blocks|key|134867|text|当对DB或某些网络绑定调用执行某些I\O操作时，async操作最有帮助，在这些操作中，处理请求的线程将在从您刚刚调用的DB或网络绑定调用获得响应之前停止。最好对它们使用await，它将真正提高应用程序的响应性(因为在等待DB或任何其他类似操作时，会有较少的ASP输入\输出线程停滞)。在我的所有应用程序中，每当有很多对DB的调用是非常必要的时候，我总是将它们包装在可等待的方法中，并使用await关键字调用它们。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|134868|entityMap^0|O|5|5D|5|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

<code>async</code> actions help best when the actions does some I\O operations to DB or some network bound calls where the thread that processes the request will be stalled before it gets answer from the DB or network bound call which you just invoked. It's best you use await with them and it will really improve the responsiveness of your application (because less ASP input\output threads will be stalled while waiting for the DB or any other operation like that). In all my applications whenever many calls to DB very necessary I've always wrapped them in awaiatable method and called that with <code>await</code> keyword.

blocks|key|2699371|text|当一个操作必须执行几个独立的长时间运行的操作时，异步操作方法非常有用。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|2699372|AsyncController类的典型用法是长时间运行的|2699373|服务调用。|blockquote|2699374|2699375|2699376|我的数据库调用应该是异步的吗？|2699377|与数据库服务器相比，IIS线程池通常可以处理更多的并发阻塞请求。如果数据库是瓶颈，异步调用将不会加速数据库响应。如果没有节流机制，通过使用异步调用将更多的工作有效地分派给不堪重负的数据库服务器，只会将更多的负担转移到数据库上。如果您的数据库是瓶颈，异步调用将不会是魔术子弹。|2699378|您应该了解一下1和2参考资料|2699379|来自@PanagiotisKanavos的评论：|2699380|此外，|2699381|并不意味着并行。异步执行将有价值的线程池线程从外部资源阻塞中释放出来，而不会造成复杂性或性能损失。这意味着同一台IIS机器可以处理更多的并发请求，而不是说它会运行得更快。|2699382|2699383|您还应该考虑阻塞调用是从CPU密集型的spinwait开始的。在压力期间，阻塞调用将导致延迟升级和应用程序池回收。异步调用简单地避免了这个|2699384|entityMap|0|LINK|mutability|MUTABLE|url|http://www.hanselman.com/blog/TheMagicOfUsingAsynchronousMethodsInASPNET45PlusAnImportantGotcha.aspx|1|http://tech.pro/tutorial/1252/asynchronous-controllers-in-asp-net-mvc^0|O|4|0|0|0|0|0|0|0|7|1|0|9|1|1|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1E|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1F|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|1H|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|1I|8|@]|D|@]|E|$]]|$1|O|3|P|5|6|7|1J|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1K|8|@]|D|@$9|1L|A|1M|1|1N]|$9|1O|A|1P|1|1Q]]|E|$]]|$1|S|3|T|5|6|7|1R|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|1S|8|@]|D|@]|E|$]]|$1|W|3|X|5|J|7|1T|8|@]|D|@]|E|$]]|$1|Y|3|-4|5|6|7|1U|8|@]|D|@]|E|$]]|$1|Z|3|10|5|6|7|1V|8|@]|D|@]|E|$]]|$1|11|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]]|12|$13|$5|14|15|16|E|$17|18]]|19|$5|14|15|16|E|$17|1A]]]]

Asynchronous action methods are useful when an action must perform several independent long running operations. 

<blockquote>
 A typical use for the AsyncController class is long-running Web
 service calls.
</blockquote>

Should my database calls be asynchronous ?

The IIS thread pool can often handle many more simultaneous blocking requests than a database server. If the database is the bottleneck, asynchronous calls will not speed up the database response. Without a throttling mechanism, efficiently dispatching more work to an overwhelmed database server by using asynchronous calls merely shifts more of the burden to the database. If your DB is the bottleneck, asynchronous calls won’t be the magic bullet.

