Migration from REST to GraphQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main difference between REST and GraphQL in terms of data fetching?

REST uses multiple endpoints for different resources, often requiring multiple requests. GraphQL uses a single endpoint and allows clients to request exactly the data they need in one query.

beginner

Why might migrating from REST to GraphQL improve client performance?

Because GraphQL lets clients request only the data they want, it reduces over-fetching and under-fetching, which can lower the number of requests and the amount of data transferred.

intermediate

What is a common first step when migrating a REST API to GraphQL?

Identify the existing REST endpoints and their data structures, then design a GraphQL schema that represents the same data and relationships in a flexible way.

intermediate

How does GraphQL handle versioning compared to REST?

GraphQL typically avoids versioning by evolving the schema with new fields and deprecating old ones, while REST often uses versioned endpoints like /v1/, /v2/.

intermediate

What is a resolver in GraphQL and why is it important during migration?

A resolver is a function that fetches the data for a specific field in a GraphQL query. During migration, resolvers connect the GraphQL schema to the existing data sources, including REST endpoints or databases.

What does GraphQL use to allow clients to specify exactly what data they want?

A A single flexible query language

B Multiple fixed endpoints

C HTTP headers

D URL parameters

GraphQL uses a single query language that lets clients specify exactly which fields and data they want.

When migrating from REST to GraphQL, what is the first thing you should do?

A Change the frontend without backend changes

B Rewrite all backend code from scratch

C Remove all REST endpoints immediately

D Design the GraphQL schema based on existing REST endpoints

Designing the GraphQL schema based on existing REST endpoints helps map current data structures before migrating.

How does GraphQL typically handle API versioning?

A By creating new endpoints for each version

B By using query parameters to specify versions

C By evolving the schema and deprecating fields

D By requiring clients to upgrade manually

GraphQL evolves the schema over time and deprecates old fields instead of creating new endpoints.

What role do resolvers play in a GraphQL API?

A They fetch and return data for query fields

B They format the HTTP response headers

C They handle user authentication

D They define the API version

Resolvers are functions that fetch and return the data for each field requested in a GraphQL query.

Which of the following is a benefit of migrating from REST to GraphQL?

A Clients can over-fetch data easily

B Clients can get all needed data in a single request

C Multiple endpoints reduce complexity

D GraphQL requires no backend changes

GraphQL allows clients to get all the data they need in one request, reducing multiple calls.

Explain the key steps involved in migrating a REST API to GraphQL.

Think about how you map existing data and connect it to GraphQL.

Describe how GraphQL improves client-server communication compared to REST.

Focus on how clients ask for data and how much data they get.

Practice

1. What is a key advantage of migrating from REST to GraphQL for database queries?

easy

A. REST automatically optimizes data fetching without changes.

B. You can request exactly the data you need in a single query.

C. GraphQL requires multiple requests to get all data.

D. GraphQL does not support querying nested data.

Solution

1. Step 1: Understand REST vs GraphQL data fetching

   REST often requires multiple requests or returns extra data, while GraphQL lets you specify exactly what you want.

2. Step 2: Identify the main benefit of GraphQL

   GraphQL reduces over-fetching and under-fetching by allowing precise queries in one request.

3. Final Answer:

   You can request exactly the data you need in a single query. -> Option B

4. Quick Check:

   GraphQL precise data fetching = C [OK]

Hint: GraphQL = one request, exact data [OK]

Common Mistakes:

• Thinking GraphQL needs multiple requests
• Believing REST auto-optimizes data fetching
• Assuming GraphQL can't query nested data

2. Which of the following is the correct way to define a simple GraphQL query to get a user's name and email?

easy

A. { user: { name, email } }

B. GET /user { name, email }

C. query { user { name, email } }

D. SELECT name, email FROM user

Solution

1. Step 1: Recognize GraphQL query syntax

   GraphQL queries start with the keyword 'query' followed by the fields requested inside braces.

2. Step 2: Compare options to GraphQL syntax

   query { user { name, email } } matches the correct GraphQL query format; others are REST, SQL, or invalid syntax.

3. Final Answer:

   query { user { name, email } } -> Option C

4. Quick Check:

   GraphQL query syntax = D [OK]

Hint: GraphQL queries start with 'query' keyword [OK]

Common Mistakes:

• Using REST or SQL syntax instead of GraphQL
• Omitting the 'query' keyword
• Incorrect braces placement

3. Given this GraphQL query:
query { book(id: "1") { title author { name } } }
What is the expected shape of the returned data?

medium

A. {"data": {"book": {"title": "Book Title", "author": {"name": "Author Name"}}}}

B. {"book": {"title": "Book Title", "author": "Author Name"}}

C. {"data": {"book": {"title": "Book Title", "author": "Author Name"}}}

D. {"data": {"book": ["title", "author"]}}

Solution

1. Step 1: Understand GraphQL nested response format

   GraphQL returns data inside a 'data' object, preserving nested structure matching the query.

2. Step 2: Match query fields to response structure

   The query requests 'title' and nested 'author' with 'name', so response nests 'author' as an object with 'name'.

3. Final Answer:

   {"data": {"book": {"title": "Book Title", "author": {"name": "Author Name"}}}} -> Option A

4. Quick Check:

   Nested data in 'data' key = A [OK]

Hint: GraphQL response nests data under 'data' key [OK]

Common Mistakes:

• Returning author as string instead of object
• Missing 'data' wrapper in response
• Using arrays instead of objects for fields

4. You migrated a REST endpoint to GraphQL but get an error: Cannot query field 'userName' on type 'User'. What is the likely cause?

medium

A. GraphQL does not support querying user names.

B. The REST endpoint is down.

C. The query syntax is missing curly braces.

D. The GraphQL schema does not define a 'userName' field on 'User' type.

Solution

1. Step 1: Analyze the error message

   The error says 'userName' is not a valid field on 'User' type, indicating schema mismatch.

2. Step 2: Understand GraphQL schema role

   GraphQL queries must match fields defined in the schema; missing fields cause errors.

3. Final Answer:

   The GraphQL schema does not define a 'userName' field on 'User' type. -> Option D

4. Quick Check:

   Schema field missing = A [OK]

Hint: Check schema fields match query fields [OK]

Common Mistakes:

• Blaming REST endpoint for GraphQL schema errors
• Ignoring schema definitions
• Assuming GraphQL can't query user names

5. During migration from REST to GraphQL, you want to avoid over-fetching user data. Which approach best achieves this?

hard

A. Define a GraphQL query that requests only the needed user fields like name and email.

B. Keep using the REST endpoint but add query parameters to limit fields.

C. Fetch all user data and filter unwanted fields on the client side.

D. Use multiple GraphQL queries to get each user field separately.

Solution

1. Step 1: Understand over-fetching in REST vs GraphQL

   REST often returns full objects; GraphQL lets you specify exactly which fields to fetch.

2. Step 2: Choose the best method to limit data fetched

   Defining a GraphQL query with only needed fields avoids over-fetching efficiently in one request.

3. Final Answer:

   Define a GraphQL query that requests only the needed user fields like name and email. -> Option A

4. Quick Check:

   Selective field query = B [OK]

Hint: Request only needed fields in GraphQL query [OK]

Common Mistakes:

• Fetching all data then filtering client-side
• Using multiple queries instead of one precise query
• Relying on REST query parameters only