retrywhen.md

retryWhen

signature: retryWhen(receives: (errors: Observable) => Observable, the: scheduler): Observable

Retry an observable sequence on error based on custom criteria.

Examples

Example 1: Trigger retry after specified duration

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { timer, interval } from 'rxjs';
import { map, tap, retryWhen, delayWhen } from 'rxjs/operators';

//emit value every 1s
const source = interval(1000);
const example = source.pipe(
  map(val => {
    if (val > 5) {
      //error will be picked up by retryWhen
      throw val;
    }
    return val;
  }),
  retryWhen(errors =>
    errors.pipe(
      //log error message
      tap(val => console.log(`Value ${val} was too high!`)),
      //restart in 6 seconds
      delayWhen(val => timer(val * 1000))
    )
  )
);
/*
  output:
  0
  1
  2
  3
  4
  5
  "Value 6 was too high!"
  --Wait 6 seconds then repeat
*/
const subscribe = example.subscribe(val => console.log(val));

Example 2: Customizable retry with increased duration

( StackBlitz )

import { Observable, throwError, timer } from 'rxjs';
import { mergeMap, finalize } from 'rxjs/operators';

export const genericRetryStrategy = ({
  maxRetryAttempts = 3,
  scalingDuration = 1000,
  excludedStatusCodes = []
}: {
  maxRetryAttempts?: number,
  scalingDuration?: number,
  excludedStatusCodes?: number[]
} = {}) => (attempts: Observable<any>) => {
  return attempts.pipe(
    mergeMap((error, i) => {
      const retryAttempt = i + 1;
      // if maximum number of retries have been met
      // or response is a status code we don't wish to retry, throw error
      if (
        retryAttempt > maxRetryAttempts ||
        excludedStatusCodes.find(e => e === error.status)
      ) {
        return throwError(error);
      }
      console.log(
        `Attempt ${retryAttempt}: retrying in ${retryAttempt *
          scalingDuration}ms`
      );
      // retry after 1s, 2s, etc...
      return timer(retryAttempt * scalingDuration);
    }),
    finalize(() => console.log('We are done!'))
  );
};

import { Component, OnInit } from '@angular/core';
import { catchError, retryWhen  } from 'rxjs/operators';
import { of } from 'rxjs';
import { genericRetryStrategy } from './rxjs-utils';
import { AppService } from './app.service';

@Component({
  selector: 'my-app',
  templateUrl: './app.component.html',
  styleUrls: [ './app.component.css' ]
})
export class AppComponent implements OnInit  {
  constructor(private _appService: AppService) {}

  ngOnInit() {
    this._appService
      .getData(500)
      .pipe(
        retryWhen(genericRetryStrategy()),
        catchError(error => of(error))
      )
      .subscribe(console.log);

    // excluding status code, delay for logging clarity
    setTimeout(() => {
    this._appService
      .getData(500)
      .pipe(
        retryWhen(genericRetryStrategy({
          scalingDuration: 2000,
          excludedStatusCodes: [500]
        })),
        catchError(error => of(error))
      )
      .subscribe(e => console.log('Exluded code:', e.status));

    }, 8000);
  }
}

Additional Resources

• retryWhen 📰 - Official docs
• retryWhen - In Depth Dev Reference
• Error handling operator: retry and retryWhen 🎥 💵 - André Staltz

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/retryWhen.ts