Top GraphQL Interview Questions and Answers

What is GraphQL?

GraphQL is a query language for APIs and a runtime for fulfilling those queries with your existing data. Developed by Facebook and now maintained by a large community, it's designed to provide a more efficient and flexible alternative to RESTful APIs, particularly for complex data interactions.

Reasons for GraphQL's Development

GraphQL was created to address limitations of REST APIs, such as over-fetching (retrieving more data than needed) and under-fetching (requiring multiple requests to get all the data). GraphQL allows clients to specify exactly what data they need, resulting in more efficient data transfer.

Companies Using GraphQL

Many companies utilize GraphQL, including:

• Facebook
• GitHub
• Pinterest
• Shopify
• And many more

Data Loading in GraphQL

GraphQL's client-driven nature means that clients specify precisely which fields they want to receive in their queries. This avoids over-fetching and reduces the amount of data transferred.

Is GraphQL a Database Technology?

No, GraphQL is not a database. It's a query language for APIs. It sits on top of your existing data and can work with various database technologies (SQL and NoSQL).

GraphQL and Programming Languages

GraphQL is language-agnostic. GraphQL servers can be implemented in many programming languages (Java, Python, JavaScript, C#, etc.). This allows integration into various existing applications and systems.

GraphQL vs. REST: Why Choose GraphQL?

While REST APIs are well-established, GraphQL offers benefits in complex scenarios:

• Single endpoint: Reduces network overhead.
• Efficient data fetching: Clients request only necessary data.
• Flexible data shaping: Clients specify data format.
• Faster development: Often easier to implement and maintain.
• High consistency: Easier to maintain consistency across different platforms.

GraphQL Protocols

While GraphQL typically runs over HTTP, it's protocol-agnostic. It can be implemented with various protocols depending on the application's requirements.

Authentication and Authorization in GraphQL

Authentication verifies the user's identity (often using OAuth or similar technologies). Authorization determines what a user can access (typically handled in the business logic layer of your application, not directly within GraphQL).

Error Handling in GraphQL

GraphQL returns errors as a separate field within the JSON response (the errors field). This allows the client to handle errors gracefully without necessarily needing to abandon the complete request.

{
  "data": { /* ... successful data ... */ },
  "errors": [
    { "message": "An error occurred" }
  ]
}
        

Server-Side Caching in GraphQL

Server-side caching is more challenging with GraphQL than with REST due to the flexibility of GraphQL queries. Strategies like caching based on query hashes or using specialized caching libraries can be implemented, but the approach must carefully consider the dynamic nature of GraphQL requests.

GraphQL Response Format

GraphQL responses are JSON objects. The structure of the response directly matches the query requested by the client.

Over-fetching and Under-fetching in GraphQL

• Over-fetching: Receiving more data than needed.
• Under-fetching: Requiring multiple requests to obtain all needed data.

GraphQL helps mitigate these issues by allowing clients to request only the specific data they require.

Advantages of GraphQL over REST

REST and Query Languages

REST is not a query language. It's an architectural style for building web services. GraphQL is a query language designed specifically for APIs.

GraphQL and Database Types

GraphQL can be used with various databases (SQL and NoSQL); it's database agnostic.

Schema Definition Language (SDL) in GraphQL

SDL is used to define the schema (types and their relationships) of your GraphQL API. The schema acts as a contract for your API.

Offline Usage with GraphQL

GraphQL is not designed for offline use; solutions involve client-side caching.

GraphiQL

GraphiQL is an in-browser IDE tool used to interact with and explore GraphQL APIs.

GraphQL Operations

• Query: For fetching data.
• Mutation: For modifying data.
• Subscription: For receiving real-time updates.

Modifying Data with Queries

While mutations are best practice for modifying data, it might be possible to design queries that trigger data changes on the server-side.

Apollo Client

Apollo Client is a popular JavaScript client for interacting with GraphQL APIs. It provides features like caching, data management, and error handling.

Apollo GraphQL

Apollo is a popular open-source platform and set of tools and libraries for building GraphQL servers and clients. It simplifies the process of creating and using GraphQL APIs. Apollo Client (for the front end) helps in fetching data efficiently and handles caching and other operations. Apollo Server (for the back end) assists in building scalable GraphQL servers.

Mutations in GraphQL

Mutations are used to modify data on the server. They're analogous to POST, PUT, DELETE requests in REST APIs. They are used for creating, updating, or deleting data.

Subscriptions in GraphQL

Subscriptions provide a mechanism for real-time updates from the server to clients. When a client subscribes to an event, the server pushes notifications to the client whenever that event occurs.

Queries in GraphQL

Queries are used to fetch data from the server. They're similar to GET requests in REST APIs. Clients specify the data they need, and the server returns a JSON response.

Fields in GraphQL

Fields are the specific data points or attributes you request within a GraphQL query. They are the keys in the JSON object returned by the GraphQL server.

{
  employee {
    name
    salary
  }
}
        

Object Types in GraphQL

Object types represent the resources or data structures accessible through the GraphQL API. They define the fields that can be queried.

Interfaces in GraphQL

Interfaces define a set of fields that multiple object types can implement. They enable polymorphism, allowing clients to query different objects using the same fields.

Unions in GraphQL

Unions specify that a field can return one of several possible object types. This allows for flexibility in representing different data structures.

Enums in GraphQL

Enums define a specific set of allowed values for a field, enhancing type safety and improving data validation.

Resolvers in GraphQL

Resolvers are functions that handle GraphQL queries and return the requested data. They retrieve the data from your data sources and map it to the structure defined in your GraphQL schema.

Arguments in GraphQL

Arguments are used to provide additional information to queries and fields, enabling you to filter, sort, or otherwise customize data retrieval.

{
  employee(id: "123") {
    name
    salary
  }
}
        

Non-Null Type Modifier

The non-null type modifier (!) indicates that a field or argument must have a value; it cannot be null. This enhances type safety and improves data validation.

Aliases in GraphQL

Aliases allow you to rename fields in a query result. This is useful for avoiding naming conflicts or improving readability.

{
  employee1: employee(id: 1) { name },
  employee2: employee(id: 2) { name }
}
        

Fragments in GraphQL

Fragments allow for reusing selections of fields across multiple queries, improving code organization and reducing redundancy. They define reusable pieces of a query.

GraphQL Voyager

GraphQL Voyager is a tool for visually exploring and documenting GraphQL schemas.

GraphQL Validation and Execution

The GraphQL runtime performs validation to ensure that queries are syntactically correct and authorized before execution.

GraphQL Libraries

Hosting a GraphQL Server

GraphQL servers can be hosted similarly to other web servers, typically using a chosen framework or language.

Remote Schemas in Apollo GraphQL

Remote schemas in Apollo allow you to create a unified GraphQL schema that combines data from different GraphQL APIs.

Schema Stitching in Apollo

Schema stitching is used to combine multiple GraphQL schemas into a single, unified schema, providing a single entry point for clients to access data from multiple sources.