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What is a Sequence?

A Sequence is a stateless grouping of Actions that control how a Server responds to requests.

The contract of a Sequence is simple: it must produce a response to a request. Creating your own Sequence gives you full control over how your Server instances handle requests and responses. The DefaultSequence looks like this:

class DefaultSequence {
  async handle(context: RequestContext) {
    try {
      const route = this.findRoute(context.request);
      const params = await this.parseParams(context.request, route);
      const result = await this.invoke(route, params);
      await this.send(context.response, result);
    } catch (error) {
      await this.reject(context, error);


In the example above, route, params, and result are all Elements. When building sequences, you use LoopBack Elements to respond to a request:


Actions are JavaScript functions that only accept or return Elements. Since the input of one action (an Element) is the output of another action (Element) you can easily compose them. Below is an example that uses several built-in Actions:

class MySequence extends DefaultSequence {
  async handle(context: RequestContext) {
    // findRoute() produces an element
    const route = this.findRoute(context.request);
    // parseParams() uses the route element and produces the params element
    const params = await this.parseParams(context.request, route);
    // invoke() uses both the route and params elements to produce the result (OperationRetVal) element
    const result = await this.invoke(route, params);
    // send() uses the result element
    await this.send(context.response, result);

Custom Sequences

Most use cases can be accomplished with DefaultSequence or by slightly customizing it. When an app is generated by the command lb4 app, a sequence file extending DefaultSequence at src/sequence.ts is already generated and bound for you so that you can easily customize it.

Here is an example where the application logs out a message before and after a request is handled:

import {DefaultSequence, Request, Response} from '@loopback/rest';

class MySequence extends DefaultSequence {
  log(msg: string) {
  async handle(context: RequestContext) {
    this.log('before request');
    await super.handle(context);
    this.log('after request');

In order for LoopBack to use your custom sequence, you must register it before starting your Application:

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

const app = new RestApplication();


Advanced topics

Customizing Sequence Actions

There might be scenarios where the default sequence ordering is not something you want to change, but rather the individual actions that the sequence will execute.

To do this, you’ll need to override one or more of the sequence action bindings used by the RestServer, under the RestBindings.SequenceActions constants.

As an example, we’ll implement a custom sequence action to replace the default “send” action. This action is responsible for returning the response from a controller to the client making the request.

To do this, we’ll register a custom send action by binding a Provider to the RestBindings.SequenceActions.SEND key.

First, let’s create our CustomSendProvider class, which will provide the send function upon injection.



import {Send, Response} from '@loopback/rest';
import {Provider, BoundValue, inject} from '@loopback/context';
import {writeResultToResponse, RestBindings, Request} from '@loopback/rest';

// Note: This is an example class; we do not provide this for you.
import {Formatter} from '../utils';

export class CustomSendProvider implements Provider<Send> {
  // In this example, the injection key for formatter is simple
    @inject('utils.formatter') public formatter: Formatter,
    @inject(RestBindings.Http.REQUEST) public request: Request,
  ) {}

  value() {
    // Use the lambda syntax to preserve the "this" scope for future calls!
    return (response: Response, result: OperationRetval) => {
      this.action(response, result);
   * Use the mimeType given in the request's Accept header to convert
   * the response object!
   * @param response The response object used to reply to the  client.
   * @param result The result of the operation carried out by the controller's
   * handling function.
  action(response: Response, result: OperationRetval) {
    if (result) {
      // Currently, the headers interface doesn't allow arbitrary string keys!
      const headers = (this.request.headers as any) || {};
      const header = headers.accept || 'application/json';
      const formattedResult = this.formatter.convertToMimeType(result, header);
      response.setHeader('Content-Type', header);
    } else {

Our custom provider will automatically read the Accept header from the request context, and then transform the result object so that it matches the specified MIME type.

Next, in our application class, we’ll inject this provider on the RestBindings.SequenceActions.SEND key.


import {RestApplication, RestBindings} from '@loopback/rest';
import {
} from '@loopback/repository';
import {CustomSendProvider} from './providers/custom-send.provider';
import {Formatter} from './utils';
import {BindingScope} from '@loopback/context';

export class YourApp extends RepositoryMixin(RestApplication) {
  constructor() {
    // Assume your controller setup and other items are in here as well.

As a result, whenever the send action of the DefaultSequence is called, it will make use of your function instead! You can use this approach to override any of the actions listed under the RestBindings.SequenceActions namespace.

Query string parameters

How to get query string param values.

Parsing Requests

Parsing and validating arguments from the request url, headers, and body. See page Parsing requests

Invoking controller methods

  • How to use invoke() in simple and advanced use cases.
  • Explain what happens when you call invoke()
  • Mention caching use case
  • Can I call invoke multiple times?

Writing the response

  • Must call sendResponse() exactly once
  • Streams?

