在 ASP.NET MVC 3 Action 方法中并行运行任务

Question

我在 ASP.NET MVC 3 应用程序中有一个搜索操作，它返回包含给定关键字的建议和建议：

[HttpPost]
public ActionResult Search(string query, int pg = 0)
{
    var keywords = query.Split(new[] { ' ', ',', ';' }, 
        StringSplitOptions.RemoveEmptyEntries);

    var containing = (from s in _readonlySession.All<Suggestion>()
                      from k in keywords
                      where (s.Text.ToLower().Contains(k.ToLower()))
                      orderby s.Text
                      select s).Distinct();

    var tagged = (from t in _readonlySession.All<Tag>()
                  from s in t.Suggestions
                  from k in keywords
                  where t.Text.ToLower().Contains(k.ToLower())
                  orderby s.Text
                  select s).Distinct();

    var model = new SearchViewModel
    {
        Query = query,
        Containing = containing.ToList(),
        Tagged = tagged.ToList()
    };

    return View(model);
}

我认为包含和标记查询可以完美地并行运行。

同时触发这两个查询、等待结果并仅在两个查询完成时才返回的最佳方法是什么？

Answer 1

任务并行库是您的最佳选择。通过 Google 可以获得大量信息，但以下是您的实现可能的样子。

[HttpPost]
public ActionResult Search(string query, int pg = 0)
{
    var keywords = query.Split(new[] { ' ', ',', ';' }, 
        StringSplitOptions.RemoveEmptyEntries);

    IEnumerable containing=null;
    Task t1 = Task.Factory.StartNew(() =>
    {
        containing = (from s in _readonlySession.All<Suggestion>()
                        from k in keywords
                        where (s.Text.ToLower().Contains(k.ToLower()))
                        orderby s.Text
                        select s).Distinct().ToList();

    });

    IEnumerable tagged=null;
    Task t2 = Task.Factory.StartNew(() =>
    {
        var tagged = (from t in _readonlySession.All<Tag>()
                        from s in t.Suggestions
                        from k in keywords
                        where t.Text.ToLower().Contains(k.ToLower())
                        orderby s.Text
                        select s).Distinct().ToList();
    });

    t1.Wait();
    t2.Wait();

    var model = new SearchViewModel
    {
        Query = query,
        Containing = containing.ToList(),
        Tagged = tagged.ToList()
    };

    return View(model);
}

请记住，如果您的应用程序收到大量请求，您最好不要异步执行查询 - 消耗 2 个额外线程来服务一个请求会占用其他传入请求的资源。只有当您的流量很大或者您的硬件不足时，这才会成为问题。

Answer 2

您在网页中使用 TPL 时要小心，因为 TPL 将利用原本用于服务其他 HTTP 请求的线程。本质上，您可以权衡单个请求的较低响应延迟与请求总体吞吐量的降低。

有关权衡的更多说明，请参阅以下文章：

http: //blogs.msdn.com/b/pfxteam/archive/2010/02/08/9960003.aspx

Answer 3

这可能是异步控制器的良好候选者。关于异步控制器的重要一点是，如果您想在这种情况下获得净性能增益，您需要修改存储库层并提供两种方法的异步版本，这两种方法在与远程系统（例如，一个数据库。

public class SearchController: AsyncController
{
    public void SearchAsync(string query, int pg = 0)
    {
        // TODO: Write a custom model binder to do this job
        // and have your controller action directly take an
        // IEnumerable<string> argument.
        var keywords = query.Split(new[] { ' ', ',', ';' }, 
            StringSplitOptions.RemoveEmptyEntries);

        AsyncManager.OutstandingOperations.Increment();
        AsyncManager.OutstandingOperations.Increment();

        _readonlySession.GetContainingCompleted += (sender, e) =>
        {
            AsyncManager.Parameters["containing"] = e.Value;
            AsyncManager.OutstandingOperations.Decrement();
        };

        _readonlySession.GetTaggedCompleted += (sender, e) =>
        {
            AsyncManager.Parameters["tagged"] = e.Value;
            AsyncManager.OutstandingOperations.Decrement();
        };

        _readonlySession.GetContainingAsync(keywords);
        _readonlySession.GetTaggedAsync(keywords);

        AsyncManager.Parameters["query"] = query;
    }

    public ActionResult SearchCompleted(string query, IEnumerable<Suggestion> containing, IEnumerable<Tag> tagged)
    {
        var model = new SearchViewModel
        {
            Query = query,
            Containing = containing.ToList(),
            Tagged = tagged.ToList()
        };
        return View(model);
    }
}