What is GraphQL and how does it differ from REST?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Define the schema

   Describe your types, fields, and their relationships in the schema so the server has a typed contract.

2. Step 2

   Client writes a query

   The client sends a query listing exactly the fields (and nested fields) it wants from the single endpoint.

3. Step 3

   Server validates

   GraphQL validates the query against the schema, rejecting unknown or malformed fields before execution.

4. Step 4

   Resolvers run

   Each requested field is resolved by a resolver function that fetches its value from a database, service, or other source.

5. Step 5

   Shaped response returns

   The server assembles a JSON tree matching the query shape and returns it in one round trip.

Code Example

query {
  user(id: "42") {
    name
    email
    posts(last: 2) {
      title
    }
  }
}

// GET /users/42            -> full user object (over-fetch)
// GET /users/42/posts      -> all posts (over-fetch)
// Then filter to last 2 posts and pick title on the client

Follow-up Questions

• How does GraphQL handle caching compared to REST?
• What is over-fetching and under-fetching?
• What is the N+1 problem in GraphQL and how do you solve it?
• Can GraphQL and REST coexist in the same system?
• How does introspection help API tooling?