You should have a look at <a href="http://www.hanselman.com/blog/TheMagicOfUsingAsynchronousMethodsInASPNET45PlusAnImportantGotcha.aspx" rel="noreferrer">1</a> and <a href="http://tech.pro/tutorial/1252/asynchronous-controllers-in-asp-net-mvc" rel="noreferrer">2</a> references

Derived from @PanagiotisKanavos comments:

<blockquote>
 Moreover, async doesn't mean parallel. Asynchronous execution frees a
 valuable threadpool thread from blocking for an external resource, for
 no complexity or performance cost. This means the same IIS machine can
 handle more concurrent requests, not that it will run faster.
 
 You should also consider that blocking calls start with a
 CPU-intensive spinwait. During stress times, blocking calls will
 result in escalating delays and app pool recycling. Asynchronous calls
 simply avoid this
</blockquote>

blocks|key|135450|text|在ASP.NET+|type|unstyled|depth|inlineStyleRanges|entityRanges|data|135451|中到处使用异步操作好吗？|blockquote|135452|135453|135454|与通常的编程一样，这取决于。当走上一条特定的道路时，总会有一个权衡。|135455|在你知道你会收到对你的服务的并发请求，并且你希望能够很好地向外扩展的地方，async-await大放异彩。async-await如何帮助向外扩展？事实上，当您同步调用异步IO调用时，例如网络调用或命中数据库时，负责执行的当前线程将被阻塞，等待请求完成。当您使用async-await时，您可以让框架为您创建一个状态机，以确保在IO调用完成后，您的方法将从它停止的地方继续执行。|offset|length|style|CODE|135456|需要注意的是，这个状态机有一个微妙的开销。让一个方法异步并不能让它执行更快的，这是一个需要理解的重要因素，也是许多人的误解。|BOLD|135457|在使用async-await时要考虑的另一件事是，它完全是异步的，这意味着您将看到异步贯穿整个调用堆栈。这意味着如果您想公开同步应用编程接口，您经常会发现自己复制了一定数量的代码，因为async和sync不能很好地混合。|135458|当我想要查询数据库(通过EF/NHibernate/其他|135459|+)时，我应该使用async/await关键字吗？|135460|135461|135462|如果您选择使用异步IO调用，那么async-await将是一个很好的选择，因为越来越多的现代数据库提供商公开了实现TAP+(任务异步模式)的异步方法。|135463|135464|在一个操作方法中，我可以使用await关键字异步查询数据库多少次？|135465|135466|135467|只要你遵循数据库提供商规定的规则，你想要多少就有多少。您可以进行的异步调用数量没有限制。如果您有相互独立且可以并发执行的查询，则可以为每个查询旋转一个新任务，并使用await+Task.WhenAll等待这两个查询完成。|135468|entityMap|0|LINK|mutability|MUTABLE|url|http://blogs.msdn.com/b/pfxteam/archive/2012/03/24/10287244.aspx|1|http://blogs.msdn.com/b/pfxteam/archive/2012/04/13/10293638.aspx^0|0|0|0|0|0|11|B|1H|B|3M|B|0|S|A|0|3|B|2K|5|0|2Q|4|1|0|0|0|0|0|G|B|0|0|0|0|0|2A|I|0^^$0|@$1|2|3|4|5|6|7|1I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1J|8|@]|9|@]|A|$]]|$1|E|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|1M|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|1N|8|@$K|1O|L|1P|M|N]|$K|1Q|L|1R|M|N]|$K|1S|L|1T|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1U|8|@$K|1V|L|1W|M|Q]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1X|8|@$K|1Y|L|1Z|M|N]]|9|@$K|20|L|21|1|22]|$K|23|L|24|1|25]]|A|$]]|$1|T|3|U|5|6|7|26|8|@]|9|@]|A|$]]|$1|V|3|W|5|D|7|27|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|28|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|29|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|2A|8|@$K|2B|L|2C|M|N]]|9|@]|A|$]]|$1|11|3|-4|5|6|7|2D|8|@]|9|@]|A|$]]|$1|12|3|13|5|D|7|2E|8|@]|9|@]|A|$]]|$1|14|3|-4|5|6|7|2F|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|2G|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|2H|8|@$K|2I|L|2J|M|N]]|9|@]|A|$]]|$1|18|3|-4|5|6|7|2K|8|@]|9|@]|A|$]]]|19|$1A|$5|1B|1C|1D|A|$1E|1F]]|1G|$5|1B|1C|1D|A|$1E|1H]]]]

