GraphQL Queries

Queries in GraphQL

We have already covered 2 parts of our GraphQL series, Part-1 which gives an introduction to GraphQL, and Part-2 which covers the schema types in GraphQL.

If we compare REST API standards where we had GET, POST, PUT, PATCH, and DELETE methods, in GraphQL we only have 2 standards: Queries and Mutations. In this Part-3, we will dive deeper into Queries in GraphQL as Mutation is out of scope from this blog.

Understanding GraphQL Queries

At its core, GraphQL revolves around the concept of queries. A query in GraphQL is a request for specific data, allowing clients to precisely define the shape and structure of the response they desire. Unlike traditional REST APIs where clients often receive excessive or incomplete data, GraphQL empowers clients to request only the data they need, reducing over-fetching and under-fetching of information. We can relate Queries to the GET method of REST standards.

Syntax and Structure

GraphQL queries are written in a declarative syntax, making them highly readable and self-explanatory. Here’s an example of a basic GraphQL query:

query {
  user(id: 123) {
    name
    email
    posts {
      title
      published
    }
  }
}

In this example, the query is requesting the name and email fields of a user with the ID 123, along with the title and published fields of their posts.

Understanding @QueryMapping

@QueryMapping is a powerful annotation used in GraphQL frameworks like GraphQL Java to map GraphQL queries to specific methods in Java classes. It acts as a bridge between the GraphQL schema and the implementation logic. By utilizing @QueryMapping, developers can establish a clear mapping between the GraphQL query fields and the corresponding methods that will handle the data retrieval.

Here’s an example to illustrate the usage of @QueryMapping:

type query {
  getUserById(id: String)
}

@QueryMapping
public User getUserById(String id) {
  // Logic to retrieve user by ID from a data source
  // and return the User object
}

In this example, the getUserById method is annotated with @QueryMapping, indicating that it corresponds to a GraphQL query. When a client sends a query requesting the user data, the GraphQL framework will invoke this method and pass the necessary arguments (in this case, the user ID). The method then retrieves the user data from a data source and returns the User object, which will be serialized and sent back as the query response.

Query Resolvers

Query Resolvers are the heart of GraphQL’s data retrieval mechanism. They are responsible for executing the logic associated with resolving the requested data fields in a GraphQL query. Query resolvers handle the mapping between the query fields and the actual data retrieval and processing.

In GraphQL Java, query resolvers are typically implemented by defining a class that implements the GraphQLQueryResolver interface.

Here’s an example of a query resolver:

@Component
public class UserResolver implements GraphQLQueryResolver {

  public User getUserById(String id) {
    // Logic to retrieve user by ID from a data source
    // and return the User object
  }
}

In this example, the UserResolver class implements the GraphQLQueryResolver interface, indicating that it contains methods to resolve GraphQL queries. The getUserById method corresponds to the respective query field defined in the GraphQL schema. When the corresponding queries are executed, the GraphQL framework will invoke these methods, and the resolvers will retrieve the data from the appropriate data sources, perform any required processing, and return the results.

Utilizing @QueryMapping and Query Resolvers offers several advantages when working with GraphQL queries.

Conclusion

Queries form the backbone of GraphQL’s powerful data retrieval mechanism. By allowing clients to define their precise data requirements, GraphQL empowers developers to optimize performance, reduce network overhead, and enhance the overall developer experience. Understanding the syntax, leveraging variables and fragments, and following best practices will enable you to unlock the full potential of GraphQL queries in your projects.