为什么对同一个 ASP.NET MVC 操作的多个同时 AJAX 调用会导致浏览器阻塞? [英] Why would multiple simultaneous AJAX calls to the same ASP.NET MVC action cause the browser to block?

问题描述

几天前我问了这个问题:

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对 ASP.NET 会话状态的访问是每个会话独占的，这意味着如果两个不同的用户并发请求，则同时授予对每个单独会话的访问权限.但是，如果为同一个会话发出两个并发请求(通过使用相同的 SessionID 值)，第一个请求将获得对会话信息的独占访问权.第二个请求仅在第一个请求完成后执行.

令人讨厌的是，几周前我略读了这一段，并没有真正理解粗体句子的全部影响.我已经读过，因为对会话状态的访问是序列化的"，而不是所有请求，无论您是否接触会话状态，都会被序列化"，如果请求来自同一个会话.

幸运的是，在 ASP.NET MVC3 中有一个解决方法，它可以创建无会话控制器.Scott Guthrie 在这里谈到了这些:

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宣布 ASP.NET MVC 3 (Release Candidate 2)

我安装了 MVC3 RC2 并升级了项目.使用 [SessionState(SessionStateBehavior.Disabled)] 装饰有问题的控制器即可解决问题.

当然，通常我几分钟前才在 Stack Overflow 中找到了这个:

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异步控制器在 ASP 中阻塞请求.NET MVC 通过 jQuery

A few days back I asked this question:

Why does $.getJSON() block the browser?

I fire six jQuery async ajax requests at the same controller action pretty much all at once. Each request takes 10 seconds to return.

Through debugging and logging requests to the action method I notice that the requests are serialised and never run in parallel. i.e. I see a timeline in my log4net logs like this:

2010-12-13 13:25:06,633 [11164] INFO   - Got:1156
2010-12-13 13:25:16,634 [11164] INFO   - Returning:1156
2010-12-13 13:25:16,770 [7124] INFO   - Got:1426
2010-12-13 13:25:26,772 [7124] INFO   - Returning:1426
2010-12-13 13:25:26,925 [11164] INFO   - Got:1912
2010-12-13 13:25:36,926 [11164] INFO   - Returning:1912
2010-12-13 13:25:37,096 [9812] INFO   - Got:1913
2010-12-13 13:25:47,098 [9812] INFO   - Returning:1913
2010-12-13 13:25:47,283 [7124] INFO   - Got:2002
2010-12-13 13:25:57,285 [7124] INFO   - Returning:2002
2010-12-13 13:25:57,424 [11164] INFO   - Got:1308
2010-12-13 13:26:07,425 [11164] INFO   - Returning:1308

Looking at the network timeline in FireFox I see this:

Both the log sample above and the Firefox network timeline are for the same set of requests.

Are requests to the same action from the same page serialised? I'm aware of serialised access to the Session object in the same session, but no session data is being touched.

I stripped the client side code down to a single request (the longest running one) but this still blocks the browser, i.e. only when the ajax request completes does the browser respond to any link clicking.

What I also observe here (in Chrome's developer tools) is that upon clicking on a link when a long running ajax request is executing it reports a Failed to load resource error immediately which suggests that the browser has killed (or is attempting to kill and waiting?) the ajax request:

However the browser still takes an age to redirect to the new page.

Are ajax requests really asynchronous or is this sleight of hand because javascript is actually single threaded?

Are my requests just taking too long for this to work?

The problem occurs in Firefox and IE as well.

I also changed the script to use $.ajax directly and explicitly set async: true.

I'm running this on IIS7.5, both the Windows 2008R2 and Windows 7 flavours do the same thing.

Debug and release builds also behave the same.

解决方案

The answer was staring me in the face.

ASP.NET Session State Overview:

Access to ASP.NET session state is exclusive per session, which means that if two different users make concurrent requests, access to each separate session is granted concurrently. However, if two concurrent requests are made for the same session (by using the same SessionID value), the first request gets exclusive access to the session information. The second request executes only after the first request is finished.

Annoyingly I'd skimmed paragraph this a couple of weeks ago not really taking in the full impact of the bold sentences. I had read that simply as "access to session state is serialised" and not "all requests, no matter whether you touch session state or not, are serialised" if the requests came from the same session.

Fortunately there is a work around in ASP.NET MVC3 and its possible to create session-less controllers. Scott Guthrie talks about these here:

Announcing ASP.NET MVC 3 (Release Candidate 2)

I installed MVC3 RC2 and upgraded the project. Decorating the controller in question with [SessionState(SessionStateBehavior.Disabled)] solves the problem.

And of course typically I just found this in Stack Overflow a few minutes ago:

Asynchronous Controller is blocking requests in ASP.NET MVC through jQuery

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