<blockquote>
 is it good to use async action everywhere in ASP.NET MVC?
</blockquote>

As usual in programming, it depends. There is always a trade-off when going down a certain path.

<code>async-await</code> shines in places where you know you'll receiving concurrent requests to your service and you want to be able to scale out well. How does <code>async-await</code> help with scaling out? In the fact that when you invoke a async IO call synchronously, such as a network call or hitting your database, the current thread which is responsible for the execution is blocked waiting for the request to finish. When you use <code>async-await</code>, you enable the framework to create a state machine for you which makes sure that after the IO call is complete, your method continues executing from where it left off.

A thing to note is that this state machine has a subtle overhead. Making a method asynchronous does not make it execute faster, and that is an important factor to understand and a misconception many people have.

Another thing to take under consideration when using <code>async-await</code> is the fact that it is async all the way, meaning that you'll see async penetrate your entire call stack, top to buttom. This means that if you want to expose synchronous API's, you'll often find yourself duplicating a certain amount of code, as <a href="http://blogs.msdn.com/b/pfxteam/archive/2012/03/24/10287244.aspx">async</a> and <a href="http://blogs.msdn.com/b/pfxteam/archive/2012/04/13/10293638.aspx">sync</a> don't mix very well.

<blockquote>
 Shall I use async/await keywords when I want to query database (via
 EF/NHibernate/other ORM)?
</blockquote>

If you choose to go down the path of using async IO calls, then yes, <code>async-await</code> will be a good choice, as more and more modern database providers expose async method implementing the TAP (Task Asynchronous Pattern).

<blockquote>
 How many times I can use await keywords to query database
 asynchronously in ONE single action method?
</blockquote>

As many as you want, as long as you follow the rules stated by your database provider. There is no limit to the amount of async calls you can make. If you have queries which are independent of each other and can be made concurrently, you can spin a new task for each and use <code>await Task.WhenAll</code> to wait for both to complete.

blocks|key|2911528|text|正如你所知道的，MVC支持异步控制器，你应该利用它。如果您的控制器执行了一个冗长的操作(可能是基于磁盘的I/o或对另一个远程服务的网络调用)，如果请求是以同步方式处理的，则IIS线程将始终处于忙碌状态。因此，线程只是在等待冗长的操作完成。当first中请求的操作正在进行时，通过服务其他请求可以更好地利用它。这将有助于为更多并发请求提供服务。您的will服务将具有高度可伸缩性，并且不会轻易运行到C10k+problem中。对数据库查询使用async/await是一个好主意。是的，你可以根据你认为合适的次数来使用它们。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|2911529|看看here，你会得到很好的建议。|2911530|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/C10k_problem|1|https://web.archive.org/web/20150516055057/http://tech.pro/tutorial/1252/asynchronous-controllers-in-asp-net-mvc^0|5I|C|0|0|2|4|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

As you know, MVC supports asynchronous controllers and you should take advantage of it. In case your Controller, performs a lengthy operation, (it might be a disk based I/o or a network call to another remote service), if the request is handled in synchronous manner, the IIS thread is busy the whole time. As a result, the thread is just waiting for the lengthy operation to complete. It can be better utilized by serving other requests while the operation requested in first is under progress. This will help in serving more concurrent requests.
Your webservice will be highly scalable and will not easily run into <a href="http://en.wikipedia.org/wiki/C10k_problem" rel="nofollow noreferrer">C10k problem</a>.
It is a good idea to use async/await for db queries. and yes you can use them as many number of times as you deem fit.

