Question

我正在尝试编写一些代码，这些代码会使Web服务并行调用多个不同的服务器，因此TPL似乎是一个明显的选择。

我的Web服务调用中只有一个会返回我想要的结果，而其他所有调用都不会。我正在努力找到一种有效地拥有Task.WaitAny的方法，但只有在与条件匹配的第一个Task返回时才会解锁。

我尝试使用WaitAny，但无法找出放置过滤器的位置。我到目前为止：

public void SearchServers()
{
    var servers = new[] {"server1", "server2", "server3", "server4"};
    var tasks = servers
                 .Select(s => Task<bool>.Factory.StartNew(server => CallServer((string)server), s))
                 .ToArray();

    Task.WaitAny(tasks); //how do I say "WaitAny where the result is true"?

    //Omitted: cancel any outstanding tasks since the correct server has been found
}

private bool CallServer(string server)
{
    //... make the call to the server and return the result ...
}

修改：快速澄清，以防上述内容有任何混淆。我正在尝试执行以下操作：

对于每台服务器，请启动Task进行检查
要么等到服务器返回true（只有最多1个服务器将返回true）
或者，等到所有服务器都返回false，即没有匹配。

Answer 1

我能想到的最好的事情是为每个ContinueWith指定一个Task，检查结果，以及true是否取消其他任务。要取消任务，您可能需要使用CancellationToken。

var tasks = servers
    .Select(s => Task.Run(...)
        .ContinueWith(t =>
            if (t.Result) {
                // cancel other threads
            }
        )
    ).ToArray();

更新：替代解决方案将是WaitAny，直到正确的任务完成（但它有一些缺点，例如从列表中删除已完成的任务并从剩余的任务中创建一个新阵列是相当沉重的操作）：

List<Task<bool>> tasks = servers.Select(s => Task<bool>.Factory.StartNew(server => CallServer((string)server), s)).ToList();

bool result;
do {
    int idx = Task.WaitAny(tasks.ToArray());
    result = tasks[idx].Result;
    tasks.RemoveAt(idx);
} while (!result && tasks.Count > 0);

// cancel other tasks

更新2：现在我会用Rx做到这一点：

[Fact]
public async Task AwaitFirst()
{
    var servers = new[] { "server1", "server2", "server3", "server4" };
    var server = await servers
        .Select(s => Observable
            .FromAsync(ct => CallServer(s, ct))
            .Where(p => p)
            .Select(_ => s)
        )
        .Merge()
        .FirstAsync();
    output.WriteLine($"Got result from {server}");
}

private async Task<bool> CallServer(string server, CancellationToken ct)
{
    try
    {
        if (server == "server1")
        {
            await Task.Delay(TimeSpan.FromSeconds(1), ct);
            output.WriteLine($"{server} finished");
            return false;
        }
        if (server == "server2")
        {
            await Task.Delay(TimeSpan.FromSeconds(2), ct);
            output.WriteLine($"{server} finished");
            return false;
        }
        if (server == "server3")
        {
            await Task.Delay(TimeSpan.FromSeconds(3), ct);
            output.WriteLine($"{server} finished");
            return true;
        }
        if (server == "server4")
        {
            await Task.Delay(TimeSpan.FromSeconds(4), ct);
            output.WriteLine($"{server} finished");
            return true;
        }
    }
    catch(OperationCanceledException)
    {
        output.WriteLine($"{server} Cancelled");
        throw;
    }

    throw new ArgumentOutOfRangeException(nameof(server));
}

我的机器上的测试需要3.32秒（这意味着它没有等到第4台服务器），我得到了以下输出：

server1 finished
server2 finished
server3 finished
server4 Cancelled
Got result from server3

Answer 2

您可以使用OrderByCompletion() from the AsyncEx library，它会在完成任务时返回这些任务。您的代码可能类似于：

var tasks = servers
    .Select(s => Task.Factory.StartNew(server => CallServer((string)server), s))
    .OrderByCompletion();

foreach (var task in tasks)
{
    if (task.Result)
    {
        Console.WriteLine("found");
        break;
    }
    Console.WriteLine("not found yet");
}

// cancel any outstanding tasks since the correct server has been found

Answer 3

使用Interlocked.CompareExchange就可以做到这一点，只有一个Task能够在serverReturedData上写入

    public void SearchServers()
        {
            ResultClass serverReturnedData = null;
            var servers = new[] {"server1", "server2", "server3", "server4"};
            var tasks = servers.Select(s => Task<bool>.Factory.StartNew(server => 
            {
               var result = CallServer((string)server), s);
               Interlocked.CompareExchange(ref serverReturnedData, result, null);

            }).ToArray();

            Task.WaitAny(tasks); //how do I say "WaitAny where the result is true"?
        //
        // use serverReturnedData as you want.
        // 
        }

编辑：正如Jasd所说，上面的代码可以在变量serverReturnedData具有有效值之前返回（如果服务器返回空值，则可能发生这种情况），以确保您可以将结果包装在自定义对象中。 / p>

Answer 4

这是基于svick答案的通用解决方案：

public static async Task<T> GetFirstResult<T>(
this IEnumerable<Func<CancellationToken, Task<T>>> taskFactories, 
Action<Exception> exceptionHandler,
Predicate<T> predicate)
{
    T ret = default(T);
    var cts = new CancellationTokenSource();
    var proxified = taskFactories.Select(tf => tf(cts.Token)).ProxifyByCompletion();
    int i;
    for (i = 0; i < proxified.Length; i++)
    {
        try
        {
            ret = await proxified[i].ConfigureAwait(false);
        }
        catch (Exception e)
        {
            exceptionHandler(e);
            continue;
        }
        if (predicate(ret))
        {
            break;
        }
    }

    if (i == proxified.Length)
    {
        throw new InvalidOperationException("No task returned the expected value");
    }
    cts.Cancel(); //we have our value, so we can cancel the rest of the tasks
    for (int j = i+1; j < proxified.Length; j++)
    {
        //observe remaining tasks to prevent process crash 
        proxified[j].ContinueWith(
         t => exceptionHandler(t.Exception), TaskContinuationOptions.OnlyOnFaulted)
                   .Forget();
    }
    return ret;
}

将ProxifyByCompletion实施为：

public static Task<T>[] ProxifyByCompletion<T>(this IEnumerable<Task<T>> tasks)
{
    var inputTasks = tasks.ToArray();
    var buckets = new TaskCompletionSource<T>[inputTasks.Length];
    var results = new Task<T>[inputTasks.Length];
    for (int i = 0; i < buckets.Length; i++)
    {
        buckets[i] = new TaskCompletionSource<T>();
        results[i] = buckets[i].Task;
    }
    int nextTaskIndex = -1;
    foreach (var inputTask in inputTasks)
    {
        inputTask.ContinueWith(completed =>
        {
            var bucket = buckets[Interlocked.Increment(ref nextTaskIndex)];
            if (completed.IsFaulted)
            {
                Trace.Assert(completed.Exception != null);
                bucket.TrySetException(completed.Exception.InnerExceptions);
            }
            else if (completed.IsCanceled)
            {
                bucket.TrySetCanceled();
            }
            else
            {
                bucket.TrySetResult(completed.Result);
            }
        }, CancellationToken.None, 
           TaskContinuationOptions.ExecuteSynchronously, TaskScheduler.Default);
    }
    return results;
}

而Forget是一种禁止CS4014的空方法：

public static void Forget(this Task task) //suppress CS4014
{
}

具有指定结果的任务并行库WaitAny

4 个答案: