Page Contents

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/core';
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 Record<string, string>) || {};
      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/core';

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 and path parameters

OAI 3.0.x describes the data from a request’s header, query and path in an operation specification’s parameters property. In a Controller method, such an argument is typically decorated by @param(). We’ve made multiple shortcuts available to the @param() decorator in the form of @param.<http_source>.<OAI_primitive_type>. Using this notation, path parameters can be described as @param.path.string. Here is an example of a controller method which retrieves a Note model instance by obtaining the id from the path object.

@get('/notes/{id}', {
  responses: {
    '200': {
      description: 'Note model instance',
      content: {
        'application/json': {
          schema: getModelSchemaRef(Note, {includeRelations: true}),
async findById(
  @param.path.string('id') id: string,
  @param.filter(Note, {exclude: 'where'}) filter?: FilterExcludingWhere<Note>
): Promise<Note> {
  return this.noteRepository.findById(id, filter);

(Notice: the filter for findById() method only supports the include clause for now.)

You can also specify a parameter which is an object value encoded as a JSON string or in multiple nested keys. For a JSON string, a sample value would be location={"lang": 23.414, "lat": -98.1515}. For the same location object, it can also be represented as location[lang]=23.414&location[lat]=-98.1515. Here is the equivalent usage for @param.query.object() decorator. It takes in the name of the parameter and an optional schema or reference object for it.

@param.query.object('location', {
  type: 'object',
  properties: {lat: {type: 'number', format: 'float'}, long: {type: 'number', format: 'float'}},

The parameters are retrieved as the result of parseParams Sequence action. Please note that deeply nested properties are not officially supported by OAS yet and is tracked by OAI/OpenAPI-Specification#1706. Therefore, our REST API Explorer does not allow users to provide values for such parameters and unfortunately has no visible indication of that. This problem is tracked and discussed in swagger-api/swagger-js#1385.

Parsing Requests

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

Invoking controller methods

The invoke sequence action simply takes the parsed request parameters from the parseParams action along with non-decorated arguments, calls the corresponding controller method or route handler method, and returns the value from it. The default implementation of invoke action calls the handler function for the route with the request specific context and the arguments for the function. It is important to note that controller methods use invokeMethod from @loopback/core and can be used with global and custom interceptors. See Interceptor docs for more details. The request flow for two route flavours is explained below.

For controller methods:

  • A controller instance is instantiated from the context. As part of the instantiation, constructor and property dependencies are injected. The appropriate controller method is invoked via the chain of interceptors.
  • Arguments decorated with @param are resolved using data parsed from the request. Arguments decorated with @inject are resolved from the context. Arguments with no decorators are set to undefined, which is replaced by the argument default value if it’s provided.

For route handlers, the handler function is invoked via the chain of interceptors. The array of method arguments is constructed using OpenAPI spec provided at handler registration time (either via .api() for full schema or .route() for individual route registration).

Writing the response

The send sequence action is responsible for writing the result of the invoke action to the HTTP response object. The default sequence calls send with (transformed) data. Under the hood, send performs all steps required to send back the response, from content-negotiation to serialization of the response body. In Express, the handler is responsible for setting response status code, headers and writing the response body. In LoopBack, controller methods and route handlers return data describing the response and it’s the responsibility of the Sequence to send that data back to the client. This design makes it easier to transform the response before it is sent.

LoopBack 4 does not yet provide first-class support for streaming responses, see Issue#2230. As a short-term workaround, controller methods are allowed to send the response directly, effectively bypassing send action. The default implementation of send is prepared to handle this case here.

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 response 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

For most use cases, the default sequence supplied with LoopBack 4 applications is good enough for request-response handling pipeline. Check out Custom Sequences on how to extend it and implement custom actions.