When should I use Async Controllers in ASP NET MVC

Question

I have some concerns using async actions in ASP NET MVC  When does it improve performance of my apps  and when does it not    Is it good to use async action everywhere in ASP NET MVC  Regarding awaitable methods  shall I use async await keywords when I want to query a database  via EF NHibernate other ORM   How many times can I use await keywords to query the database asynchronously in one single action method

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

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

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