Take a look <a href="https://web.archive.org/web/20150516055057/http://tech.pro/tutorial/1252/asynchronous-controllers-in-asp-net-mvc" rel="nofollow noreferrer">here</a> for excellent advise.

blocks|key|2699480|text|您可能会找到我的MSDN+article+on+the+subject+helpful；我在那篇文章中花了很多篇幅描述什么时候应该在ASP.NET上使用async，而不仅仅是如何在ASP.NET上使用async。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2699481|在ASP.NET+|2699482|中使用异步操作我有一些顾虑。当它提高我的应用程序的性能时，以及何时不是。|blockquote|2699483|2699484|2699485|首先，要理解async/await就是为了释放线程。在GUI应用程序中，它主要是关于释放GUI线程，以便用户体验更好。在服务器应用程序(包括ASP.NET+MVC)上，它主要是关于释放请求线程，以便服务器可以扩展。|2699486|特别是，它不会：|2699487|2699488|可让您更快地完成个人请求。事实上，当你点击一个await时，它们会完成(只有一点点)对调用者/浏览器的slower.|unordered-list-item|2699489|Return。await只向ASP.NET线程池“让步”，而不向浏览器“让步”。|2699490|2699491|2699492|的第一个问题是-在ASP.NET+MVC中到处使用异步操作好吗？|2699493|2699494|2699495|我想说在你做I/O的任何地方使用它都是很好的，尽管它可能不一定是有益的(见下文)。|2699496|然而，将它用于CPU绑定的方法是不好的。有时开发人员认为他们可以通过在控制器中调用Task.Run来获得async的好处，这是一个可怕的想法。因为该代码最终通过占用另一个线程来释放请求线程，所以根本没有好处(事实上，它们正在承担额外线程切换的惩罚)！|2699497|当我想要查询数据库(通过EF/NHibernate/其他|2699498|+)时，我应该使用async/await关键字吗？|2699499|2699500|2699501|您可以使用任何可用的可等待的方法。现在大多数主流软件都支持async，但也有一些不支持，如果你的对象关系映射不支持async，那么就不要尝试用Task.Run或其他类似的东西来包装它(见上)。|2699502|请注意，我说的是“你可以使用”。如果您正在讨论只有一个数据库后端的ASP.NET+MVC，那么您(几乎可以肯定)不会从async中获得任何可伸缩性优势。这是因为IIS可以处理的并发请求比单个SQL+server实例(或其他传统RDBMS)多得多。然而，如果你的后端更现代-+SQL+server集群，Azure+SQL，NoSQL等等-并且你的后端可以扩展，你的可伸缩性瓶颈是IIS，那么你可以从async中获得可伸缩性的好处。|2699503|2699504|第三个问题--在一个单独的操作方法中，我可以使用await关键字异步查询数据库多少次？|2699505|2699506|2699507|你想要多少都行。但是，请注意，许多ORM都有每个连接一个操作的规则。特别是，EF只允许每个DbContext执行一个操作；无论操作是同步还是异步，都是如此。|2699508|此外，请再次牢记后端的可伸缩性。如果您正在访问SQL+Server的单个实例，而您的IIS已经能够使SQLServer保持满容量，那么加倍或增加两倍的SQLServer压力对您没有任何帮助。|2699509|entityMap|0|LINK|mutability|MUTABLE|url|https://msdn.microsoft.com/en-us/magazine/dn802603.aspx^0|24|5|2S|5|8|Z|0|0|0|0|0|0|6|5|C|5|0|0|0|N|5|0|7|5|0|0|0|0|0|0|0|15|8|1G|5|0|0|0|0|0|T|5|1L|5|1Z|8|0|1N|5|5I|5|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1X|8|@$9|1Y|A|1Z|B|C]|$9|20|A|21|B|C]]|D|@$9|22|A|23|1|24]]|E|$]]|$1|F|3|G|5|6|7|25|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|26|8|@]|D|@]|E|$]]|$1|K|3|-4|5|6|7|27|8|@]|D|@]|E|$]]|$1|L|3|-4|5|6|7|28|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|29|8|@$9|2A|A|2B|B|C]|$9|2C|A|2D|B|C]]|D|@]|E|$]]|$1|O|3|P|5|6|7|2E|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|2F|8|@]|D|@]|E|$]]|$1|R|3|S|5|T|7|2G|8|@$9|2H|A|2I|B|C]]|D|@]|E|$]]|$1|U|3|V|5|T|7|2J|8|@$9|2K|A|2L|B|C]]|D|@]|E|$]]|$1|W|3|-4|5|6|7|2M|8|@]|D|@]|E|$]]|$1|X|3|-4|5|6|7|2N|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|J|7|2O|8|@]|D|@]|E|$]]|$1|10|3|-4|5|6|7|2P|8|@]|D|@]|E|$]]|$1|11|3|-4|5|6|7|2Q|8|@]|D|@]|E|$]]|$1|12|3|13|5|6|7|2R|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|2S|8|@$9|2T|A|2U|B|C]|$9|2V|A|2W|B|C]]|D|@]|E|$]]|$1|16|3|17|5|6|7|2X|8|@]|D|@]|E|$]]|$1|18|3|19|5|J|7|2Y|8|@]|D|@]|E|$]]|$1|1A|3|-4|5|6|7|2Z|8|@]|D|@]|E|$]]|$1|1B|3|-4|5|6|7|30|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|6|7|31|8|@$9|32|A|33|B|C]|$9|34|A|35|B|C]|$9|36|A|37|B|C]]|D|@]|E|$]]|$1|1E|3|1F|5|6|7|38|8|@$9|39|A|3A|B|C]|$9|3B|A|3C|B|C]]|D|@]|E|$]]|$1|1G|3|-4|5|6|7|3D|8|@]|D|@]|E|$]]|$1|1H|3|1I|5|J|7|3E|8|@]|D|@]|E|$]]|$1|1J|3|-4|5|6|7|3F|8|@]|D|@]|E|$]]|$1|1K|3|-4|5|6|7|3G|8|@]|D|@]|E|$]]|$1|1L|3|1M|5|6|7|3H|8|@]|D|@]|E|$]]|$1|1N|3|1O|5|6|7|3I|8|@]|D|@]|E|$]]|$1|1P|3|-4|5|6|7|3J|8|@]|D|@]|E|$]]]|1Q|$1R|$5|1S|1T|1U|E|$1V|1W]]]]

