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

You may find my MSDN article on the subject helpful; I took a lot of space in that article describing when you should use async on ASP.NET, not just how to use async on ASP.NET.

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

First, understand that async/await 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:

Make your individual requests complete faster. In fact, they will complete (just a teensy bit) slower.

Return to the caller/browser when you hit an await. await only "yields" to the ASP.NET thread pool, not to the browser.

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

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 async by just calling Task.Run 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)!

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

You could use whatever awaitable methods you have available. Right now most of the major players support async, but there are a few that don't. If your ORM doesn't support async, then don't try to wrap it in Task.Run 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 async. 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 async.

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

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.

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

A typical use for the AsyncController class is long-running Web service calls.

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 1 and 2 references

Derived from @PanagiotisKanavos comments:

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

is it good to use async action everywhere in ASP.NET MVC?

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

async-await 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 async-await 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 async-await, 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 async-await 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 async and sync don't mix very well.

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

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

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

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 await Task.WhenAll to wait for both to complete.

async 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 await keyword.

My 5 cents:

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

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:

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

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 C10k problem. 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 here for excellent advise.

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

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

The reason is, as Stephen Cleary and others already mentioned, it can introduce performance issues, rather than resolving them, and it will help you only in a specific scenario:

High-traffic controllers

Scalable backend

Is it good to use async action everywhere in ASP.NET MVC?

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.

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

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:

.ToListAsync()
.FirstOrDefaultAsync()
.SaveChangesAsync()
.FindAsync()

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

The sky is the limit