RxJS を使用した Exponential Backoff カウンター

岩佐 孝浩

2021年8月22日

2 min read

Angular Exponential Backoff RxJS

AWS は、サービスとの連携時に Exponential Backoff の使用を推奨しています。 最近、 Angular アプリケーションプロジェクトに取り組んでいたので、 RxJS を使用してカウンターを実装してみました。

docs.aws.amazon.com

Retry behavior - AWS SDKs and Tools

Retry behavior includes settings regarding how the SDKs attempt to recover from failures resulting from requests made to AWS services. Configure this functionality by using the following settings.

この投稿のサンプルは、 GitHub リポジトリから取得できます。

github.com

GitHub - iwstkhr/exp-backoff-counter: Exponential backoff counter using rxjs

Exponential backoff counter using rxjs. Contribute to iwstkhr/exp-backoff-counter development by creating an account on GitHub.

import { BehaviorSubject, EMPTY, interval, Observable, of, share } from 'rxjs';
import { scan, switchMap, takeWhile } from 'rxjs/operators';

export class ExpBackoffCounter {

  private readonly counterSubject$ = new BehaviorSubject<number>(0);

  /**
   * Get a hot observable which streams a counter value.
   *
   * @remarks
   * Make sure that this observable will be unsubscribed when it is not needed.
   */
  get counter$(): Observable<number> {
    return this.counterSubject$.asObservable().pipe(
      switchMap(count => count <= 0 ? EMPTY : of(count)),
      switchMap(count => interval(1000).pipe(
        scan(acc => acc - 1, count),
        takeWhile(value => value >= 0),
      )),
      share(),
    );
  }

  /**
   * Stream a counter value.
   *
   * @param count Specify a starting value, otherwise the previous starting value + 1
   */
  next(count?: number): void {
    if (count == null) {
      this.counterSubject$.next(2 ** (this.counterSubject$.value + 1));
    } else {
      this.counterSubject$.next(2 ** count);
    }
  }

}