You may find my <a href="https://msdn.microsoft.com/en-us/magazine/dn802603.aspx">MSDN article on the subject helpful</a>; I took a lot of space in that article describing when you should use <code>async</code> on ASP.NET, not just how to use <code>async</code> on ASP.NET.

<blockquote>
 I have some concerns using async actions in ASP.NET MVC. When it improves performance of my apps, and when - not.
</blockquote>

First, understand that <code>async</code>/<code>await</code> is all about freeing up threads. On GUI applications, it's mainly about freeing up the GUI thread so the user experience is better. On server applications (including ASP.NET MVC), it's mainly about freeing up the request thread so the server can scale.

In particular, it won't:

<ul>
<li>Make your individual requests complete faster. In fact, they will complete (just a teensy bit) slower.</li>
<li>Return to the caller/browser when you hit an <code>await</code>. <code>await</code> only "yields" to the ASP.NET thread pool, not to the browser.</li>
</ul>

<blockquote>
 First question is - is it good to use async action everywhere in ASP.NET MVC?
</blockquote>

I'd say it's good to use it everywhere you're doing I/O. It may not necessarily be beneficial, though (see below).

However, it's bad to use it for CPU-bound methods. Sometimes devs think they can get the benefits of <code>async</code> by just calling <code>Task.Run</code> in their controllers, and this is a horrible idea. Because that code ends up freeing up the request thread by taking up another thread, so there's no benefit at all (and in fact, they're taking the penalty of extra thread switches)!

