app-vnext / polly-samples

Provides sample implementations of the Polly library. The intent of this project is to help newcomers kick-start their use of Polly within their own projects.

Polly-Samples

This repository provides sample implementations of using the Polly library in a .NET application.

The intent of this project is to help newcomers kick-start the use of Polly within their own projects.

The samples demonstrate the policies in action, against faulting endpoints.

Projects

The solution contains three applications and one class library:

• PollyTestWebApi: This application is a web API with several endpoints. (Further information)
• PollyDemos: This library contains the Polly demos. (Further information)
• PollyTestClientConsole: This application provides a CLI to walk through the demos. (Further information)
• PollyTestClientWpf: This application provides a GUI to walk through the demos. (Further information)

flowchart LR
    console{{PollyTestClientConsole}}
    wpf{{PollyTestClientWPF}}
    lib>PollyDemos]
    api[/PollyTestWebApi\]

    console -- uses --> lib
    wpf -- uses --> lib
    lib -- invokes --> api

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Demos

General information

• The demos run against an example 'faulting server'.
  • To simulate failure, the dummy server rejects more than 3 calls in any five-second period.
• Be sure to read the <summary> at the top of each demo.
  • This explains the intent of that demo, and what resilience it adds to its handling of the calls to the 'faulting server'.
• Sometimes the <summary> also highlights what this demo doesn't achieve, which is often picked up in the following demo.
• Explore the demos in sequence for best understanding.

Sequence

#	Description	Link
00	No strategy	Code
01	Retry N times	Code
02	Wait and retry N times	Code
03	Wait and retry N times, N big enough to guarantee success	Code
04	Wait and retry forever	Code
05	Wait and retry with exponential back-off	Code
06	Wait and retry nesting circuit breaker	Code
07	Wait and retry chaining with circuit breaker by using Pipeline	Code
08	Fallback, Retry, and CircuitBreaker in a Pipeline	Code
09	Fallback, Timeout, and Retry in a Pipeline	Code
10	Without isolation: Faulting calls swamp resources, also prevent good calls	Code
11	With isolation: Faulting calls separated, do not swamp resources, good calls still succeed	Code
12	Hedging in latency mode	Code
13	Hedging in fallback mode: retry only	Code
14	Hedging in fallback mode: retry with fallback	Code
15	Hedging in parallel mode	Code
16	Entity Framework with retry N times	Code

Want further information?

• For any questions about the operation of the demos, ask on this repo with an issue.
• For any questions about Polly, ask in the Polly repository.
• For full Polly syntax, see Polly repository and the Polly documentation.

Slide decks

View the slides presented at NDC, DevIntersections and other conferences.

You are welcome to use and adapt this presentation for not-for-profit presentations of Polly to co-workers, user groups and similar, subject to the condition that references to the .NET Foundation, App-vNext and the individual members of the Polly team are retained.

polly-samples's Issues

Minor Issue: Can never be Null

Just trying to promote good programming practices.

The first line of each test reads:
if (cancellationToken == null) throw new ArgumentNullException(nameof(cancellationToken));

CancellationToken is a struct and thus can never be null.

CS0037 - Cannot convert null to 'type' because it is a non-nullable value type.

See: cs0037

While I love testing arguments for valid state and it is better to check than not to check, this check is redundant and more importantly is teaching junior developers, who may not know any better, that structs might be null - which by definition they can't.

Finalize (OnSuccess Action) the WaitAndRetryAsync with a WrapAsync Policy

I have a FallbackAsync Policy combined with a WaitAndRetryAsync Policy.
Both Policies are wrapped together with WrapAsync.
FallbackPolicy.WrapAsync(RetryPolicy). This works.

Is have a need to add some Finalizer eq. When the WaitAndRetryAsync succeeds like the fallbackAction but in the WaitAndRetryAsync

Async demo 09: Never stops after running 14+ requests

After running the demo a few times and allowing it to run for several request, 14+ times), the screen fills up with

Object name: 'System.Net.Http.HttpClient'.

