Discuss how you would add a GraphQL API to an existing Spring Boot RESTful service.

Question

Accepted Answer

Adding a GraphQL API to an existing Spring Boot REST service means layering a schema-driven query interface alongside the existing REST endpoints, typically using the spring-boot-starter-graphql module built on GraphQL Java.

Key Points:
• Add the spring-boot-starter-graphql dependency, which brings in GraphQL Java and Spring's integration support.
• Define the API contract in a schema file, conventionally src/main/resources/graphql/schema.graphqls, describing types, queries, and mutations.
• Implement data fetchers as @Controller classes with @QueryMapping and @MutationMapping methods that delegate to existing services.
• The GraphQL endpoint, by default /graphql, can coexist with existing REST controllers without conflict.
• Test the API with tools like GraphiQL, Postman, or Spring's own GraphQL test client before rolling it out.

Example:
An existing ProductService used by REST controllers is reused unchanged; a new @QueryMapping method named "product" calls the same service to resolve GraphQL queries, so no business logic is duplicated between the REST and GraphQL layers.

Code Example:
```
type Query {
    product(id: ID!): Product
}

type Product {
    id: ID!
    name: String!
    price: Float!
}
```

```
@Controller
public class ProductGraphQLController {

private final ProductService productService;

@QueryMapping
    public Product product(@Argument String id) {
        return productService.findById(id);
    }
}
```

Interview Tip:
A concise interview answer is:

"I'd add spring-boot-starter-graphql, define the schema in a .graphqls file, and write data fetcher controllers with @QueryMapping and @MutationMapping that call into the same service layer the REST controllers already use. That way GraphQL and REST coexist without duplicating business logic."