Overview

This module provides APIs for connecting and interacting with GraphQL endpoints.

GraphQL is an open-source data query and manipulation language for APIs. GraphQL allows clients to define the structure of the data required and the same structure of the data is returned from the server, preventing the returning of excessively large amounts of data or reducing the number of requests sent to the server.

The Ballerina GraphQL implementation is using HTTP as the underlying protocol.

Listener

The graphql:Listener is used to listen to a given IP/Port. To create a graphql:Listener, an http:Listener or a port number can be used.

Create a standalone graphql:Listener

import ballerina/graphql;

listener graphql:Listener graphqlListener = check new(4000);

Create a graphql:Listener using an http:Listener

import ballerina/graphql;
import ballerina/http;

listener http:Listener httpListener = check new(4000);
listener graphql:Listener graphqlListener = check new(httpListener);

Service

The Ballerina GraphQL service represents the GraphQL schema. When a service is attached to a graphql:Listener, a GraphQL schema will be auto-generated.

The GraphQL services are exposed through a single endpoint. The path of the GraphQL service endpoint can be provided via the service path of the GraphQL service. The endpoint of the following Ballerina GraphQL service will be /graphql.

import ballerina/graphql;

service graphql:Service /graphql on graphqlListener {
    // ...
}

The GraphQL service endpoint URL will be <host>:<port>/graphql.

Alternatively, a Ballerina graphql service can not have a path, in which case the endpoint will be the host URL and the port as following example.

import ballerina/graphql;

service graphql:Service on graphqlListener {
    // ...
}

The GraphQL service endpoint URL will be <host>:<port>

Alternatively, the listener and service initialization can be combined into a single statement as follows:

service graphql:Service on new graphql:Listener(9090) {

}

Query type

The resource functions inside the GraphQL service can represent the resolvers of the Query root type.

When a resource function is defined inside a GraphQL service with the get accessor, the generated schema will have a Query root type and the resource function will be a field of the Query object.

Note: A GraphQL service must have at least one resource function defined. Otherwise, it will result in a compilation error.

The resource method must use the get accessor for a field to be considered as a Query field. The resource function name will become the name of the particular field in the GraphQL schema. The return type of the resource function will be the type of the corresponding field.

import ballerina/graphql;

service graphql:Service /graphql on new graphql:Listener(4000) {
    resource function get greeting(string name) returns string {
        return "Hello, " + name;
    }
}

The above can be queried using the GraphQL document below:

{
    greeting(name: "John")
}

The result will be the following JSON.

{
    "data": {
        "greeting": "Hello, John"
    }
}

Mutation type

The remote functions inside the GraphQL service represent the resolvers of the Mutation root type.

When a remote function is defined inside a GraphQL service, the schema will have a Mutation operation and the remote function will be a field of the Mutation object.

For example, consider the following service that has a Person record named profile. It has a Query field named profile, which returns the Person record. It also has two remote functions named updateName and updateCity, which are used as mutations.

import ballerina/graphql;

public type Person record {|
    string name;
    int age;
    string city;
|};

service /graphql on new graphql:Listener(4000) {
    private Person profile;

    function init() {
        self.profile = { name: "Walter White", age: 50, city: "Albuquerque" };
    }

    resource function get profile() returns Person {
        return self.profile;
    }

    remote function updateName(string name) returns Person {
        self.profile.name = name;
        return self.profile;
    }

    remote function updateCity(string city) returns Person {
        self.profile.city = city;
        return self.profile;
    }
}

This will generate the following schema:

type Query {
    profile: Person!
}

type Mutation {
    updateName(name: String!): Person!
    updateCity(city: String!): Person!
}

type Person {
    name: String!
    age: Int!
    city: String!
}

Note: A GraphQL schema must have a root Query type. Therefore, a Ballerina GraphQL service must have at least one resource function defined.

This can be mutated using the following document.

mutation updatePerson {
    updateName(name: "Mr. Lambert") {
        ... ProfileFragment
    }
    updateCity(city: "New Hampshire") {
        ... ProfileFragment
    }
}

fragment ProfileFragment on Person {
    name
    city
}

Note: This document uses two mutations and each mutation requests the same fields from the service using a fragment (ProfileFragment).

Result:

{
    "data": {
        "updateName": {
            "name": "Mr. Lambert",
            "city": "Albuquerque"
        },
        "updateCity": {
            "name": "Mr. Lambert",
            "city": "New Hampshire"
        }
    }
}

See how the result changes the Person record. The first mutation changes only the name and it populates the result of the updateName field. Then, it will execute the updateCity operation and populate the result. This is because the execution of the mutation operations will be done serially in the same order as they are specified in the document.

