Destroy a wcf thread - multithreading

I'm using multithreaded wcf maxConcurrentCalls = 10. By logging calls to my service I see that 10 different threads are executing in my service class and that they are reused in the following calls.
Can I tell WCF to destroy/delete a thread so it will create a new one on the next call?
This is because I have thread-static state that I sometimes want to be cleared (on unexpected exceptions). I am using the thread-static scope to gain performance.

WCF doesn't create new threads. It uses threads from a thread pool to service requests. So when a request begins it draws a thread from this pool to execute the request and after it finishes it returns the thread to the pool. The way that WCF uses threads underneath is an implementation detail that you should not rely on. You should never use Thread Static in ASP.NET/WCF to store state.
In ASP.NET you should use HttpContext.Items and in WCF OperationContext to store some state that would be available through the entire request.
Here's a good blog post you may take a look at which illustrates a nice way to abstract this.

Related

How exactly thread pools are working in ASP.NET Core?

I suppose, there is a thread pool which the web server are using to serve requests. So the controllers are running within one of the thread of this thread pool. Say it is the 'serving' pool.
In one of my async action method I use an async method,
var myResult = await myObject.MyMethodAsync();
// my completion logic here
As explained many places, we are doing this, to not block the valuable serving pool thread, instead execute MyMethodAsync in an other background thread... then continue the completion logic in again a serving pool thread, probably in other one, but having the http context, and some othe minor things marshaled there correctly.
So the background thread in which MyMethodAsync runs must be from an other thread pool, unless the whole thing makes no sense.
Question
Please confirm or correct my understanding and in case if it is correct, I still miss why would one thread in one pool more valuable resource than other thread in another pool? In the end of the day the whole thing runs on a same particular hardware with given number of cores and CPU performance...
There is only one thread pool in a .NET application. It has both worker threads and I/O threads, which are treated differently, but there is only one pool.
I suppose, there is a thread pool which the web server are using to serve requests. So the controllers are running within one of the thread of this thread pool. Say it is the 'serving' pool.
ASP.NET uses the .NET thread pool to serve requests, yes.
As explained many places, we are doing this, to not block the valuable serving pool thread, instead execute MyMethodAsync in an other background thread... then continue the completion logic in again a serving pool thread, probably in other one, but having the http context, and some othe minor things marshaled there correctly.
So the background thread in which MyMethodAsync runs must be from an other thread pool, unless the whole thing makes no sense.
This is the wrong part.
With truly asynchronous methods, there is no thread (as described on my blog). While the code within MyMethodAsync will run on some thread, there is no thread dedicated to running MyMethodAsync until it completes.
You can think about it this way: asynchronous code usually deals with I/O, so lets say for example that MyMethodAsync is posting something to an API. Once the post is sent, there's no point in having a thread just block waiting for a response. Instead, MyMethodAsync just wires up a continuation and returns. As a result, most asynchronous controller methods use zero threads while waiting for external systems to respond. There's no "other thread pool" because there's no "other thread" at all.
Which is kind of the point of asynchronous code on ASP.NET: to use fewer threads to serve more requests. Also see this article.

Safe to spawn a new thread inside request thread in spring

I have a spring controller. The request thread from the controller is passed to the #Service annotated Service class. Now I want to do some background work and the request thread must some how trigger the background thread and continue with it's own work and should not wait for the background thread to complete.
My first question : is this safe to do this.?
Second question : how to do this.?
Is this safe
Not really. If you have many concurrent users, you'll spawn a thread for everyone of them, and the high number of threads could bring your server to its knees. The app server uses a pool of threads, precisely to avoid this problem.
How to do this
I would do this by using the asynchronous capabilities of Spring. Call a service method annotated with #Async, and the service method will be executed by another thread, from a configurable pool.

WCF Operation.Context not Thread safe?

I'm code reviewing a WCF service.
In the header of each message we inject data that the service is going to use later to build a connection string to a DB.
That's because the service is going to be used by a number of different sites, each with its own DB that the service has to query.
We use wcf extensibility. We have a custom MessageInspector that, after receiving the request, extracts the data from the message header, creates a context (that implements IExtension) and adds it to OperationContext.Current.Extensions.
Before sending the reply the custom context is removed from the Extencions collection.
This is a fairly common pattern, as discussed here:
Where to store data for current WCF call? Is ThreadStatic safe?
and here:
http://social.msdn.microsoft.com/Forums/vstudio/en-US/319cac66-66e8-4dfe-9a82-dfd289c9df1f/wcf-doesnt-have-session-storage-so-where-should-one-store-call-specific-data?forum=wcf
This all works fine as long as the service receives a request, processes it, sends the reply and receives the next request.
But what if the service receives a request and before being able to reply it gets a second request? I built a small console application to test it. I send 2 messages from 2 different threads, I made the wcf service wait for 2 seconds, to ensure the second request comes in before the first one is completed and this is what I get:
Site Id : test1450 ; Session: uuid:2caf47cf-7d46-4d72-9275-d9c037fa0e70;id=2 : Thread Id: 6
Site Id : test1450 ; Session: uuid:2caf47cf-7d46-4d72-9275-d9c037fa0e70;id=3 : Thread Id: 22
It looks like wcf creates 2 sessions executing on 2 different threads, but Site Id is the same. It shouldn't. Judging from this it looks like OperationContext.Current.Extensions is a collection shared between threads.
Right now I'm inclined to think my test is wrong and I missed something.
Has anyone tried something similar and found out that OperationContext.Current is not thread safe?
OperationContext.Current like other similar properties such as HttpContext.Current have thread affine (or thread static) values. So they are thread safe in the sense that multiple threads can read them, but different threads will get different instances. They can be thought of as dictionaries between specific threads and instances.
So in this context they are not thread safe.
Requests are served by a thread pool so concurrent requests will have different thread ids. (up to a point where the thread pool is full, then requests will be put on hold)

