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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.


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

In order to use it in your application, all you need to do is have your application class extend RestApplication, and it will provide you with an instance of RestServer listening on port 3000. The following shows how to make use of it:

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

export class HelloWorldApp extends RestApplication {
  constructor() {
    // 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')}`);


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 miminum 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 => {
  await app.start();

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

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() {
    // 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