Subscription Type

The subscription type can be used to continuously fetch the data from a GraphQL service.

The resource functions inside the GraphQL service with the subscribe accessor can represent the resolvers of the Subscription root type.

When a resource function is defined inside a GraphQL service with the subscribe accessor, the generated schema will have a Subscription root type and the resource function will be a field of the Subscription object.

The resource method must use the subscribe accessor for a field to be considered as a Subscription field. The resource function name will become the name of the particular field in the GraphQL schema. The return type of the resource function will be the type of the corresponding field.

The resource functions that belong to Subscription type must return a stream of any type. Any other return type will result in a compilation error.

The return type

import ballerina/graphql;

service graphql:Service /graphql on new graphql:Listener(4000) {
    resource function subscribe messages() returns stream<string> {
        return ["Walter", "Jesse", "Mike"].toStream();
    }
}

The above can be queried using the GraphQL document below:

subscription {
    messages
}

When a subscription type is defined, a WebSocket service will be created to call the subscription. The above service will create the service as follows:

ws://<host>:4000/graphql

This can be accessed using a WebSocket client. When the returned stream has a new entry, it will be broadcast to the subscribers.

Types

The Ballerina GraphQL resolver (resource/remote) methods can return the following types:

Return types

Scalar types

When a Ballerina primitive type is returned from a resource or a remote method, it will be mapped to a GraphQL NON_NULL type of the corresponding GraphQL scalar type.

The following Ballerina types are considered as Scalar types:

• int
• string
• boolean
• float

resource function get greeting() returns string {
    return "Hello, World";
}

This can be queried using the following document:

{
    greeting
}

Result:

{
    "data": {
        "greeting": "Hello, World"
    }
}

Enums

When a resource or a remote function returns an enum value, it will be mapped to a GraphQL ENUM type.

import ballerina/graphql;

public enum Color {
    RED,
    GREEN,
    BLUE
}

service on new graphql:Listener(4000) {
    resource function get color(int code) returns Color {
        // ...
    }
}

The above service will generate the following GraphQL schema.

type Query {
    color: Color!
}

enum Color {
    RED
    GREEN
    BLUE
}

Record types

When a resource or remote method is returning a record type, each field of the record can be queried separately. Each record type is mapped to a GraphQL OBJECT type and the fields of the record type are mapped to the fields of the OBJECT type.

public type Person record {|
    string name;
    int age;
|};

resource function get profile() returns Person {
    return { name: "Walter White", age: 51 };
}

This will generate the following schema.

type Query {
    profile: Person!
}

type Person {
    name: String!
    age: Int!
}

This can be queried using the following document:

{
    profile {
        name
        age
    }
}

Result:

{
    "data": {
        "profile": {
            "name": "Walter White",
            "age": 51
        }
    }
}

Each field can be queried separately as shown in the following document:

{
    profile {
        name
    }
}

Result:

{
    "data": {
        "profile": {
            "name": "Walter White"
        }
    }
}

Service types

When a resource function returns a service type, the service type is mapped to a GraphQL OBJECT type and the resource methods of the service type will be mapped as the fields of the OBJECT.

When a service type is returned from a graphql:Service, the returning service type should also follow the rules of the graphql:Service explained above.

import ballerina/graphql;

service graphql:Service /graphql on new graphql:Listener(4000) {
    resource function get profile() returns Person {
        return new("Walter White", 51);
    }
}

service class Person {
    private string name;
    private int age;

    public function init(string name, int age) {
        self.name = name;
        self.age = age;
    }

    resource function get name() returns string => self.name;

    resource function get age() returns int => self.age;
}

This will generate the following schema:

type Query {
    profile: Person!
}

type Person {
    name: String!
    age: Int!
}

This can be queried using the following document:

query getProfile {
    profile {
        name
    }
}

The above will result in the following JSON:

{
    "data": {
        "profile": {
            "name": "Walter White"
        }
    }
}

Arrays

A GraphQL resource function can return an array of the types mentioned above. When a resource method returns an array, the corresponding GraphQL field will be the type of LIST.

public type Person record {|
    string name;
    int age;
|};

resource function get people() returns Person[] {
    Person p1 = { name: "Walter White", age: 51 };
    Person p2 = { name: "James Moriarty", age: 45 };
    Person p3 = { name: "Tom Marvolo Riddle", age: 71 };
    return [p1, p2, p3];
}

This will generate the following schema:

type Query {
    profile: [Person!]!
}

type Person {
    name: String!
    age: Int!
}

This can be queried using the following document:

{
    people {
        name
    }
}

Result:

{
    "data": {
        "people": [
            {
                "name": "Walter White"
            },
            {
                "name": "James Moriarty"
            },
            {
                "name": "Tom Marvolo Riddle"
            }
        ]
    }
}

Note: Each element in the array consists only of the required name field.

