RetryWithBackOffStrategy.cs

// Licensed under the MIT license with <3 by GitHub

/// <summary>
/// An exponential back off strategy which starts with 1 second and then 4, 9, 16...
/// </summary>
[SuppressMessage("Microsoft.Security", "CA2104:DoNotDeclareReadOnlyMutableReferenceTypes")]
public static readonly Func<int, TimeSpan> ExponentialBackoff = n => TimeSpan.FromSeconds(Math.Pow(n, 2));

/// <summary>
/// Returns a cold observable which retries (re-subscribes to) the source observable on error up to the 
/// specified number of times or until it successfully terminates. Allows for customizable back off strategy.
/// </summary>
/// <param name="source">The source observable.</param>
/// <param name="retryCount">The number of attempts of running the source observable before failing.</param>
/// <param name="strategy">The strategy to use in backing off, exponential by default.</param>
/// <param name="retryOnError">A predicate determining for which exceptions to retry. Defaults to all</param>
/// <param name="scheduler">The scheduler.</param>
/// <returns>
/// A cold observable which retries (re-subscribes to) the source observable on error up to the 
/// specified number of times or until it successfully terminates.
/// </returns>
[SuppressMessage("Microsoft.Reliability", "CA2000:Dispose objects before losing scope")]
public static IObservable<T> RetryWithBackoffStrategy<T>(
    this IObservable<T> source, 
    int retryCount = 3,
    Func<int, TimeSpan> strategy = null,
    Func<Exception, bool> retryOnError = null,
    IScheduler scheduler = null)
{
    strategy = strategy ?? ExpontentialBackoff;
    scheduler = scheduler ?? RxApp.TaskpoolScheduler;

    if (retryOnError == null)
        retryOnError = e => true;

    int attempt = 0;

    return Observable.Defer(() =>
    {
        return ((++attempt == 1) ? source : source.DelaySubscription(strategy(attempt - 1), scheduler))
            .Select(item => new Tuple<bool, T, Exception>(true, item, null))
            .Catch<Tuple<bool, T, Exception>, Exception>(e => retryOnError(e)
                ? Observable.Throw<Tuple<bool, T, Exception>>(e)
                : Observable.Return(new Tuple<bool, T, Exception>(false, default(T), e)));
    })
    .Retry(retryCount)
    .SelectMany(t => t.Item1
        ? Observable.Return(t.Item2)
        : Observable.Throw<T>(t.Item3));
}

RetryWithBackOffStrategyTests.cs

// Licensed under the MIT license with <3 by GitHub

static readonly Func<int, TimeSpan> LinearMsStrategy = n => TimeSpan.FromMilliseconds(1*n);

[Fact]
public void DoesNotRetryInCaseOfSuccess()
{
    new TestScheduler().With(sched =>
    {
        int tryCount = 0;

        var source = Observable.Defer(() =>
        {
            tryCount++;
            return Observable.Return("yolo");
        });

        source.RetryWithBackoffStrategy(
            retryCount: 3,
            strategy: LinearMsStrategy,
            scheduler: sched
            );

        source.Subscribe();

        Assert.Equal(1, tryCount);

        sched.AdvanceByMs(1);
        Assert.Equal(1, tryCount);
    });
}

[Fact]
public void PropagatesLastObservedExceptionIfAllTriesFail()
{
    new TestScheduler().With(sched =>
    {
        int tryCount = 0;

        var source = Observable.Defer(() =>
        {
            tryCount++;
            return Observable.Throw<string>(new InvalidOperationException(tryCount.ToString()));
        });

        var observable = source.RetryWithBackoffStrategy(
            retryCount: 3,
            strategy: LinearMsStrategy,
            scheduler: sched
            );

        Exception lastError = null;
        observable.Subscribe(_ => {}, e => {lastError = e;});
                    
        Assert.Equal(1, tryCount);

        sched.AdvanceByMs(1);
        Assert.Equal(2, tryCount);

        sched.AdvanceByMs(2);
        Assert.Equal(3, tryCount);

        Assert.Null(lastError);
        Assert.Equal("3", lastError.Message);
    });
}

[Fact]
public void RetriesOnceIfSuccessBeforeRetriesRunOut()
{
    new TestScheduler().With(sched =>
    {
        int tryCount = 0;

        var source = Observable.Defer(() =>
        {
            if (tryCount++ < 1) return Observable.Throw<string>(new InvalidOperationException());
            return Observable.Return("yolo " + tryCount);
        });

        var observable = source.RetryWithBackoffStrategy(
            retryCount: 5,
            strategy: LinearMsStrategy,
            scheduler: sched
            );

        string lastValue = null;
        observable.Subscribe(n => {lastValue = n;});

        Assert.Equal(1, tryCount);
        Assert.Null(lastValue);

        sched.AdvanceByMs(1);
        Assert.Equal(2, tryCount);
        Assert.Equal("yolo 2", lastValue);
    });
}

[Fact]
public void UnsubscribingDisposesSource()
{
    new TestScheduler().With(sched =>
    {
        int c = -1;

        var neverEndingSource = Observable.Defer(() =>
        {
            return Observable.Timer(TimeSpan.FromMilliseconds(1), TimeSpan.FromMilliseconds(1), sched)
                .Do(_ => c++)
                .Select(_ => Unit.Default);
        });

        var observable = neverEndingSource.RetryWithBackoffStrategy(scheduler: sched);

        // Cold
        Assert.Equal(-1, c);

        var disp = observable
            .Take(2)
            .Subscribe();

        sched.AdvanceByMs(1);
        Assert.Equal(0, c);

        sched.AdvanceByMs(1);
        Assert.Equal(1, c);

        sched.AdvanceByMs(10);
        Assert.Equal(1, c);
    });
}

Samples.cs

public IObservable<IWebResponse> MakeWebRequest() { ... }

// Retry 4 times with default back off strategy for all exceptions
return Observable.Defer(() => MakeWebRequest())
  .RetryWithBackoffStrategy(retryCount: 4)

// Retry 4 times with default back off strategy for all web exceptions
return Observable.Defer(() => MakeWebRequest())
  .RetryWithBackoffStrategy(retryCount: 4, retryOnError: e => e is WebException)

// Retry 4 times with default back off strategy on 202s
return Observable.Defer(() => MakeWebRequest())
  .RetryWithBackoffStrategy(retryCount: 4, retryOnError: e => e is WebException && ((WebException)e).StatusCode == 202)