The exception happens on line 159.

Adding sample code for hedging

After the V8 migration is done we could add new samples for missing strategies.

As far as I can see all strategies are covered with one or multiple demos, except Hedging (since it is newly introduced in V8 :))

timeout policy not working

I'm trying out the timeout policy with polly. I expect an exception to be thrown after 1 sec passed because that is the timeout upper limit I set up. But currently, no exception will be thrown and the method LongOperation() returns normally after around 5 secs.

Why does the timeout policy not work in this case?

static void Main(string[] args)
{
    Stopwatch stopwatch = new Stopwatch();
    stopwatch.Start();

    timeoutPolicy().GetAwaiter().GetResult();

    stopwatch.Stop();
    Console.WriteLine("Time elapsed: {0}", stopwatch.Elapsed);
    Console.ReadKey();
}

static async Task timeoutPolicy()
{
    AsyncTimeoutPolicy<HttpResponseMessage> timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(1); // setup the timeout limit to be 1 sec

    HttpResponseMessage response = await timeoutPolicy.ExecuteAsync((ct) => LongOperation(), CancellationToken.None);

    Console.WriteLine(response);



}

static Task<HttpResponseMessage> LongOperation()
{
    return Task<HttpResponseMessage>.Factory.StartNew(() =>
    {
        Thread.Sleep(5000); // Sleep 5 seconds
        return new HttpResponseMessage()
        {
            StatusCode = HttpStatusCode.BadRequest
        };

    });
}

Discussion about getting rid of the sync demos

I'm proposing to get rid of the sync demos. Here is my rationale:

• Prior V8 you have to define separate policies for sync and async methods
• Since V8 you have a unified strategy which can be executed both in sync and in async way
• After the V8 migration the sync demos look like code-duplicates rather than great additions

Is there any objection to delete the sync demos?

Documentation - SSL issue

I had troubles with SSL on Linux although I ran the below command.

dotnet dev-certs https --trust

I had this issue for any http or https for localhost connections only.

On further investigation, I was able to mitigate this, by the fix here . I think this can be udated in the README.

Also, I saw that pressing any key during the demo run can trigger the demo run summary till then and stop the run. I am assuming it is nice to have this documented in the README as well.

circuitBreakerStrategy never opens

In the Demo06_WaitAndRetryNestingCircuitBreaker the only message logged is "Strategy logging: ...". the "circuit breaker strategy: break if the action fails at least 4 times in a row" never happens?

Am I missing something?

Exception not re-thrown

In the following code the inner method InvokeAsync is throwing an OnlineException but I can never catch it what is re-thrown is a simple Exception:

//-- in a singleton
CircuitBreakerPolicy = Policy
    .Handle<Exception>()
    .CircuitBreakerAsync(
        exceptionsAllowedBeforeBreaking: ExceptionsAllowedForCircuiteBreaker,
        durationOfBreak: TimeSpan.FromSeconds(BreakTime),
        onBreak: (ex, breakDelay) =>
        {
            Debug.Log($".Breaker logging: Breaking the circuit for {breakDelay.TotalMilliseconds} ms!");
            Debug.Log($"..due to: + {ex.Message}");
        },
        onReset: () => Debug.Log(".Breaker logging: Call ok! Closed the circuit again!"),
        onHalfOpen: () => Debug.Log(".Breaker logging: Half-open: Next call is a trial!")
    );
//--

public virtual async Task<IOnlineResponse> Process(CancellationToken ct)
{
    Debug.Log($"Processing = {Type}");

    State = new RequestState();
    U serviceResponse = null;

    AsyncTimeoutPolicy timeoutPolicy = Policy.TimeoutAsync(TimeOut);

    AsyncRetryPolicy waitAndRetryPolicy = Policy
            .Handle<Exception>(e => !(e is BrokenCircuitException))
            .WaitAndRetryAsync(
            retryCount: DefaultRetryCount,
            attempt => TimeSpan.FromMilliseconds(AttemptDelay),
            (exception, calculatedWaitDuration) =>
            {
                Debug.Log($".Request exception (will retry): " + exception.Message);
            });