Handling errors

There are many reasons why the application may not be able to handle an incoming request:

  • The requested endpoint (method + URL path) was not found.
  • Parameters provided by the client were not valid.
  • A backend database or a service cannot be reached.
  • The response object cannot be converted to JSON because of cyclic dependencies.
  • A programmer made a mistake and a TypeError is thrown by the runtime.
  • And so on.

In the Sequence implementation described above, all errors are handled by a single catch block at the end of the sequence, using the Sequence Action called reject.

The default implementation of reject does the following steps:

  • Call strong-error-handler to send back an HTTP response describing the error.
  • Log the error to stderr if the status code was 5xx (an internal server error, not a bad request).

To prevent the application from leaking sensitive information like filesystem paths and server addresses, the error handler is configured to hide error details.

  • For 5xx errors, the output contains only the status code and the status name from the HTTP specification. For example:

      "error": {
        "statusCode": 500,
        "message": "Internal Server Error"
  • For 4xx errors, the output contains the full error message (error.message) and the contents of the details property (error.details) that ValidationError typically uses to provide machine-readable details about validation problems. It also includes error.code to allow a machine-readable error code to be passed through which could be used, for example, for translation.

      "error": {
        "statusCode": 422,
        "name": "Unprocessable Entity",
        "message": "Missing required fields",

During development and testing, it may be useful to see all error details in the HTTP responsed returned by the server. This behavior can be enabled by enabling the debug flag in error-handler configuration as shown in the code example below. See strong-error-handler docs for a list of all available options.

app.bind(RestBindings.ERROR_WRITER_OPTIONS).to({debug: true});

An example error message when the debug mode is enabled:

  "error": {
    "statusCode": 500,
    "name": "Error",
    "message": "ENOENT: no such file or directory, open '/etc/passwords'",
    "errno": -2,
    "syscall": "open",
    "code": "ENOENT",
    "path": "/etc/passwords",
    "stack": "Error: a test error message\n    at Object.openSync (fs.js:434:3)\n    at Object.readFileSync (fs.js:339:35)"

Keeping your Sequences

  • Try and use existing actions
  • Implement your own version of built in actions
  • Publish reusable actions to npm

Working with Express middleware

Under the hood, LoopBack leverages Express framework and its concept of middleware. To avoid common pitfalls, it is not possible to mount Express middleware directly on a LoopBack application. Instead, LoopBack provides and enforces a higher-level structure.

In a typical Express application, there are four kinds of middleware invoked in the following order:

  1. Request-preprocessing middleware like cors or body-parser.
  2. Route handlers handling requests and producing responses.
  3. Middleware serving static assets (files).
  4. Error handling middleware.

In LoopBack, we handle the request in the following steps:

  1. The built-in request-preprocessing middleware is invoked.
  2. The registered Sequence is started. The default implementation of findRoute and invoke actions will try to match the incoming request against the following resources:
    1. Native LoopBack routes (controller methods, route handlers).
    2. External Express routes (registered via mountExpressRouter API)
    3. Static assets
  3. Errors are handled by the Sequence using reject action.

Let’s see how different kinds of Express middleware can be mapped to LoopBack concepts:

Request-preprocessing middleware

At the moment, LoopBack does not provide API for mounting arbitrary middleware, we are discussing this feature in issues #1293 and #2035. Please up-vote them if you are interested in using Express middleware in LoopBack applications.

All applications come with cors enabled, this middleware can be configured via RestServer options - see Customize CORS.

While it is not possible to add additional middleware to a LoopBack application, it is possible to mount the entire LoopBack application as component of a parent top-level Express application where you can add arbitrary middleware as needed. You can find more details about this approach in Creating an Express Application with LoopBack REST API

Route handlers

In Express, a route handler is a middleware function that serves the response and does not call next(). Handlers can be registered using APIs like app.get(),, but also a more generic app.use().

In LoopBack, we typically use Controllers and Route handlers to implement request handling logic.

To support interoperability with Express, it is also possible to take an Express Router instance and add it to a LoopBack application as an external router - see Mounting an Express Router. This way it is possible to implement server endpoints using Express APIs.

Static files

LoopBack provides native API for registering static assets as described in Serve static files. Under the hood, static assets are served by serve-static middleware from Express.

The main difference between LoopBack and vanilla Express applications: LoopBack ensures that static-asset middleware is always invoked as the last one, only when no other route handled the request. This is important for performance reasons to avoid costly filesystem calls.

Error handling middleware

In Express, errors are handled by a special form of middleware, one that’s accepting four arguments: err, request, response, next. It’s up to the application developer to ensure that error handler is registered as the last middleware in the chain, otherwise not all errors may be routed to it.

In LoopBack, we use async functions instead of callbacks and thus can use simple try/catch flow to receive both sync and async errors from individual sequence actions. A typical Sequence implementation then passes these errors to the Sequence action reject.

You can learn more about error handling in Handling errors.