Question 1

What is GraphQL?

Accepted Answer

GraphQL is a query language and runtime for APIs developed by Facebook that allows clients to request exactly the data they need in a single request. It solves problems like over-fetching and under-fetching by enabling clients to specify their data requirements declaratively through a strongly-typed schema.

Question 2

How does GraphQL differ from REST APIs?

Accepted Answer

GraphQL uses a single endpoint with a flexible query language where clients specify exactly what data they need, while REST uses multiple endpoints with fixed data structures determined by the server. GraphQL eliminates over-fetching and under-fetching by allowing clients to request nested, related data in one query, whereas REST often requires multiple round trips.

Question 3

What is a GraphQL schema?

Accepted Answer

A GraphQL schema is a contract between client and server that defines the complete structure of your API, including available types, fields, queries, mutations, and their relationships. It uses the Schema Definition Language (SDL) and serves as both documentation and validation, enabling better tooling, auto-completion, and type safety.

Question 4

What are the scalar types in GraphQL?

Accepted Answer

GraphQL has five built-in scalar types: Int (32-bit integer), Float (floating-point number), String (UTF-8 character sequence), Boolean (true/false), and ID (unique identifier). Custom scalars like DateTime, Email, or URL can also be defined for domain-specific validation and serialization needs.

Question 5

What is a GraphQL query?

Accepted Answer

A GraphQL query is a read operation that requests specific data from a GraphQL server. Unlike REST APIs where you get all fields from an endpoint, a query lets you specify exactly which fields you need in a JSON-like syntax that mirrors the shape of the response you'll receive.

Question 6

What is a GraphQL mutation?

Accepted Answer

A GraphQL mutation is an operation that modifies data on the server, such as creating, updating, or deleting records. While queries are for reading data, mutations are for writing data and typically return the modified data to confirm the operation succeeded. Unlike queries which can be executed in parallel, mutations are executed sequentially by default.

Question 7

What are GraphQL subscriptions?

Accepted Answer

GraphQL subscriptions maintain a real-time connection between client and server, allowing the server to push data to clients when specific events occur. Unlike queries (one-time data fetching) and mutations (one-time data modification), subscriptions establish a persistent connection (typically via WebSockets) that streams updates over time, ideal for chat apps, live notifications, or real-time dashboards.

Question 8

What is a resolver in GraphQL?

Accepted Answer

A resolver is a function responsible for populating the data for a single field in your GraphQL schema. When a GraphQL query is executed, each field calls its corresponding resolver function to fetch or compute its value. Resolvers act as the bridge between the declarative GraphQL query language and your actual data sources.

Question 9

What are fragments in GraphQL?

Accepted Answer

Fragments are reusable units of fields that you can include in multiple queries. They're defined with 'fragment FragmentName on TypeName' and used with the spread operator '...FragmentName', helping you avoid repetition and maintain consistency across queries. Fragments are particularly valuable in component-based architectures where each component can define its own data requirements.

Question 10

What is the N+1 problem in GraphQL?

Accepted Answer

The N+1 problem occurs when a GraphQL resolver makes 1 database query to fetch a list of N items, then makes N additional queries to fetch related data for each item. This results in N+1 total queries instead of 2 optimized queries, causing severe performance degradation. The solution is to use batching and caching tools like DataLoader.

Question 11

What is DataLoader and how does it solve the N+1 problem?

Accepted Answer

DataLoader is a batching and caching utility that solves the N+1 problem by collecting all individual data requests made during a single execution tick and batching them into one efficient request. It also provides per-request caching to ensure the same object is only fetched once per query, dramatically improving performance.

Question 12

What are nullable and non-nullable types in GraphQL?

Accepted Answer

In GraphQL, types are nullable by default, meaning they can return null. Adding an exclamation mark (!) makes a type non-nullable, guaranteeing it will always return a value. For example, 'String' can be null, but 'String!' must always have a value. This applies to both field types and list items.

Question 13

What is the difference between queries and mutations?