    AsyncPolicyWrap policyWrap = Policy.WrapAsync(timeoutPolicy, waitAndRetryPolicy, CircuitBreaker.Instance.CircuitBreakerPolicy);

    try
    {
        serviceResponse = await policyWrap.ExecuteAsync(async (cancellationToken) =>
        {
            await CheckConnectivity(cancellationToken);
            return await InvokeAsync(cancellationToken);
        }, ct, continueOnCapturedContext: true);

        State.Status = RequestStatus.Success;
        State.Message = "";
    }
    catch (OnlineException ex)
    {
        State.Status = OnlineExceptionStatusToRequestStatus(ex.StatusCode);
        State.Message = ex.ToString();
    }
    catch (Exception ex)
    {
        State.Status = RequestStatus.Failed;
        State.Message = ex.ToString();
    }

    Debug.Log($"Response = {Type} - {State.Status}");

    return new V { State = State, Response = serviceResponse };
}

AsyncDemo09_Wrap_Fallback_Timeout_WaitAndRetry throws

This doesn't seem to be part of the demo.

Demo was canceled: AsyncDemo09_Wrap_Fallback_Timeout_WaitAndRetry
.Log,then retry: Cannot access a disposed object.
Object name: 'System.Net.Http.HttpClient'.
Demo already stopped.
Unobserved task exception: System.AggregateException: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread. ---> System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.ThrowObjectDisposedException()
   at System.Threading.CancellationTokenSource.InternalRegister(Action`1 callback, Object stateForCallback, SynchronizationContext targetSyncContext, ExecutionContext executionContext)
   at System.Threading.CancellationToken.Register(Action`1 callback, Object state, Boolean useSynchronizationContext, Boolean useExecutionContext)
   at System.Threading.CancellationToken.InternalRegisterWithoutEC(Action`1 callback, Object state)
   at System.Threading.Tasks.Task.Delay(Int32 millisecondsDelay, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Delay(TimeSpan delay, CancellationToken cancellationToken)
   at Polly.Retry.AsyncRetryEngine.<ImplementationAsync>d__0`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Polly.AsyncPolicy.<ExecuteAsync>d__21`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Polly.Wrap.AsyncPolicyWrapEngine.<>c__DisplayClass1_0`1.<<ImplementationAsync>b__0>d.MoveNext()
   --- End of inner exception stack trace ---
---> (Inner Exception #0) System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.ThrowObjectDisposedException()
   at System.Threading.CancellationTokenSource.InternalRegister(Action`1 callback, Object stateForCallback, SynchronizationContext targetSyncContext, ExecutionContext executionContext)
   at System.Threading.CancellationToken.Register(Action`1 callback, Object state, Boolean useSynchronizationContext, Boolean useExecutionContext)
   at System.Threading.CancellationToken.InternalRegisterWithoutEC(Action`1 callback, Object state)
   at System.Threading.Tasks.Task.Delay(Int32 millisecondsDelay, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Delay(TimeSpan delay, CancellationToken cancellationToken)
   at Polly.Retry.AsyncRetryEngine.<ImplementationAsync>d__0`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Polly.AsyncPolicy.<ExecuteAsync>d__21`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Polly.Wrap.AsyncPolicyWrapEngine.<>c__DisplayClass1_0`1.<<ImplementationAsync>b__0>d.MoveNext()<---
.

Is there any OnSuccess Action like OnRetry for policy to notify the attempt when it got fixed

Hi

I am trying to reuse creating policy and track the attempts and log exceptions while retry.

Is there a way to handle on success in policy itself to notify the user that call was successful in particular attempt

No Mac user interface

Hello

Something I noted while experimenting with Polly Samples is the lack of UI for Mac users, just the command line utility. I created a Blazor web app connected to the api. It needs a bit of polishing but would you be interested in a PR being submitted to add this to the repo?

How to create web version of audio chatbot using polly

How to create web version of audio chatbot using polly