<blockquote>
 Shall I use async/await keywords when I want to query database (via EF/NHibernate/other ORM)?
</blockquote>

You could use whatever awaitable methods you have available. Right now most of the major players support <code>async</code>, but there are a few that don't. If your ORM doesn't support <code>async</code>, then don't try to wrap it in <code>Task.Run</code> or anything like that (see above).

Note that I said "you could use". If you're talking about ASP.NET MVC with a single database backend, then you're (almost certainly) not going to get any scalability benefit from <code>async</code>. This is because IIS can handle far more concurrent requests than a single instance of SQL server (or other classic RDBMS). However, if your backend is more modern - a SQL server cluster, Azure SQL, NoSQL, etc - and your backend can scale, and your scalability bottleneck is IIS, then you can get a scalability benefit from <code>async</code>.

<blockquote>
 Third question - How many times I can use await keywords to query database asynchronously in ONE single action method?
</blockquote>

As many as you like. However, note that many ORMs have a one-operation-per-connection rule. In particular, EF only allows a single operation per DbContext; this is true whether the operation is synchronous or asynchronous.

Also, keep in mind the scalability of your backend again. If you're hitting a single instance of SQL Server, and your IIS is already capable of keeping SQLServer at full capacity, then doubling or tripling the pressure on SQLServer is not going to help you at all.

blocks|key|2911591|text|我的5分：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2911592|当且仅当您执行IO操作时，|2911593|才会使用async/await。每次查询DB时，外部服务webservice.|ordered-list-item|offset|length|style|CODE|2911594|Always都会优先使用对DB的异步调用。|2911595|。|2911596|2911597|附注:第1点有例外情况，但您需要对异步内部有很好的了解。|2911598|作为额外的优势，如果需要，您可以并行执行很少的IO调用：|2911599|Task+task1+=+FooAsync();+//+launch+it,+but+don't+wait+for+result
Task+task2+=+BarAsync();+//+launch+bar;+now+both+foo+and+bar+are+running
await+Task.WhenAll(task1,+task2);+//+this+is+better+in+regard+to+exception+handling
//+use+task1.Result,+task2.Result|code-block|syntax|javascript|2911600|entityMap^0|0|0|4|B|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|12|8|@$G|13|H|14|I|J]]|9|@]|A|$]]|$1|K|3|L|5|F|7|15|8|@]|9|@]|A|$]]|$1|M|3|N|5|F|7|16|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|17|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|18|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|19|8|@]|9|@]|A|$]]|$1|T|3|U|5|V|7|1A|8|@]|9|@]|A|$W|X]]|$1|Y|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|Z|$]]

My 5 cents:

<ol>
<li>Use <code>async/await</code> if and only if you do an IO operation, like DB or external service webservice.</li>
<li>Always prefer async calls to DB.</li>
<li>Each time you query the DB.</li>
</ol>

P.S. There are exceptional cases for point 1, but you need to have a good understanding of async internals for this.

As an additional advantage, you can do few IO calls in parallel if needed:

<pre><code>Task task1 = FooAsync(); // launch it, but don't wait for result
Task task2 = BarAsync(); // launch bar; now both foo and bar are running
await Task.WhenAll(task1, task2); // this is better in regard to exception handling
// use task1.Result, task2.Result
</code></pre>

