We Keep Coding

TextReader and TextWriter over WCF

I'm trying to write this question for the fourth time, as I'm unfortunately not sure what I actually want. Please excuse me being vague or a bit off.
I have a command line .NET4.5 C# application. I'll run it at the same time at two geographically distant locations. I want to be able to turn one instance into host and the other into client.
In the heart of it, I want a TextReader implementation, that upon Read/ReadLine awaits for an input from a remote client. I also want a TextWriter that upon Write/WriteLine will call a remote client and pass text to it.
The TextWriter is of course fairly simple matter, as I basically do that: on write, call client.SendText(s); to the other instance.
How do I go about implementing TextReader.ReadLine() in that scenario? A naive aproach would be to have, in the WCF service, a method:
class Service : IService{
void SendText(string s){
Console.WriteLine(s); // or whatever is the destination TextWriter
}
string ReadLine(){
return Console.ReadLine(); // or whatever is the source TextReader
}
}
but is keeping the WCF operation call open for minutes or even hours a good idea?
Another way I can see is to have:
class Service : IService{
void RegisterClient(string url){/* url points to an IService endpoint in client*/}
void SendString(string s){...}
}
And then host it in both instances, call RegisterClient from client providing own endpoint url and then if host wants to return some data to client - just calls SendString, no problem. But if hosts must wait for input from client, how to solve that reasonably?
class WCFTextReader : TextReader{
public override string ReadLine(){
// what here? it should return whatever comes in the next SendString call from client...
}
}
I could have a Queue<string> of incoming messages and then do a while(true) and either return first from Queue or Thread.Sleep for a short while and re-check, but every time I feel that I need while(true) and Thread.Sleep I'd rather come here, because I think I'm missing something....

The default WCF message pattern is request/response; a client sends a request, and the server responds to it.
There is, however, a duplex message pattern, in which the service interface contract identifies another interface as its callback interface contract; the client must implement this callback interface contract, similar to how the service must implement the service interface contract.
This message pattern should allow your server to arbitrarily send a message to the client without the client having to prompt it to do so.
http://msdn.microsoft.com/en-us/library/ms731064%28v=vs.110%29.aspx
http://msdn.microsoft.com/en-us/library/ms731184%28v=vs.110%29.aspx
** Fair warning: in my experience, you potentially still have to deal with the issue of keeping the connection alive, in case you plan to have the server send a message back to the client after a significant period of inactivity.
If keeping the connection open is acceptable to you, then you can implement some kind of keep-alive mechanism.
If keeping the connection open is not acceptable, then I think you'd need to look at a more complex approach that would involve the client periodically checking in with the server in order to give the server an opportunity to send the client any queued up messages that it has for the client.

Can I stop WCF hanging at 100% CPU by making it time out? [duplicate]

Is there a way to tell a WCF service to response to a request (with or without aborting it's processing) after a certain amount of time, even if it didn't finish yet, something like a server-side timeout policy?

I suppose you could do this by starting a new Thread as soon as the WCF operation starts. The real work then happens on the new thread and the original WCF request thread waits using a Thread.Join() with a specific timeout. If the timeout occurs the worker thread can be canceled using a Thread.Abort().
Something like this:
public string GetData(int value)
{
string result = "";
var worker = new Thread((state) =>
{
// Simulate l0ng running
Thread.Sleep(TimeSpan.FromSeconds(value));
result = string.Format("You entered: {0}", value);
});
worker.Start();
if (!worker.Join(TimeSpan.FromSeconds(5)))
{
worker.Abort();
throw new FaultException("Work took to long.");
}
return result;
}

I have solved the same problem and created a blog post:
http://kanchengcao.blogspot.com/2012/06/adding-timeout-and-congestion.html
In short:
WCF server timeout config do not work
You could implement a timeout as others said
Such a timeout does not guarantee a timely response to the client, since the request could be queued for long before entering your code with timeout.
So I implemented method to drop requests if the server is overloaded and is expected to cause more timeouts.

I don't know why you want to do this - you should probably edit your question to say what you're trying to accomplish.
If I had to do this, then I would have the web service pass the request off to a separate Windows Service, possibly by using WCF over MSMQ. I would have a timeout on that request. If the request didn't finish in time, I'd simply return a Timeout fault. The actual request would not be impacted.