Nullable types

A Ballerina GraphQL resource function can return a union of a type and nil. When a resource method returns a union of a type and nil, the corresponding GraphQL field will be of a nullable type.

public type Person record {|
    string name;
    int age;
|};

resource function get profile(int id) returns Person? {
    if (id == 1) {
        return { name: "Walter White", age: 51 };
    }
}

This will generate the following schema:

type Query {
    profile: Person
}

type Person {
    name: String!
    age: Int!
}

This can be queried using the following document:

{
    profile(id: 1) {
        name
    }
}

Result:

{
    "data": {
        "profile": {
            "name": "Walter White"
        }
    }
}

If the following document is used:

{
    profile(id: 4) {
        name
    }
}

This will be the result:

{
    "data": {
        "profile": null
    }
}

Union types

The Ballerina GraphQL service can return a union of distinct service types. This will be mapped to a GraphQL UNION type.

Note: If a union type is returned directly without providing a type name (returns T1|T2|T3), the type name will be T1_T2_T3.

import ballerina/graphql;

public type Profile Student|Teacher;

service /graphql on new graphql:Listener(4000) {
    resource function get profile(int purity) returns Profile {
        if (purity < 90) {
            return new Student(1, "Jesse Pinkman");
        } else {
            return new Teacher(737, "Walter White", "Chemistry");
        }
    }
}

distinct service class Student {
    private int id;
    private string name;

    public function init(int id, string name) {
        self.id = id;
        self.name = name;
    }

    resource function get id() returns int {
        return self.id;
    }

    resource function get name() returns string {
        return self.name;
    }
}

distinct service class Teacher {
    private int id;
    private string name;
    private string subject;

    public function init(int id, string name, string subject) {
        self.id = id;
        self.name = name;
        self.subject = subject;
    }

    resource function get id() returns int {
        return self.id;
    }

    resource function get name() returns string {
        return self.name;
    }

    resource function get subject() returns string {
        return self.subject;
    }
}

This will generate the following schema:

type Query {
    profile(purity: Int!): Profile!
}

type Student {
    id: Int!
    name: String!
}

type Teacher {
    id: Int!
    name: String!
    subject: String!
}

union Profile = Student|Teacher

This can be queried using the following document:

query {
    profile(purity: 75) {
        ... on Student {
            name
        }
        ... on Teacher {
            name
            subject
        }
    }
}

The result will be:

{
    "data": {
        "profile": {
            "name": "Jesse Pinkman"
        }
    }
}

If the following document is used:

query {
    profile(purity: 99) {
        ... on Student {
            name
        }
        ... on Teacher {
            name
            subject
        }
    }
}

The result will be:

{
    "data": {
        "profile": {
            "name": "Walter White",
            "subject": "Chemistry"
        }
    }
}

Errors

A Ballerina GraphQL resource or remote method can return an error with the union of the types mentioned above.

Note: A resource or a remote function cannot return only an error, any subtype of an error, or, an error?, which will result in a compilation error.

public type Person record {|
    string name;
    int age;
|};

resource function get profile(int id) returns Person|error {
    if (id == 1) {
        return { name: "Walter White", age: 51 };
    } else {
        return error(string `Invalid ID provided: ${id}`);
    }
}

This can be queried using the following document:

{
    profile(id: 5) {
        name
    }
}

Result:

{
    "errors": [
        {
            "message": "Invalid ID provided: 5",
            "locations": [
                {
                    "line": 2,
                    "column": 4
                }
            ]
        }
    ]
}

Note: The error message will be logged to the console and the error will be returned to the client.

Hierarchical resource paths

A resource method inside a GraphQL service can have hierarchical paths.

When a hierarchical path is present, each level of the hierarchical path maps to the GraphQL field of the same name, and the type of that field will be mapped to an OBJECT type with the same name.

import ballerina/graphql;

service graphql:Service /graphql on new graphq:Listener(4000) {
    resource function profile/name/first() returns string {
        return "Walter";
    }

    resource function profile/name/last() returns string {
        return "White"
    }

    resource function profile/age() returns int {
        return 51;
    }
}

The above service will create the following schema:

type Query {
    profile: profile!
}

type profile {
    name: name!
    age: Int!
}

type name {
    first: String!
    last: String!
}

Note: The field name and the type names are equal.

Refer to the Ballerina GraphQL specification for more information.