Page Contents

Overview

The Server interface defines the minimal required functions (start and stop) and a ‘listening’ property to implement for a LoopBack application. Servers in LoopBack 4 are used to represent implementations for inbound transports and/or protocols such as REST over http, gRPC over http2, graphQL over https, etc. They typically listen for requests on a specific port, handle them, and return appropriate responses. A single application can have multiple server instances listening on different ports and working with different protocols.

Usage

LoopBack 4 offers the @loopback/rest package out of the box, which provides an HTTP/HTTPS-based server called RestServer for handling REST requests.

In order to use it in your application, your application class needs to extend RestApplication to provide an instance of RestServer listening on port 3000. The following example shows how to use RestApplication:

import {RestApplication, RestServer} from '@loopback/rest';

export class HelloWorldApp extends RestApplication {
  constructor() {
    super();
    // give our RestServer instance a sequence handler function which
    // returns the Hello World string for all requests
    // with RestApplication, handler function can be registered
    // at app level
    this.handler((sequence, request, response) => {
      sequence.send(response, 'Hello World!');
    });
  }

  async start() {
    // call start on application class, which in turn starts all registered
    // servers
    await super.start();

    // get a singleton HTTP server instance
    const rest = await this.getServer(RestServer);
    console.log(`REST server running on port: ${await rest.get('rest.port')}`);
  }
}

Configuration

The REST server can be configured by passing a rest property inside your RestApplication options. For example, the following code customizes the port number that a REST server listens on.

const app = new RestApplication({
  rest: {
    port: 3001,
  },
});

Customize How OpenAPI Spec is Served

There are a few options under rest.openApiSpec to configure how OpenAPI spec is served by the given REST server.

  • servers: Configure servers for OpenAPI spec
  • setServersFromRequest: Set servers based on HTTP request headers, default to false
  • endpointMapping: Maps urls for various forms of the spec. Default to:
    {
      '/openapi.json': {version: '3.0.0', format: 'json'},
      '/openapi.yaml': {version: '3.0.0', format: 'yaml'},
    }
const app = new RestApplication({
  rest: {
    openApiSpec: {
      servers: [{url: 'http://127.0.0.1:8080'}],
      setServersFromRequest: false,
      endpointMapping: {
        '/openapi.json': {version: '3.0.0', format: 'json'},
        '/openapi.yaml': {version: '3.0.0', format: 'yaml'},
      },
    },
  },
});

Configure the API Explorer

LoopBack allows externally hosted API Explorer UI to render the OpenAPI endpoints for a REST server. Such URLs can be specified with rest.apiExplorer:

  • url: URL for the hosted API Explorer UI, default to https://loopback.io/api-explorer.
  • httpUrl: URL for the API explorer served over plain http to deal with mixed content security imposed by browsers as the spec is exposed over http by default. See https://github.com/strongloop/loopback-next/issues/1603. Default to the value of url.
const app = new RestApplication({
  rest: {
    apiExplorer: {
      url: 'https://petstore.swagger.io',
      httpUrl: 'http://petstore.swagger.io',
    },
  },
});

Enable HTTPS

Enabling HTTPS for the LoopBack REST server is just a matter of specifying the protocol as https and specifying the credentials.

In the following app, we configure HTTPS for a bare minimum app using a key + certificate chain variant.

import {RestApplication, RestServer, RestBindings} from '@loopback/rest';
import * as fs from 'fs';

export async function main() {
  const options = {
    rest: {
      protocol: 'https',
      key: fs.readFileSync('./key.pem'),
      cert: fs.readFileSync('./cert.pem'),
    },
  };
  const app = new RestApplication(options);
  app.handler(handler => {
    handler.response.send('Hello');
  });
  await app.start();

  const url = app.restServer.url;
  console.log(`Server is running at ${url}`);
}

Customize CORS

CORS is enabled by default for REST servers with the following options:

{
  origin: '*',
  methods: 'GET,HEAD,PUT,PATCH,POST,DELETE',
  preflightContinue: false,
  optionsSuccessStatus: 204,
  maxAge: 86400,
  credentials: true,
}

The application code can customize CORS via REST configuration:

export async function main() {
  const options = {
    rest: {
      cors: {...},
    },
  };
  const app = new RestApplication(options);
}

For a complete list of CORS options, see https://github.com/expressjs/cors#configuration-options.

rest options

Property Type Purpose
port number Specify the port on which the RestServer will listen for traffic.
protocol string (http/https) Specify the protocol on which the RestServer will listen for traffic.
key string Specify the SSL private key for https.
cert string Specify the SSL certificate for https.
cors CorsOptions Specify the CORS options.
sequence SequenceHandler Use a custom SequenceHandler to change the behavior of the RestServer for the request-response lifecycle.
openApiSpec OpenApiSpecOptions Customize how OpenAPI spec is served
apiExplorer ApiExplorerOptions Customize how API explorer is served

Add servers to application instance

You can add server instances to your application via the app.server() method individually or as an array using app.servers() method. Using app.server() allows you to uniquely name your binding key for your specific server instance. The following example demonstrates how to use these functions:

import {Application} from '@loopback/core';
import {RestServer} from '@loopback/rest';

export class HelloWorldApp extends Application {
  constructor() {
    super();
    // This server instance will be bound under "servers.fooServer".
    this.server(RestServer, 'fooServer');
    // Creates a binding for "servers.MQTTServer" and a binding for
    // "servers.SOAPServer";
    this.servers([MQTTServer, SOAPServer]);
  }
}

You can also add multiple servers in the constructor of your application class as shown here.

Next Steps