Implement your service using the asynchronous model and have some code monitoring your outstanding requests to see if they've taken too long.
Then, if a timeout occurs before the request can be answered in the real way, then call their callback. The WCF stack provides this when it calls your
BeginFoo( fooParam1, fooParam2, AsyncCallback callback, object state)
Then throw or return your fault/timeout exception or response in the correponding EndFoo() method.
Make sure to not call their callback again if the real answer comes along eventually.
It'll take some getting used to asynchronous wcf programming, but no, apparently there is no server side setting.
Also, you should try to use a client that supports timeout or cancellable requests because you might not be able to rely on the server to time out the request for you. There might not be connectivity or the server machine might have another problem.
Cheers,
Chris

WCF Windows Service - Long operations/Callback to calling module

I have a Windows Service that takes the name of a bunch of files and do operations on them (zip/unzip, updating db etc). The operations can take time depending on size and number of files sent to the service.
(1) The module that is sending a request to this service waits until the files are processed. I want to know if there is a way to provide a callback in the service that will notify the calling module when it is finished processing the files. Please note that multiple modules can call the service at a time to process files so the service will need to provide some kind of a TaskId I guess.
(2) If a service method is called and is running and another call is made to the same service, then how will that call be processed(I think there is only one thread asociated with the service). I have seen that when the service is taking time in processing a method, the threads associated with the service begin to increase.

WCF does indeed offer duplex bindings which allow you to specify a callback contract, so that the service can call back to the calling client to notify.
However, in my opinion, this mechanism is rather flaky and not really to be recommended.
In such a case, when the call causes a fairly long running operation to happen, I would do something like this:
If you want to stick to HTTP/NetTcp bindings, I would:
drop off the request with the service, and then "let go" - this would be a one-way call, you just drop off what you want to have done, and then your client is done
have a status call that the client could call after a given time to find out whether or not the results of the request are ready by now
if they are, there should be a third service call to retrieve the results
So in your case, you could drop off the request to zip some files. The service would go off and do its work and store the resulting ZIP in a temporary location. Then later on the client could check to see whether the ZIP is ready, and if so, retrieve it.
This works even better over a message queue (MSMQ) which is present in every Windows server machine (but not a lot of people seem to know about it or use it):
your client drops off the request on a request queue
the service listens on that request queue and fetches request after request and does it works
the service can then post the results to a result queue, on which your callers in turn are listening
Check out how to do all of this efficiently by reading the excellent MSDN article Foudnations: Build a queue WCF Response Service - highly recommended!
A message-queue based systems tends to be much more stable and less error-prone that a duplex-/callback-contract based system, in my opinion.

(1) The simplest way to achieve this is with a taskId as you note, and then have another method called IsTaskComplete with which client can check whether the task has been completed.
(2) Additional calls made to the service will start new threads.
edit: the default service behaviour is to start new threads per call. The configurable property is Instance Context Mode, and can be set to PerCall, PerSession, or Shareable.

The question has a solution, but I'm using a WCF duplex service to get the result of a long operation, and even though I found a problem that has cost me several hours to solve (and that's why I searched this question earlier), now it works perfectly, and I believe it is a simple solution within the WCF duplex service framework.
What is the problem with a long operation? The main problem is blocking the client interface while the server performs the operation, and with the WCF duplex service we can use a call back to the client to avoid the blockage (It is an old method to avoid blocking but it can easily be transformed into the async/await framework using a TaskCompletionSource).
In short, the solution uses a method to start the operation asynchronously on the server and returns immediately. When the results are ready, the server returns them by means of the client call back.
First, you have to follow any standard guide to create WCF duplex services and clients, and I found these two useful:
msdn duplex service
Codeproject Article WCF Duplex Service
Then follow these steps adding your own code:
Define the call back interface with an event manager method to send results from the server and receive them in the client.
public interface ILongOperationCallBack
{
[OperationContract(IsOneWay = true)]
void OnResultsSend(....);
}
Define the Service Interface with a method to pass the parameters needed by the long operation (refer the previous ILongOperationCallBack interface in the CallBackContractAttribute)
[ServiceContract(CallbackContract=typeof(ILongOperationCallBack))]
public interface ILongOperationService
{
[OperationContract]
bool StartLongOperation(...);
}
In the Service class that implements the Service Interface, first get the proxy of the client call back and save it in a class field, then start the long operation work asynchronously and return the bool value immediately. When the long operation work is finished send the results to the client using the client call back proxy field.
public class LongOperationService:ILongOperationService
{
ILongOperationCallBack clientCallBackProxy;
public ILongOperationCallBack ClientCallBackProxy
{
get
{
return OperationContext.Current.GetCallbackChannel<ITrialServiceCallBack>());
}
}
public bool StartLongOperation(....)
{
if(!server.IsBusy)
{
//set server busy state
//**Important get the client call back proxy here and save it in a class field.**
this.clientCallBackProxy=ClientCallBackProxy;
//start long operation in any asynchronous way
......LongOperationWorkAsync(....)
return true; //return inmediately
}
else return false;
}
private void LongOperationWorkAsync(.....)
{
.... do work...
//send results when finished using the cached client call back proxy
this.clientCallBackProxy.SendResults(....);
//clear server busy state
}
....
}
In the client create a class that implements ILongOperationCallBack to receive results and add a method to start the long operation in the server (the start method and the event manager don't need to be in the same class)
public class LongOperationManager: ILongOperationCallBack
{
public busy StartLongOperation(ILongOperationService server, ....)
{
//here you can make the method async using a TaskCompletionSource
if(server.StartLongOperation(...)) Console.WriteLine("long oper started");
else Console.Writeline("Long Operation Server is busy")
}
public void OnResultsSend(.....)
{
... use long operation results..
//Complete the TaskCompletionSource if you used one
}
}
NOTES:
I use the bool return in the StartLongOperation method to indicate that the server is Busy as opposed to down, but it is only necessary when the long operation can't be concurrent as in my actual application, and maybe there are best ways in WCF to achieve non concurrency (to discover if the server is down, add a Try/Catch block as usual).
The important quote that I didn't see documented is the need to cache the call back client proxy in the StartLongOperation method. My problem was that I was trying to get the the proxy in the working method (yes, all the examples use the call back client proxy in the service method, but it isn't explicity stated in the documentation, and in the case of a long operation we must delay the call back until the operation ends).
Do not get and cache twice the call back Proxy after a service method has returned and before the next one.
Disclaimer: I haven't added code to control errors, etc.

