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IReliableConcurrentQueue<T>.TryDequeueAsync 方法
定义
重要
一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。 对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。
暂时将某个值从队列中取消排队。 如果队列为空，则取消排队操作将等待项变得可用。
public System.Threading.Tasks.Task<Microsoft.ServiceFabric.Data.ConditionalValue<T>> TryDequeueAsync(Microsoft.ServiceFabric.Data.ITransaction tx, System.Threading.CancellationToken cancellationToken = default, TimeSpan? timeout = default);abstract member TryDequeueAsync : Microsoft.ServiceFabric.Data.ITransaction * System.Threading.CancellationToken * Nullable<TimeSpan> -> System.Threading.Tasks.Task<Microsoft.ServiceFabric.Data.ConditionalValue<'T>>Public Function TryDequeueAsync (tx As ITransaction, Optional cancellationToken As CancellationToken = Nothing, Optional timeout As Nullable(Of TimeSpan) = Nothing) As Task(Of ConditionalValue(Of T))参数
- tx
- ITransaction
要与此操作关联的事务。
- cancellationToken
- CancellationToken
要监视取消请求的标记。 默认为 None。
返回
表示异步取消排队操作的任务。 任务的结果是 T 类型的 ConditionalValue。如果某个值在给定时间内取消排队，则返回 HasValue 为 false 的 ConditionalValue，否则它将返回 HasValue 为 true 的 ConditionalValue，将 Value 作为 T 类型的取消排队项
例外
副本 (replica) 不再位于 中。
副本 (replica) 当前不可读。
副本 (replica) 出现暂时性故障。 对新事务重试操作
副本 (replica) 看到了除上面定义的类型以外的不可重试的故障。 清理并重新引发异常
操作无法在给定的超时时间内完成。 应中止事务，并创建一个新事务以重试。
tx 为 null。 请勿处理此异常。
操作已通过 cancellationToken取消。
事务在内部由系统出错。 对新事务重试操作
当方法调用对对象的当前状态无效时引发。 例如，使用的事务已终止：用户已提交或中止。 如果引发此异常，则很可能使用事务的服务代码中存在 bug。
示例
此示例演示如何取消排队，并在重试后无限次记录，直到取消令牌。
protected override async Task RunAsync(CancellationToken cancellationToken)
{
var concurrentQueue = await this.StateManager.GetOrAddAsync<IReliableConcurrentQueue<long>>(new Uri("fabric:/concurrentQueue"));
// Assumption: values are being enqueued by another source (e.g. the communication listener).
while (true)
{
cancellationToken.ThrowIfCancellationRequested();
try
{
using (var tx = this.StateManager.CreateTransaction())
{
var dequeueOutput = await concurrentQueue.TryDequeueAsync(tx, cancellationToken, TimeSpan.FromMilliseconds(100));
await tx.CommitAsync();
if (dequeueOutput.HasValue)
{
Console.WriteLine("Dequeue # " + dequeueOutput);
}
else
{
Console.WriteLine("Could not dequeue in the given time");
}
}
}
catch (TransactionFaultedException e)
{
// This indicates that the transaction was internally faulted by the system. One possible cause for this is that the transaction was long running
// and blocked a checkpoint. Increasing the "ReliableStateManagerReplicatorSettings.CheckpointThresholdInMB" will help reduce the chances of running into this exception
Console.WriteLine("Transaction was internally faulted, retrying the transaction: " + e);
}
catch (FabricNotPrimaryException e)
{
// Gracefully exit RunAsync as the new primary should have RunAsync invoked on it and continue work.
// If instead dequeue was being executed as part of a client request, the client would be signaled to re-resolve.
Console.WriteLine("Replica is not primary, exiting RunAsync: " + e);
return;
}
catch (FabricNotReadableException e)
{
// Retry until the queue is readable or a different exception is thrown.
Console.WriteLine("Queue is not readable, retrying the transaction: " + e);
}
catch (FabricObjectClosedException e)
{
// Gracefully exit RunAsync as this is happening due to replica close.
// If instead dequeue was being executed as part of a client request, the client would be signaled to re-resolve.
Console.WriteLine("Replica is closing, exiting RunAsync: " + e);
return;
}
catch (TimeoutException e)
{
Console.WriteLine("Encountered TimeoutException during DequeueAsync, retrying the transaction: " + e);
}
catch (FabricTransientException e)
{
// Retry until the queue is writable or a different exception is thrown.
Console.WriteLine("Queue is currently not writable, retrying the transaction: " + e);
}
// Delay and retry.
await Task.Delay(TimeSpan.FromMilliseconds(100), cancellationToken);
}
}
注解
虽然 TryDequeueAsync(ITransaction, CancellationToken, Nullable<TimeSpan>) 只能返回已提交相应 EnqueueAsync(ITransaction, T, CancellationToken, Nullable<TimeSpan>) 的值， TryDequeueAsync(ITransaction, CancellationToken, Nullable<TimeSpan>) 但操作不会相互隔离。 将某个值取消排队后，其他事务无法将其取消排队，但不会阻止其他值取消排队。
当包含一个或多个 TryDequeueAsync(ITransaction, CancellationToken, Nullable<TimeSpan>) 操作的事务中止时，取消排队的值将以任意顺序重新添加到队列的头部。 这将确保这些值很快再次取消排队，从而提高数据结构的公平性，但不强制实施严格的排序 (这需要降低允许的并发性，如 IReliableQueue<T>) 中所示。
