以下是两种方法
CreateRetryTransformBlock和
CreateRetryActionBlock，它们基于以下假设运作：
- 调用者希望处理所有项目，即使其中一些项目反复失败。
- 调用者对所有发生的异常感兴趣，即使对于最终成功的项目也是如此（对于
CreateRetryActionBlock不适用）。
- 调用者可能希望设置总重试次数的上限，之后该块应转换为故障状态。
- 调用者希望能够设置正常块的所有可用选项，包括
MaxDegreeOfParallelism、BoundedCapacity、CancellationToken和EnsureOrdered，以及与重试功能相关的选项。
下面的实现使用
SemaphoreSlim来控制首次尝试的操作和在其延迟持续时间过去后重试的先前失败操作之间的并发级别。
public class RetryExecutionDataflowBlockOptions : ExecutionDataflowBlockOptions
{
public int MaxAttemptsPerItem { get; set; } = 1;
public TimeSpan RetryDelay { get; set; } = TimeSpan.Zero;
public int MaxRetriesTotal { get; set; } = -1;
}
public readonly struct RetryResult<TInput, TOutput>
{
public readonly TInput Input { get; }
public readonly TOutput Output { get; }
public readonly bool Success { get; }
public readonly Exception[] Exceptions { get; }
public bool Failed => !Success;
public Exception FirstException => Exceptions != null ? Exceptions[0] : null;
public int Attempts =>
Exceptions != null ? Exceptions.Length + (Success ? 1 : 0) : 1;
public RetryResult(TInput input, TOutput output, bool success,
Exception[] exceptions)
{
Input = input;
Output = output;
Success = success;
Exceptions = exceptions;
}
}
public class RetryLimitException : Exception
{
public RetryLimitException(string message, Exception innerException)
: base(message, innerException) { }
}
public static IPropagatorBlock<TInput, RetryResult<TInput, TOutput>>
CreateRetryTransformBlock<TInput, TOutput>(
Func<TInput, Task<TOutput>> transform,
RetryExecutionDataflowBlockOptions dataflowBlockOptions)
{
if (transform == null) throw new ArgumentNullException(nameof(transform));
if (dataflowBlockOptions == null)
throw new ArgumentNullException(nameof(dataflowBlockOptions));
int maxAttemptsPerItem = dataflowBlockOptions.MaxAttemptsPerItem;
int maxRetriesTotal = dataflowBlockOptions.MaxRetriesTotal;
TimeSpan retryDelay = dataflowBlockOptions.RetryDelay;
if (maxAttemptsPerItem < 1) throw new ArgumentOutOfRangeException(
nameof(dataflowBlockOptions.MaxAttemptsPerItem));
if (maxRetriesTotal < -1) throw new ArgumentOutOfRangeException(
nameof(dataflowBlockOptions.MaxRetriesTotal));
if (retryDelay < TimeSpan.Zero) throw new ArgumentOutOfRangeException(
nameof(dataflowBlockOptions.RetryDelay));
var cancellationToken = dataflowBlockOptions.CancellationToken;
var exceptionsCount = 0;
var semaphore = new SemaphoreSlim(
dataflowBlockOptions.MaxDegreeOfParallelism);
async Task<(TOutput, Exception)> ProcessOnceAsync(TInput item)
{
await semaphore.WaitAsync();
try
{
var result = await transform(item).ConfigureAwait(false);
return (result, null);
}
catch (Exception ex)
{
if (maxRetriesTotal != -1)
{
if (Interlocked.Increment(ref exceptionsCount) > maxRetriesTotal)
{
throw new RetryLimitException($"The max retry limit " +
$"({maxRetriesTotal}) has been reached.", ex);
}
}
return (default, ex);
}
finally
{
semaphore.Release();
}
}
async Task<Task<RetryResult<TInput, TOutput>>> ProcessWithRetryAsync(
TInput item)
{
var (result, firstException) = await ProcessOnceAsync(item);
if (firstException == null) return Task.FromResult(
new RetryResult<TInput, TOutput>(item, result, true, null));
return RetryStageAsync();
async Task<RetryResult<TInput, TOutput>> RetryStageAsync()
{
var exceptions = new List<Exception>();
exceptions.Add(firstException);
for (int i = 2; i <= maxAttemptsPerItem; i++)
{
await Task.Delay(retryDelay, cancellationToken);
var (result, exception) = await ProcessOnceAsync(item);
if (exception != null)
exceptions.Add(exception);
else
return new RetryResult<TInput, TOutput>(item, result,
true, exceptions.ToArray());
}
return new RetryResult<TInput, TOutput>(item, default, false,
exceptions.ToArray());
};
}
var input = new TransformBlock<TInput, Task<RetryResult<TInput, TOutput>>>(
item => ProcessWithRetryAsync(item), dataflowBlockOptions);
var output = new TransformBlock<Task<RetryResult<TInput, TOutput>>,
RetryResult<TInput, TOutput>>(t => t, dataflowBlockOptions);
input.LinkTo(output, new DataflowLinkOptions { PropagateCompletion = true });
PropagateFailure(output, input);
return DataflowBlock.Encapsulate(input, output);
async void PropagateFailure(IDataflowBlock block1, IDataflowBlock block2)
{
try { await block1.Completion.ConfigureAwait(false); }
catch (Exception ex) { block2.Fault(ex); }
}
}
public static ITargetBlock<TInput> CreateRetryActionBlock<TInput>(
Func<TInput, Task> action,
RetryExecutionDataflowBlockOptions dataflowBlockOptions)
{
if (action == null) throw new ArgumentNullException(nameof(action));
var block = CreateRetryTransformBlock<TInput, object>(async input =>
{
await action(input).ConfigureAwait(false); return null;
}, dataflowBlockOptions);
var nullTarget = DataflowBlock.NullTarget<RetryResult<TInput, object>>();
block.LinkTo(nullTarget);
return block;
}