WCF - Client callback vs. polling for "keep list of subscribers"

I want to create a simple client-server example in WCF. I did some testing with callbacks, and it works fine so far. I played around a little bit with the following interface:
[ServiceContract(SessionMode = SessionMode.Required, CallbackContract = typeof(IStringCallback))]
public interface ISubscribeableService
{
[OperationContract]
void ExecuteStringCallBack(string value);
[OperationContract]
ServerInformation Subscribe(ClientInformation c);
[OperationContract]
ServerInformation Unsubscribe(ClientInformation c);
}
Its a simple example. a little bit adjusted. You can ask the server to "execute a string callback" in which case the server reversed the string and calls all subscribed client callbacks.
Now, here comes the question: If I want to implement a system where all clients "register" with the server, and the server can "ask" the clients if they are still alive, would you implement this with callbacks (so instead of this "stringcallback" a kind of TellTheClientThatIAmStillHereCallback). By checking the communication state on the callback I can also "know" if a client is dead. Something similar to this:
Subscribers.ForEach(delegate(IStringCallback callback)
{
if (((ICommunicationObject)callback).State == CommunicationState.Opened)
{
callback.StringCallbackFunction(new string(retVal));
}
else
{
Subscribers.Remove(callback);
}
});
My problem, put in another way:
The server might have 3 clients
Client A dies (I pull the plug of the laptop)
The server dies and comes back online
A new client comes up
So basically, would you use callbacks to verify the "still living state" of clients, or would you use polling and keep track "how long I havent heard of a client"...

You can detect most changes to the connection state via the Closed, Closing, and Faulted events of ICommunicationObject. You can hook them at the same time that you set up the callback. This is definitely better than polling.
IIRC, the Faulted event will only fire after you actually try to use the callback (unsuccessfully). So if the Client just disappears - for example, a hard reboot or power-off - then you won't be notified right away. But do you need to be? And if so, why?
A WCF callback might fail at any time, and you always need to keep this in the back of your mind. Even if both the client and server are fine, you might still end up with a faulted channel due to an exception or a network outage. Or maybe the client went offline sometime between your last poll and your current operation. The point is, as long as you code your callback operations defensively (which is good practice anyway), then hooking the events above is usually enough for most designs. If an error occurs for any reason - including a client failing to respond - the Faulted event will kick in and run your cleanup code.
This is what I would refer to as the passive/lazy approach and requires less coding and network chatter than polling or keep-alive approaches.

If you enable reliable sessions, WCF internally maintains a keep-alive control mechanism. It regularly checks, via hidden infrastructure test messages, if the other end is still there. The time interval of these checks can be influenced via the ReliableSession.InactivityTimeout property. If you set the property to, say, 20 seconds, then the ICommunicationObject.Faulted event will be raised about 20 to 30 (maximum) seconds after a service breakdown has occurred on the other side.
If you want to be sure that client applications always remain "auto-connected", even after temporary service breakdowns, you may want to use a worker thread (from the thread pool) that repeatedly tries to create a new proxy instance on the client side, and calls a session-initiating operation, after the Faulted event has been raised there.
As a second approach, since you are implementing a worker thread mechanism anyway, you might also ignore the Faulted event and let the worker thread loop during the whole lifetime of the client application. You let the thread repeatedly check the proxy state, and try to do its repair work whenever the state is faulted.
Using the first or the second approach, you can implement a service bus architecture (mediator pattern), guaranteeing that all client application instances are constantly ready to receive "spontaneous" service messages whenever the service is running.
Of course, this only works if the reliable session "as such" is configured correctly to begin with (using a session-capable binding, and applying the ServiceContractAttribute.SessionMode, ServiceBehaviorAttribute.InstanceContextMode, OperationContractAttribute.IsInitiating, and OperationContractAttribute.IsTerminating properties in meaningful ways).

