Top 25 GraphQL Interview Questions And Answers

Question 1

What is GraphQL?

Accepted Answer

GraphQL is a query language created by [Facebook](http://facebook.github.io/) in 2012 which provides a **common interface between the client and the server for data fetching and manipulations**.

The client asks for various data from the GraphQL server via queries. The response format is described in the query and defined by the client instead of the server: they are called **client‐specified queries**.  
The structure of the data is not hardcoded as in traditional REST APIs - this makes retrieving data from the server more efficient for the client.

Question 2

Is GraphQL only for React/JavaScript Developers?

Accepted Answer

No. GraphQL is an API technology so it can be used in any context where an API is required.

On the _backend_, a GraphQL server can be implemented in any programming language that can be used to build a web server. Next to Javascript, there are popular reference implementations for Ruby, Python, Scala, Java, Clojure, Go and .NET.

Since a GraphQL API is usually operated over HTTP, any client that can speak HTTP is able to query data from a GraphQL server.

*Note*: GraphQL is actually transport layer agnostic, so you could choose other protocols than HTTP to implement your server.

Question 3

What is an _exclamation point_ in GraphQL?

Accepted Answer

That means that the field is non-nullable. By default, all types in GraphQL are nullable. When non-null is applied to the type of a field, it means that if the server resolves that field to `null`, the response will _fail validation_.

Question 4

Is GraphQL a Database Technology?

Accepted Answer

No. GraphQL is often confused with being a database technology. This is a misconception, GraphQL is a _query language_ for APIs - not databases. In that sense it’s database agnostic and can be used with any kind of database or even no database at all.

Question 5

How to do _Error Handling_ in GraphQL?

Accepted Answer

A successful GraphQL query is supposed to return a JSON object with a root field called `"data"`. If the request fails or partially fails (e.g. because the user requesting the data doesn’t have the right access permissions), a second root field called `"errors"` is added to the response:
```js
    {
      "data": { ... },
      "errors": [ ... ]
    }
```

Question 6

Where is GraphQL _useful_?

Accepted Answer

GraphQL helps where your **client needs a flexible response** format to avoid extra queries and/or massive data transformation with the overhead of keeping them in sync.

Using a GraphQL server makes it very easy for a client side developer to change the response format without any change on the backend.

With GraphQL, you can describe the required data in a more natural way. It can speed up development, because in application structures like **top-down rendering** in React, the required data is more similar to your component structure.

Question 7

What is difference between `Mutation` and `Query`?

Accepted Answer

Technically any GraphQL _query_ could be implemented to cause a data write. But there is a convention that any operations that cause writes should be sent explicitly via a _mutation_.

Besides the difference in the semantic, there is one important technical difference:

Query fields can be executed in parallel by the GraphQL engine while Mutation top-level fields MUST execute serially according to the spec.

Question 8

What is GraphQL `schema`?

Accepted Answer

Every GraphQL server has two core parts that determine how it works: a schema and resolve functions.

The *schema* is a model of the data that can be fetched through the GraphQL server. It defines what queries clients are allowed to make, what types of data can be fetched from the server, and what the relationships between these types are.

Consider:

```css
type Author {
  id: Int
  name: String
  posts: [Post]
}
type Post {
  id: Int
  title: String
  text: String
  author: Author
}
type Query {
  getAuthor(id: Int): Author
  getPostsByTitle(titleContains: String): [Post]
}
schema {
  query: Query
}
```

Question 9

List the _key concepts_ of the GraphQL query language

Accepted Answer

**Key concepts of the GraphQL** query language are:

*   Hierarchical
*   Product‐centric
*   Strong‐typing
*   Client‐specified queries
*   Introspective

Question 10

How to do _Server-Side Caching_ in GraphQL?

Accepted Answer

One common concern with GraphQL, especially when comparing it to REST, are the difficulties to maintain server-side cache. With REST, it’s easy to cache the data for each endpoint, since it’s sure that the _structure_ of the data will not change.

With GraphQL on the other hand, it’s not clear what a client will request next, so putting a caching layer right behind the API doesn’t make a lot of sense.

Server-side caching still is a challenge with GraphQL.

Top 25 GraphQL Interview Questions

What is GraphQL?

Is GraphQL a Database Technology?

What is an exclamation point in GraphQL?

Is GraphQL only for React/JavaScript Developers?

What is difference between `Mutation` and `Query`?

How to do Error Handling in GraphQL?

What is GraphQL `schema`?

Where is GraphQL useful?

Whether do you find GraphQL the right fit for designing microservice architecture?

Explain the main difference between REST and GraphQL

Does GraphQL support offline usage?

What kind of operations could GraphQL schema have?

How to do `Authentication` and `Authorization` in GraphQL?

How to do Server-Side Caching in GraphQL?

List the key concepts of the GraphQL query language

How to implement a set of GraphQL mutations in single transaction?

Are there any disadvantages to GraphQL?

Can you make a GraphQL type both an `input` and `output` type?

How to respond with different status codes in GraphQL?

Is it possible to use inheritance with GraphQL input types?

What the criteria set is for deciding when to use GraphQL vs. HATEOAS?

What is `AST` in GraphQL?

How do you prevent nested attack on GraphQL server?

How to query all the GraphQL type fields without writing a long query?

How would you model recursive data structures in GraphQL?

Top 25 GraphQL Interview Questions

What is GraphQL?

Is GraphQL a Database Technology?

What is an exclamation point in GraphQL?

Is GraphQL only for React/JavaScript Developers?

What is difference between Mutation and Query?

How to do Error Handling in GraphQL?

What is GraphQL schema?

Where is GraphQL useful?

Whether do you find GraphQL the right fit for designing microservice architecture?

Explain the main difference between REST and GraphQL

Does GraphQL support offline usage?

What kind of operations could GraphQL schema have?

How to do Authentication and Authorization in GraphQL?

How to do Server-Side Caching in GraphQL?

List the key concepts of the GraphQL query language

How to implement a set of GraphQL mutations in single transaction?

Are there any disadvantages to GraphQL?

Can you make a GraphQL type both an input and output type?

How to respond with different status codes in GraphQL?

Is it possible to use inheritance with GraphQL input types?

What the criteria set is for deciding when to use GraphQL vs. HATEOAS?

What is AST in GraphQL?

How do you prevent nested attack on GraphQL server?

How to query all the GraphQL type fields without writing a long query?

How would you model recursive data structures in GraphQL?

What is difference between `Mutation` and `Query`?

What is GraphQL `schema`?

How to do `Authentication` and `Authorization` in GraphQL?

Can you make a GraphQL type both an `input` and `output` type?

What is `AST` in GraphQL?