Ktor Graphql

Easily serve GraphQL over http together with Ktor.

Installation:

This project is hosted on Maven Central (from version 2.0.3 onwards). See package here

Add dependency:

Maven

<dependency>
    <groupId>com.github.excitement-engineer</groupId>
    <artifactId>ktor-graphql</artifactId>
    <version>${version}</version>
</dependency>

Gradle

build.gradle:

compile 'com.github.excitement-engineer:ktor-graphql:${version}'

build.gradle.kts:

implementation("com.github.excitement-engineer:ktor-graphql:${version}")

Setup

Add the graphQL route inside of Ktor's routing feature:

val server = embeddedServer(Netty, port = 8080) {
    routing {
        graphQL("/graphql", schema) { request ->
            Config(showExplorer = true)
        }
    }
}

server.start(wait = true)

The graphQL route has two required parameters:

• The path associated with the graphQL endpoint.
• A graphQL schema (see GraphQL-Java).

Optionally, various options can be passed in a Config in the trailing callback. The callback is invoked every time that a request is executed against the server.

Configuration

The Config class accepts the following parameters:

• showExplorer: If true, then a GraphQL-playground is presented when the graphQL endpoint is loaded in the browser. This is useful for testing your application during development.
• formatError: An optional function that it is used to format any errors that occur in completing a GraphQL operation. If not provided, GraphQL's default spec-compliant GraphQLError.toSpecification() is used.
• executeRequest: A custom function for executing queries against your schema.
• renderExplorer: A function to render a custom graphql explorer. The default is Graphql-Playground however this library also contains a way for rendering GraphiQL (see ktor.graphql.explorer.graphiQL).

HTTP usage

ktor-graphql will accept requests with the parameters:

• query: A string GraphQL document to be executed.
• variables: The runtime values to use for any GraphQL query variables as a JSON object.
• operationName: If the provided query contains multiple named operations, this specifies which operation should be executed. If not provided, a 400 error will be returned if the query contains multiple named operations.
• raw: If the showExplorer option is enabled and the raw parameter is provided raw JSON will always be returned instead of the GraphQL explorer even when loaded from a browser.

GraphQL will first look for each parameter in the URL's query-string:

/graphql?query=query+getUser($id:ID){user(id:$id){name}}&variables={"id":"4"}

If not found in the query-string, it will look in the POST request body.

ktor-graphql will interpret the POST body depending on the provided Content-Type header.

• application/json: the POST body will be parsed as a JSON object of parameters.
• application/graphql: the POST body will be parsed as GraphQL query string, which provides the query parameter.

Guides

Passing a root value or context

In order to pass a context when executing a query against the schema we can use the executeRequest option of the Config.

val graphQLExecutor = GraphQL.newGraphQL(schema).build()

val server = embeddedServer(Netty, port = 8080) {
    routing {
        graphQL("/graphql", schema) { request ->
            Config(
                executeRequest = {
                    val input = ExecutionInput.newExecutionInput()
                            .fromRequest(request)
                            .context(yourContent)
                            .root(yourRootValue)
                            .build()

                    graphQLExecutor.execute(input)
                }
            )
        }
    }
}

server.start(wait = true)

You can also pass a dataLoader instance into ExecutionInput.

Masking exceptions

An unexpected exception may occur in your application that you may not want to expose to consumers of your API because they may contain cryptic errors or may expose server-side code. For example, if the query { user(id: "1") { name } } is executed then the server will respond with Could not cast db.user to User. A more constructive error could be Internal server error. Please report this error to errors@domain.com

In order to do this we introduce an Exception subclass ClientException. This exception should be thrown whenever an error occurs that the user of the API should be notified about. An use case for ClientException is an error stating that you need to be authenticated to perform the operation.

class ClientException(message: String): Exception(message)

Next, we customize the error behavior in the formatError option by customizing the error message depending on whether the original exception is a ClientException or not.

graphQL("/graphql", schema) {
    Config(
        formatError = { error ->
            val clientMessage = if (error is ExceptionWhileDataFetching) {

                val exception = error.exception

                val formattedMessage = if (exception is ClientException) {
                    exception.message
                } else {
                    "Internal server error"
                }

                formattedMessage
            } else {
                error.message
            }

            val result = error.toSpecification()
            result["message"] = clientMessage

            result
        }
    )
}

The example repo provides an example on how to implement error masking.

Example

Refer to the ktor-graphql-example repo for an example on how to use ktor-graphql. It provides an example on how to implement:

• Authentication of a user using an access token and then passing the authenticated user to GraphQL's context.
• Masking internal server errors using the formatError function.
• Writing tests against you graphQL endpoint.

Credit

This library is based on the express-graphql library.

Part of the documentation, API and test cases in this library are based on express-graphql, credit goes to the authors of this library.