I had a similar situation using WCF and callbacks. I did not want to use polling, but I was using a "reilable" protocol, so if a client died, then it would hang the server until it timed out and crashed.
I do not know if this is the most correct or elegant solution, but what I did was create a class in the service to represent the client proxy. Each instance of this class contained a reference to the client proxy, and would execute the callback function whenever the server set the "message" property of the class. By doing this, when a client disconnected, the individual wrapper class would get the timeout excetpion, and remove itself from the server's list of listeners, but the service would not have to wait for it. This doesn't actually answer your question about determining if the client is alive, but it is another way of structuring the service to addrss the issue. If you needed to know when a client died, you would be able to pick up when the client wrapper removed itself from the listener list.

I have not tried to use WCF callbacks over the wire but i have used them for interprocess communication. I was having a problem where call of the calls that were being sent were ending up on the same thread and making the service dead lock when there were calls that were dependant on the same thread.
This may apply to the problem that you are currently have so here is what I had to do to fix the problem.
Put this attribute onto the server and client of the WCF server implemetation class
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Multiple)]
public class WCFServerClass
The ConcurrencyMode.Multiple makes each call process on its own thread which should help you with the server locking up when a client dies until it timesout.
I also made sure to use a Thread Pool on the client side to make sure that there were no threading issues on the client side

How to check the availability of a net.tcp WCF service

My WCF server needs to go up and down on a regular basis, the client sometimes uses the server, but if it is down the client just ignore it.
So each time I need to use the server services I check the connection state and if it's not open I open it.
The problem is that if I attempt to open while the server is down there is a delay which hits performance.
My question is, is there a way to do some kind of myClient.CanOpen()? so I'd know if there is any point to open the connection to the server.

There is an implementation of WS-Discovery that would allow you to listen for up/down announcements for your service. This is also a very convenient form of service address resolution because it utilizes UDP multicast messages to find the service, rather than configuring one set address on the client.
WS-Discovery for WCF
There's also an implementation done by a Microsoft employee:
WS-Discovery Sample Implementation
.NET 4.0 will include this natively. You can read about .NET 4.0's implementation on Jesus Rodriguez's blog. It has a great chart that details the ad-hoc communication that goes on in WS-Disco Using WS-Discovery in WCF 4.0
Another thing you might consider, especially if your messages are largely one-way, is a protocol that works natively disconnected, like MSMQ. I don't know what your design for your application looks like, but MSMQ would allow a client to send a message regardless of the state of the service and the service will get it when it comes back up. This way your client doesn't have to block quite so much trying to get confirmation that a service is up before communicating... it'll just fire and forget.
Hope this helps.

If you are doing a synchronous call expecting a server timeout in an application with a user interface, you should be doing it in another thread. I doubt that the performance hit is due to exception overhead.
Is your performance penalty in CPU load, gui availability or wall clock time?
You could investigate to see if you can create a custom binding on TCP, but with faster timeout.
I assume you know that "IsOneWay=true" is faster than request->response in your case because you wouldn't be expecting a response anyway, but then you are not getting confirmation or return values.
You could also implement a two-way communication that is not request->response.

If you were in a local network it might be possible to broadcast a signal to say that a new server is up. The client would need to listen for the broadcast signal and respond accordingly.

Here's what I'm using and it works like a charm. And btw, the ServiceController class lives in namespace 'System.ServiceProcess'.
try
{
ServiceController sc = new ServiceController("Service Name", "Computer's IP Address");
Console.WriteLine("The service status is currently set to {0}",
sc.Status.ToString());
if ((sc.Status.Equals(ServiceControllerStatus.Stopped)) ||
(sc.Status.Equals(ServiceControllerStatus.StopPending)))
{
Console.WriteLine("Service is Stopped, Ending the application...");
Console.Read();
EndApplication();
}
else
{
Console.WriteLine("Service is Started...");
}
}
catch (Exception)
{
Console.WriteLine("Error Occurred trying to access the Server service...");
Console.Read();
EndApplication();
}

I don't think it's possible doing a server side call to your Client to inform him that you the service has been started ... Best method i can see is having a client method figuring out where or not the service is open and in good condition. Unless I am missing some functionality of WCF ...
There is a good blogpost WCF: Availability of the WCF services if you are interested in a read.