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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.

Common tasks


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() {
    // 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')}`);

Next Steps