Polly

Fallback resilience strategy

About

• Options: FallbackStrategyOptions<T>
• Extensions: AddFallback
• Strategy Type: Reactive

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[!NOTE] Version 8 documentation for this strategy has not yet been migrated. For more information on fallback concepts and behavior, refer to the older documentation.

Usage

// Add a fallback/substitute value if an operation fails.
new ResiliencePipelineBuilder<UserAvatar>()
    .AddFallback(new FallbackStrategyOptions<UserAvatar>
    {
        ShouldHandle = new PredicateBuilder<UserAvatar>()
            .Handle<SomeExceptionType>()
            .HandleResult(r => r is null),
        FallbackAction = args => Outcome.FromResultAsValueTask(UserAvatar.Blank)
    });

// Use a dynamically generated value if an operation fails.
new ResiliencePipelineBuilder<UserAvatar>()
    .AddFallback(new FallbackStrategyOptions<UserAvatar>
    {
        ShouldHandle = new PredicateBuilder<UserAvatar>()
            .Handle<SomeExceptionType>()
            .HandleResult(r => r is null),
        FallbackAction = args =>
        {
            var avatar = UserAvatar.GetRandomAvatar();
            return Outcome.FromResultAsValueTask(avatar);
        }
    });

// Use a default or dynamically generated value, and execute an additional action if the fallback is triggered.
new ResiliencePipelineBuilder<UserAvatar>()
    .AddFallback(new FallbackStrategyOptions<UserAvatar>
    {
        ShouldHandle = new PredicateBuilder<UserAvatar>()
            .Handle<SomeExceptionType>()
            .HandleResult(r => r is null),
        FallbackAction = args =>
        {
            var avatar = UserAvatar.GetRandomAvatar();
            return Outcome.FromResultAsValueTask(UserAvatar.Blank);
        },
        OnFallback = args =>
        {
            // Add extra logic to be executed when the fallback is triggered, such as logging.
            return default; // returns an empty ValueTask
        }
    });

Defaults

Property	Default Value	Description
ShouldHandle	Predicate that handles all exceptions except OperationCanceledException.	Predicate that determines what results and exceptions are handled by the fallback strategy.
FallbackAction	Null, Required	Fallback action to be executed.
OnFallback	null	Event that is raised when fallback happens.

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