使用 Rx 和 SelectMany 限制并发请求

Question

我有一个我想使用HttpClient 同时下载的页面的 URL 列表。 URL 列表可能很大（100 个或更多！）

我目前有这个代码：

var urls = new List<string>
            {
                @"http:\\www.amazon.com",
                @"http:\\www.bing.com",
                @"http:\\www.facebook.com",
                @"http:\\www.twitter.com",
                @"http:\\www.google.com"
            };

var client = new HttpClient();

var contents = urls
    .ToObservable()
    .SelectMany(uri => client.GetStringAsync(new Uri(uri, UriKind.Absolute)));

contents.Subscribe(Console.WriteLine);

问题：由于SelectMany的使用，几乎同时创建了一大堆Task。似乎如果 URL 列表足够大，很多任务都会超时（我收到 “A Task was cancelled” 异常）。

所以，我认为应该有一种方法，也许使用某种调度程序来限制并发任务的数量，在给定的时间不允许超过 5 或 6 个。

这样我就可以获得并发下载，而不会像现在那样启动太多可能会停止的任务。

如何做到这一点，这样我就不会因为大量超时任务而饱和？

Answer 1

记住SelectMany() 实际上是Select().Merge()。虽然SelectMany 没有maxConcurrent 参数，但Merge() 有。所以你可以使用它。

从您的示例中，您可以这样做：

var urls = new List<string>
    {
        @"http:\\www.amazon.com",
        @"http:\\www.bing.com",
        @"http:\\www.facebook.com",
        @"http:\\www.twitter.com",
        @"http:\\www.google.com"
    };

var client = new HttpClient();

var contents = urls
    .ToObservable()
    .Select(uri => Observable.FromAsync(() => client.GetStringAsync(uri)))
    .Merge(2); // 2 maximum concurrent requests!

contents.Subscribe(Console.WriteLine);

Answer 2

这是一个使用DataFlow API 的示例：

private static Task DoIt()
{
    var urls = new List<string>
    {
        @"http:\\www.amazon.com",
        @"http:\\www.bing.com",
        @"http:\\www.facebook.com",
        @"http:\\www.twitter.com",
        @"http:\\www.google.com"
    };

    var client = new HttpClient();

    //Create a block that takes a URL as input
    //and produces the download result as output
    TransformBlock<string,string> downloadBlock =
        new TransformBlock<string, string>(
            uri => client.GetStringAsync(new Uri(uri, UriKind.Absolute)),
            new ExecutionDataflowBlockOptions
            {
                //At most 2 download operation execute at the same time
                MaxDegreeOfParallelism = 2
            }); 

    //Create a block that prints out the result
    ActionBlock<string> doneBlock =
        new ActionBlock<string>(x => Console.WriteLine(x));

    //Link the output of the first block to the input of the second one
    downloadBlock.LinkTo(
        doneBlock,
        new DataflowLinkOptions { PropagateCompletion = true});

    //input the urls into the first block
    foreach (var url in urls)
    {
        downloadBlock.Post(url);
    }

    downloadBlock.Complete(); //Mark completion of input

    //Allows consumer to wait for the whole operation to complete
    return doneBlock.Completion;
}

static void Main(string[] args)
{
    DoIt().Wait();
    Console.WriteLine("Done");
    Console.ReadLine();
}

Answer 3

你能看看这是否有帮助？

var urls = new List<string>
        {
            @"http:\\www.amazon.com",
            @"http:\\www.bing.com",
            @"http:\\www.google.com",
            @"http:\\www.twitter.com",
            @"http:\\www.google.com"
        };

var contents =
    urls
        .ToObservable()
        .SelectMany(uri =>
            Observable
                .Using(
                    () => new System.Net.Http.HttpClient(),
                    client =>
                        client
                            .GetStringAsync(new Uri(uri, UriKind.Absolute))
                            .ToObservable()));