/**
   * Gets the value from cache if the key is provided.
   * If no value exists in cache, then check if the same call exists
   * in flight, if so return the subject. If not create a new
   * Subject inFlightObservable and return the source observable.
   */
  get(key: string, fallback?: Observable<any>, maxAge?: number): Observable<any> | Subject<any> {

    if (this.hasValidCachedValue(key)) {
      console.log(`%cGetting from cache ${key}`, 'color: green');
      return Observable.of(this.cache.get(key).value);
    }

    if (!maxAge) {
      maxAge = this.DEFAULT_MAX_AGE;
    }

    if (this.inFlightObservables.has(key)) {
      return this.inFlightObservables.get(key);
    } else if (fallback && fallback instanceof Observable) {
      this.inFlightObservables.set(key, new Subject());
      console.log(`%c Calling api for ${key}`, 'color: purple');
      return fallback.do((value) => { this.set(key, value, maxAge); });
    } else {
      return Observable.throw('Requested key is not available in Cache');
    }

  }