Accepted Answer

Queries are for reading data and can be executed in parallel for optimal performance. Mutations are for modifying data (create, update, delete) and are executed sequentially by default to prevent race conditions. While both use similar syntax, mutations conventionally modify server state while queries do not.

Question 14

How do you handle errors in GraphQL?

Accepted Answer

GraphQL handles errors through a standardized error format in the response's 'errors' array, while partial data can still be returned in the 'data' field. Errors can be thrown in resolvers, caught and formatted, and enriched with custom extensions. GraphQL supports returning partial data with errors, unlike REST which typically returns only an error response.

Question 15

When should you use GraphQL vs REST?

Accepted Answer

Use GraphQL for complex data requirements with nested relationships, when clients need flexible data fetching, for mobile apps with bandwidth constraints, or when you want strong typing and self-documentation. Use REST for simple CRUD operations, when you need built-in HTTP caching, for public APIs with stable requirements, or when simplicity is prioritized over flexibility.

GraphQL Fundamentals(6 questions)

What is GraphQL and what problems does it solve?

How does GraphQL differ from REST APIs?

What are the main components of a GraphQL API?

What is a GraphQL schema and why is it important?

What are the advantages and disadvantages of GraphQL?

When should you use GraphQL vs REST?

Schema Definition Language (SDL)(7 questions)

What is the GraphQL Schema Definition Language (SDL)?

What are the scalar types in GraphQL?

What is the difference between type and input type?

What are nullable and non-nullable types in GraphQL?

What is the difference between [String]!, [String!], and [String!]!?

What are union types and when would you use them?

What are interface types and how do they differ from unions?

Queries(7 questions)

What is a GraphQL query?

How do you pass arguments to a query?

What are query variables and why should you use them?

How do you use aliases in GraphQL queries?

What are fragments and how do you use them?

How do you request nested data in a GraphQL query?

How do you use directives (@include, @skip) in queries?

Mutations(6 questions)

What is a GraphQL mutation?

How do mutations differ from queries?

How do you pass complex input objects to mutations?

What should a mutation return?

Are mutations executed in parallel or sequentially?

What are the best practices for designing mutations?

Subscriptions(4 questions)

What are GraphQL subscriptions?

How do subscriptions differ from queries and mutations?

When should you use subscriptions vs polling?

What is a pub/sub system in the context of subscriptions?

Resolvers(6 questions)

What is a resolver in GraphQL?

What are the four arguments passed to a resolver (parent, args, context, info)?

What is the context object and what should it contain?

How do you write a resolver for nested fields?

How do you handle async operations in resolvers?

How do you handle errors in resolvers?

Data Fetching and N+1 Problem(4 questions)

What is the N+1 problem in GraphQL?

How does DataLoader solve the N+1 problem?

How does DataLoader batching work?

What are the best practices for using DataLoader?

Error Handling(4 questions)

How does error handling work in GraphQL?

What is the structure of a GraphQL error response?

How do you return partial data with errors?

What are the best practices for GraphQL error handling?

Authentication and Authorization(5 questions)

How do you implement authentication in GraphQL?

What is the difference between authentication and authorization in GraphQL?

How do you pass authentication tokens to a GraphQL API?

How do you implement field-level authorization?

What are schema directives for authorization?

Performance and Optimization(6 questions)

What are the common performance issues in GraphQL APIs?

How do you implement query complexity analysis?

What is query depth limiting and why is it important?

What is persisted queries and how do they improve performance?

How do you implement caching in GraphQL?

How do you optimize resolver performance?

Pagination(5 questions)

What are the common pagination patterns in GraphQL?

What is cursor-based pagination?

What is the Relay Connection specification?

What are edges and nodes in Relay pagination?

When should you use offset vs cursor pagination?

Schema Design(5 questions)

What are the best practices for GraphQL schema design?

How do you version a GraphQL API?

What is Apollo Federation and how does it work?

What is schema-first vs code-first development?

How do you handle breaking changes in a GraphQL schema?

Security(4 questions)

What are the common security vulnerabilities in GraphQL?

How do you prevent denial of service attacks in GraphQL?