How to run a time consuming task on startup in a web application while deploying it

we are facing an issue with initializing our cache at server startup or application deployment. Initializing the cache involves
Querying a database to get the list of items
Making an rmi call for each item
Listening to the data on a JMS queue/topic
Constructing the cache
This initialization process is in startup code. All this is taking lot of time due to which the deployment is taking lot of time or server start time is increasing.
So what I proposed is to create a thread in the startup and run the initialization code in it. I wrote a sample application to demonstrate it.
It involves a ServletContextListener, a filter. In the listener I am creating a new thread in which the HeavyProcess will run. When it finishes an event will be fired which the filter will be listening. On receiving the event the filter will allow incoming http requests. Until then the filter redirects all clients to a default page which shows a message that the application is initializing.
I presented this approach and few concerns were raised.
We should not ideally create a thread because handling the thread will be difficult.
My question is why cant we create a thread like these in web applications.
If this is not good, then what is the best approach?
If you can use managed threads, avoid unmanaged ones. The container has no control over unmanaged threads, and unmanaged threads survive redeployments, if you do not terminate these properly. So you have to register unmanaged threads, and terminate these somehow (which is not easy as well, because you have to handle race-conditions carefully).
So one solution is to use #Startup, and something like this:
#Schedule(second = "*/45", minute = "*", hour = "*")
protected void asyncInit(final Timer timer) {
timer.cancel();
// Do init here
// Set flag that init has been completed
}
I have learned about this method here: Executing task after deployment of Java EE application
So this gives you an async managed thread, and deployment will not be delayed by #PostConstruct. Note the timer.cancel().
Looking at your actual problem: I suggest using a cache which supports "warm starts".
For example, Infinispan supports cache stores so that the cache content survives restarts. If you have a cluster, there are distributed or replicated caching modes as well.
JBoss 7 embeds Infinispan (it's an integrated service in the same JVM), but it can be operated independently as well.
Another candidate is Redis (and any other key/value store with persistence will do as well).
In general, creating unmanaged threads in a Java EE environment is a bad idea. You will loose container managed transactions, user context and many more Java EE concepts within your unmanaged thread. Additionally unmanaged threads may block the conainer on shutdown if your thread handling isn't appropriate.
Which Java EE Version are you using? Perhaps you can use Servlet 3.0's async feature?
Or call a asynchronous EJB for doing the heavy stuff at startup (#PostConstruct). The call will then set a flag when its job is done.

Ansync thread from WCF RESTful Service

We have created a WCF RESTful service for a WPF(UI) Application. The UI sends a request to the WCF Service which then invokes a suitable method in BLL, which in turn invokes a method in DAL. All these layers have been separated using IOC/DI.
Now, for a new feature, we want that when a new object of a certain type is added to the database, it should go through 3 steps which would be performed in a separate thread.
That is, if service sends a request to BLL to add a new object OBJ to the database, the BLL should save the object into database through the DAL and then initiate a new thread to perform a some actions upon the object without blocking the WCF Request.
But whenever we try to do so by starting a new thread in the BLL, the application crashes. It is so because the 'InRequestScope' object of the database context has been disposed and the thread cannot update the database. Also the WCF request does not ends until the thread is completed, although the return value has been provided and the BLL method has completed execution.
Any help would be much valued.
I have figured out the solution and explanation for this behavior. Turns out to be a rather silly one.
Since I was creating a thread from the BLL (with IsBackground = true;), the parent thread (originated by the service request) was waiting for this thread to end. And when both the threads ended, the response was sent back to the client. And the solution, well, use a BackgroundWorker instead, no rocket science, just common sense.
And for the disposing of context, since the objects were InRequestScope, and the request had ended. So every time a Repository required a UnitOfWork (uow/context), it would generate a new context and end it as soon as the database request was complete. And the solution would be, create a uow instance, store in a variable, pass it to the repository required to be used, and force all repositories to use the same uow instance than creating a new one for itself.
This seem more of a client-side concern than a service-side concern. Why not have the client make asynchronous requests to WCF service since this automatically provides multi-threaded access to the service.
The built-in System.Net.WebClient (since you're access a webHttpBinding or WCF Web API endpoint) can be used asynchronously. This blog post gives a quick overview of how it is done. Although this MSDN article seems to apply to file I/O, about three quarters down, there is a detailed explanation on coding asynchronous WebClient usage.

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