blocks|key|135565|text|在ASP.NET+|type|unstyled|depth|inlineStyleRanges|entityRanges|data|135566|中到处使用异步操作好吗？|blockquote|135567|135568|135569|在任何可以使用异步方法的地方都可以这样做，特别是当您在工作进程级别遇到性能问题时，这会发生在大量数据和计算操作中。否则，就不需要了，因为单元测试将需要强制转换。|135570|135571|关于可等待的方法:当我想要查询数据库时，我是否应该使用async/await关键字(通过EF/NHibernate/其他ORM)？|135572|135573|135574|是的，最好将异步用于任何数据库操作，以尽可能避免工作进程级别的性能问题。请注意，EF为大多数操作创建了许多异步替代方案，例如：|135575|.ToListAsync()
.FirstOrDefaultAsync()
.SaveChangesAsync()
.FindAsync()|code-block|syntax|javascript|135576|135577|在一个操作方法中，我可以使用await关键字异步查询数据库多少次？|135578|135579|135580|天高任鸟飞|135581|entityMap^0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|14|8|@]|9|@]|A|$]]|$1|E|3|-4|5|6|7|15|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|16|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|17|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|18|8|@]|9|@]|A|$]]|$1|J|3|K|5|D|7|19|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1C|8|@]|9|@]|A|$]]|$1|P|3|Q|5|R|7|1D|8|@]|9|@]|A|$S|T]]|$1|U|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]|$1|V|3|W|5|D|7|1F|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|1I|8|@]|9|@]|A|$]]|$1|11|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|12|$]]

<blockquote>
 Is it good to use async action everywhere in ASP.NET MVC?
</blockquote>

It's good to do so wherever you can use an async method especially when you have performance issues at the worker process level which happens for massive data and calculation operations. Otherwise, no need because unit testing will need casting.

<blockquote>
 Regarding awaitable methods: shall I use async/await keywords when I
 want to query a database (via EF/NHibernate/other ORM)?
</blockquote>

Yes, it's better to use async for any DB operation as could as possible to avoid performance issues at the level of worker processes. 
Note that EF has created many async alternatives for most operations, such as:

<pre><code>.ToListAsync()
.FirstOrDefaultAsync()
.SaveChangesAsync()
.FindAsync()
</code></pre>

<blockquote>
 How many times can I use await keywords to query the database
 asynchronously in one single action method?
</blockquote>

The sky is the limit

blocks|key|134999|text|我的经验是，如今很多开发人员使用async/await作为控制器的默认配置。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|135000|我的建议是，只有当你知道它会对你的有帮助的时候才使用它。|135001|原因是，正如Stephen+Cleary和其他人已经提到的那样，它可能会引入性能问题，而不是解决它们，并且它只会在特定场景中对您有所帮助：|135002|135003|高流量controllers|unordered-list-item|135004|Scalable后端|135005|135006|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/users/263693/stephen-cleary^0|G|B|0|0|6|E|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|11|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|12|8|@]|D|@$9|13|A|14|1|15]]|E|$]]|$1|J|3|-4|5|6|7|16|8|@]|D|@]|E|$]]|$1|K|3|L|5|M|7|17|8|@]|D|@]|E|$]]|$1|N|3|O|5|M|7|18|8|@]|D|@]|E|$]]|$1|P|3|-4|5|6|7|19|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|1A|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]]]

My experience is that today a lot of developers use <code>async/await</code> as a default for controllers.

My suggestion would be, use it only when you know it will help you.

The reason is, as <a href="https://stackoverflow.com/users/263693/stephen-cleary">Stephen Cleary</a> and others already mentioned, it can introduce performance issues, rather than resolving them, and it will help you only in a specific scenario:

<ul>
<li>High-traffic controllers</li>
<li>Scalable backend</li>
</ul>

I have some concerns using async actions in ASP.NET MVC. When does it improve performance of my apps, and when does it not?

<ol>
<li>Is it good to use async action everywhere in ASP.NET MVC?</li>
<li>Regarding awaitable methods: shall I use async/await keywords when I want to query a database (via EF/NHibernate/other ORM)?</li>
<li>How many times can I use await keywords to query the database asynchronously in one single action method?</li>
</ol>

When should I use Async Controllers in ASP.NET MVC?

单元测试

SQL Server

数据库

服务器

NoSQL

我对在ASP.NET MVC中使用异步操作有一些顾虑。它什么时候能提高我的应用程序的性能，什么时候不能在ASP.NET MVC中到处使用异步操作好吗？关于可等待的方法:当我想要查询数据库(通过EF/NHibernate/other ORM)时，我是否应该使用async/await关键字？我可以在一个操作方法中使用awa...

问我应该在什么时候使用ASP.NET MVC中的异步控制器？
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