GraphQL for Kotlin: Powerful Libraries for Building APIs

GraphQL Libraries for Kotlin: Complete Overview and Practical Guide

GraphQL is a query language for APIs that allows clients to receive only the data they actually need. In the Kotlin ecosystem, there are several powerful libraries for working with GraphQL, both on the server side and the client side. In this article, we will look at three key libraries: graphql-kotlin, KGraphQL, and Apollo Kotlin, and show examples of their use.

1. graphql-kotlin — Official Library from Expedia Group

graphql-kotlin is the most mature and well-maintained library for creating GraphQL servers in Kotlin. It fully utilizes the language's features such as coroutines, data classes, and null-safety.

Key Features:

• Support for Kotlin coroutines for asynchronous processing
• Automatic schema generation from data classes
• Built-in support for Subscriptions via WebSocket
• Integration with Spring Boot, Ktor, and other frameworks
• Full support for the GraphQL June 2018 specification

Example Server with graphql-kotlin:

// Define the data modeldata class User(val id: String, val name: String, val email: String)

// Create a Query resolverclass UserQuery : Query {    fun getUser(id: String): User? {        // Logic for retrieving a user        return User(id = "1", name = "Ivan Petrov", email = "ivan@example.com")    }        fun users(): List<User> = listOf(        User("1", "Ivan", "ivan@example.com"),        User("2", "Maria", "maria@example.com")    )}

// Configure and start the serverfun main() {    val schema = Schema {        query(UserQuery())    }        val server = GraphQLServer(schema)    server.start(port = 8080)}

Example Client Request:

query {  user(id: "1") {    name    email  }}

2. KGraphQL — Lightweight Alternative

KGraphQL is a minimalist library for creating GraphQL APIs. It stands out for its simplicity of setup and lack of unnecessary dependencies.

Key Features:

• Minimal size — about 100 KB
• Does not require Spring Boot or other heavy frameworks
• Support for DSL for defining the schema
• Ability to integrate with any HTTP server

Example of Using KGraphQL:

// Define the modeldata class Product(val id: Int, val title: String, val price: Double)

// Create a GraphQL schemaval schema = schema {    query("products") {        resolver { ->             listOf(                Product(1, "Laptop", 89999.99),                Product(2, "Mouse", 2499.50)            )        }    }        query("product") {        resolver { id: Int ->            Product(id, "Product #$id", 1000.0)        }    }}

// Start the serverfun main() {    val server = KGraphQLServer(schema)    server.start(port = 4000)}

3. Apollo Kotlin — Client Library from Apollo

Apollo Kotlin is a powerful client for working with GraphQL APIs. It generates type-safe code based on the GraphQL schema, which significantly simplifies development.

Advantages of Apollo Kotlin:

• Generation of Kotlin code from .graphql files
• Support for caching (normalized cache)
• Working with coroutines and Flow
• Automatic error handling
• Support for mutations, queries, and subscriptions

Example of Using Apollo Kotlin on the Client:

// First, define the query in a .graphql file// file: GetUser.graphql// query GetUser($id: ID!) {//   user(id: $id) {//     id//     name//     email//   }