Trouble implementing fallback to different service

[a082651]

I'm trying to follow the guidance in the fallback documentation, specifically under the anti-pattern "Using retry for fallback". The code for using a Fallback strategy doesn't work as written because it lacks a FallbackAction. This seems to conflict with the listed benefit of "eliminating the need for additional code like Context or ExecuteOutcomeAsync()", as FallbackAction seems to require interaction with one of those.

We are rolling out a new API that replaces an old one. I'd like to use the old one as a fallback to reduce the impact of our rollout. I can't seem to find an implementation of FallbackAction that has access to the value returned by the OnFallback callback.

Here is a highly simplified example:

async Task<string> FallbackStrategy() => "fallback";
async Task<string> FlaggedStrategy() => "flagged";

var resiliencePipeline = new ResiliencePipelineBuilder<string>()
    .AddFallback(new FallbackStrategyOptions<string>
    {
        ShouldHandle = new PredicateBuilder<string>().HandleResult(_ => true),
        OnFallback = async x => await FallbackStrategy(),
        FallbackAction = x => Outcome.FromResultAsValueTask<string>("outcome"),
    })
    .Build();

string actual = await resiliencePipeline.ExecuteAsync(async _ => await FlaggedStrategy());
Assert.Equal("fallback", actual); // fails, actual = "outcome"

[peter-csala]

Oops, that's my bad. Let me file a PR today to fix that sample first.

After that I will take a look at your other problem.

[a082651]

Changing FallbackAction to FallbackAction = async args => Outcome.FromResult(await FallbackStrategy()) has the expected result. Thank you for the quick turnaround!

[peter-csala]

I can't seem to find an implementation of FallbackAction that has access to the value returned by the OnFallback callback.

It works in the other way around:

1. The FallbackAction is executed

Polly/src/Polly.Core/Fallback/FallbackResilienceStrategy.cs

Line 22 in de7fdb0

var outcome = await StrategyHelper.ExecuteCallbackSafeAsync(callback, context, state).ConfigureAwait(context.ContinueOnCapturedContext);

2. The OnFallback args is created

Polly/src/Polly.Core/Fallback/FallbackResilienceStrategy.cs

Line 29 in de7fdb0

var onFallbackArgs = new OnFallbackArguments<T>(context, outcome);

3. The OnFallback is invoked

Polly/src/Polly.Core/Fallback/FallbackResilienceStrategy.cs

Line 35 in de7fdb0

await _onFallback(onFallbackArgs).ConfigureAwait(context.ContinueOnCapturedContext);

So, you can access the result of the FallbackAction's result inside the OnFallback via the args parameter's Outcome property.