: "Use the author field with type Author instead")
  publishedYear: Int
}

Hint

Use the @deprecated directive with the reason argument exactly as shown.

4

Add Author type and update Book

Create a new GraphQL type called Author with fields id of type ID! and name of type String!. Then update the Book type to replace the deprecated author field with a new field author of type Author!.

GraphQL

type Book {
  id: ID!
  title: String!
  author: Author!
  publishedYear: Int
}

# Define Author type here

Hint

Define the new Author type first, then update the author field in Book to use Author!.

Practice

1. What is the main purpose of schema evolution in GraphQL APIs?

easy

A. To update the API without breaking existing client applications

B. To remove all old fields immediately from the schema

C. To prevent any changes to the API once deployed

D. To make all fields mandatory for clients

Solution

1. Step 1: Understand schema evolution concept

   Schema evolution allows changes to the API while keeping existing clients working.

2. Step 2: Identify the correct purpose

   Removing fields immediately or making all fields mandatory breaks clients, so those are incorrect.

3. Final Answer:

   To update the API without breaking existing client applications -> Option A

4. Quick Check:

   Schema evolution = safe API updates [OK]

Hint: Schema evolution means safe API changes without breaking clients [OK]

Common Mistakes:

• Thinking schema evolution means removing fields immediately
• Believing all fields must be mandatory
• Assuming no changes are allowed after deployment

2. Which of the following is the correct way to mark a field as deprecated in a GraphQL schema?

easy

A. type User { name: String @remove(reason: "Use fullName instead") }

B. type User { name: String deprecated: true }

C. type User { name: String @deprecated(reason: "Use fullName instead") }

D. type User { name: String deprecated(reason: "Use fullName instead") }

Solution

1. Step 1: Recall GraphQL deprecation syntax

   GraphQL uses the @deprecated directive with a reason argument to mark fields deprecated.

2. Step 2: Check each option's syntax

   type User { name: String @deprecated(reason: "Use fullName instead") } uses correct @deprecated directive syntax; others are invalid or incorrect.

3. Final Answer:

   type User { name: String @deprecated(reason: "Use fullName instead") } -> Option C

4. Quick Check:

   @deprecated directive syntax = type User { name: String @deprecated(reason: "Use fullName instead") } [OK]

Hint: Use @deprecated(reason: "...") to mark fields deprecated [OK]

Common Mistakes:

• Using deprecated: true instead of @deprecated directive
• Using @remove directive which does not exist
• Omitting the @ symbol before deprecated

3. Given this GraphQL schema snippet:

type Query {
  user(id: ID!): User
}

type User {
  id: ID!
  name: String
  email: String @deprecated(reason: "Use contactEmail instead")
  contactEmail: String
}

What happens if a client queries for email field?

medium

A. The query succeeds but clients get a deprecation warning for email

B. The query fails because email is removed

C. The query returns null for email always

D. The query returns contactEmail value instead of email

Solution

1. Step 1: Understand @deprecated behavior in GraphQL

   Deprecated fields still exist and return data but signal clients to avoid using them.

2. Step 2: Analyze the query effect

   Querying email returns its value but clients should see a deprecation warning.

3. Final Answer:

   The query succeeds but clients get a deprecation warning for email -> Option A

4. Quick Check:

   Deprecated fields return data with warnings [OK]

Hint: Deprecated fields still return data but warn clients [OK]

Common Mistakes:

• Assuming deprecated fields are removed immediately
• Thinking deprecated fields return null
• Believing deprecated fields auto-redirect to new fields

4. Consider this schema update attempt:

type User {
  id: ID!
  name: String
  email: String
}

# Update:
type User {
  id: ID!
  name: String
  contactEmail: String
}

What is the main problem with this update?

medium

A. GraphQL does not allow adding new fields to existing types

B. Adding contactEmail without deprecating email causes syntax error

C. You must rename email to contactEmail in one step

D. Removing email field breaks existing clients still using it

Solution

1. Step 1: Identify schema evolution best practice

   Removing fields immediately breaks clients that still query those fields.

2. Step 2: Analyze the update

   The update removes email without deprecation, causing breaking changes.

3. Final Answer:

   Removing email field breaks existing clients still using it -> Option D

4. Quick Check:

   Immediate removal breaks clients [OK]

Hint: Never remove fields immediately; deprecate first [OK]

Common Mistakes:

• Thinking adding fields causes syntax errors
• Believing renaming must be one-step without deprecation
• Assuming GraphQL forbids adding new fields

5. You want to evolve a GraphQL schema by replacing a field phone with mobilePhone without breaking clients. Which strategy is best?

hard

A. Remove phone immediately and add mobilePhone

B. Add mobilePhone as optional, deprecate phone with reason, keep both for now

C. Rename phone to mobilePhone directly without deprecation

D. Keep only phone and do not add mobilePhone

Solution

1. Step 1: Apply schema evolution best practice

   Adding new fields as optional and deprecating old ones avoids breaking clients.

2. Step 2: Evaluate options

   Add mobilePhone as optional, deprecate phone with reason, keep both for now follows best practice; others cause breaking changes or no evolution.

3. Final Answer:

   Add mobilePhone as optional, deprecate phone with reason, keep both for now -> Option B

4. Quick Check:

   Deprecate old, add new optional field [OK]

Hint: Add new optional field, deprecate old with reason [OK]

Common Mistakes:

• Removing old field immediately
• Renaming fields without deprecation
• Not adding new field at all