http-retry

A configurable Angular HTTP interceptor to retry GET requests and respond to errors and flaky connections.

Installation

To install this library, run:

npm install @ngx-utilities/http-retry --save -or- yarn add @ngx-utilities/http-retry

and then import HttpRetryModule in your Angular AppModule:

// app.module.ts

import { HttpRetryModule } from '@ngx-utilities/http-retry';

@NgModule({
  imports: [
    HttpRetryModule.forRoot()
  ],
  providers: [
    networkErrorRetryStrategyProvider,
    serverUnavailableRetryStrategyProvider
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

Usage

You configure http-retry by providing (via Angular dependency injection) a collection of injectable classes that implement the HttpRequestRetryStrategy interface. These "retry strategies" tell http-retry which status codes to retry, how many times to retry, and when to stop retrying. The global HTTP interceptor provided by HttpRetryModule will do nothing if you do not provide any retry strategies.

In addition to being thrown by the http request observable like normal, the last HttpErrorResponse received when http-retry stops retrying a request is passed to the retry strategy's onFailure method and emitted on the HttpRetryService's httpRetryFailures observable.

Implementing HttpRequestRetryStrategy

// network-error.retry-strategy.ts

import { Injectable, Provider } from '@angular/core';
import { HttpRequestRetryStrategy, HTTP_REQUEST_RETRY_STRATEGIES } from '@ngx-utilities/http-retry';

import { NetworkStatusService } from './../services/network-status.service';

@Injectable()
export class NetworkErrorRetryStrategy implements HttpRequestRetryStrategy {
  // status code 0 means there was a network error (e.g. a timeout)
  readonly statuses = [0];
  readonly maxCount = 3;

  delayFn(retryNumber: number) {
    // retry immediately, wait 3 seconds and try again, and then stop due the max count
    return retryNumber === 1 ? 0 : 3000;
  }

  onFailure(error: HttpErrorResponse) {
    console.log('network error', error.status, error.url);
  }
}

export const networkErrorRetryStrategyProvider: Provider = {
  provide: HTTP_REQUEST_RETRY_STRATEGIES,
  useClass: NetworkErrorRetryStrategy,
  multi: true
};

// server-unavailable.retry-strategy.ts

@Injectable()
export class ServerUnavailableRetryStrategy implements HttpRequestRetryStrategy {
  // retry if the server is temporarily unavailable (e.g. for maintenance)
  readonly statuses = [502, 503];
  readonly maxCount = 10;

  delayFn() {
    return 3000;
  }

  onFailure(error: HttpErrorResponse) {
    console.log('server unavailable error', error.status, error.url);
  }
}

export const serverUnavailableRetryStrategyProvider: Provider = {
  provide: HTTP_REQUEST_RETRY_STRATEGIES,
  useClass: ServerUnavailableRetryStrategy,
  multi: true
};

Using HttpRetryService (optional)

// my.component.ts

import { HttpRetryService } from '@ngx-utilities/http-retry';

export class MyComponent implements OnInit {
  constructor(private readonly httpRetryService: HttpRetryService) { }

  ngOnInit() {
    this.httpRetryService.httpRetryFailures.subscribe(error => {
      console.log('global http retry error listener', error.status, error.url);
    });
  }
}

RxJS Operator

The core functionality of this library is exposed as a RxJS operator that takes an an array of retry strategies which can be instances of classes or plain objects that implement the HttpRequestRetryStrategy interface. You can use this operator directly if you do not wish to use the global interceptor provided by HttpRetryModule.

import { httpRequestRetry } from '@ngx-utilities/http-retry';

export class MyComponent implements OnInit {
  constructor(private readonly httpClient: HttpClient) { }

  ngOnInit() {
    this.httpClient.get('/some/url').pipe(httpRequestRetry(retryStrategies)).subscribe();
  }
}

Request Headers

http-retry will add a header named X-Request-Attempt-Number to each request it sends that with the number of the current attempt. This was added primarily for testing. But it's plausible that this could be useful information to track on the server in some scenarios, so this header is added in production. A future version may allow disabling this option if needed.

License

MIT © Kevin Phelps