WCF Return Multiple Function Results Asynchronously? - wcf

Question

I've been reading up on how to do this but I'm having a hard time getting my head around it.
Synchronous and Asynchronous Operations
WCF: Working with One-Way Calls, Callbacks, An...
My goal is to return results from multiple functions in a Silverlight enabled WCF service as they are discovered.
A general overview of how this service is desired to work is as follows.
User enters a url that has many large csv files listed in hyperlinks. The service gets the initial user entered url, makes a web request of that url, gets the csv file names using regex, service downloads the csv files to server, files are converted to a different format.
Currently all of that works, but no response is shown until the entire operation is done. I would like to provide feedback during each function.
I'm using VB for this app but am fluent in C# if anyone has a code suggestion.
<ServiceContract(Namespace:="http://somemadeupurl")>
<SilverLightFaultBehavior()>
<AspNetCompatibilityRequirements(RequirementsMode:=
AspNetCompatibilityRequirementsMode.Allowed)>
Public Class GetCSV
<OperationContract()>
Public Function ProcessInitialLink(ByVal strInitialLink As String)
'download the source html
'do a webrequest and extract csv links
'since dates are in the filenames I would like to send back most
'recent to user here
Dim strMostRecentCSV As String=SomeRegexMatch
> 'problem here. Would like to return strMostRecentCSV and keep processing
GetAndConvertBigCSV(strMostRecentCSV)
Return strMostRecentCSV
End Function
'actually a list but for brevity..
Private Function GetAndConvertBigCSV(ByVal strMostRecentCSV as string)
'do a bunch of downloading
'call a function to do a bunch of converting
'call a function to clean up files
End Function
End Class
If it's done like this it will return strMostRecentCSV but has to wait until GetAndConvertBigCSV is done before returning.
I've tried spawning GetAndConvertBigCSV as a new thread and it works but returns nothing that I can bind to the Silverlight client (e.Result)
At a minimum I would like to provide feedback of the first function Return then have the service continue.
Thanks a million for help.

Answer 1

I think that what you probably want is a "polling duplex http service" - what this means is that WCF will fake up a duplex style service for you (by polling). The advantage of a duplex service over a regular asynchronous service is that it is easy to call back to the client multiple times (so you can provide feedback on the progress of your task).
It is very easy to implement. Let's say you have two projects a web application and a silverlight application...
Web Application
Create your service, for example:
[ServiceContract(CallbackContract = typeof(ICallback))]
public interface ILongRunningService
{
[OperationContract]
void StartLongRunningProcess(string initialParameter);
}
[ServiceContract]
public interface ICallback
{
[OperationContract(IsOneWay = true)]
void Update(string someStateInfo);
}
public class LongRunningService : ILongRunningService
{
public void StartLongRunningProcess(string initialParameter)
{
// Get hold of the callback channel and call it once a second
// five times - you can do anything here - create a thread,
// start a timer, whatever, you just need to get the callback
// channel so that you have some way of contacting the client
// when you want to update it
var callback = OperationContext
.Current
.GetCallbackChannel<ICallback>();
ThreadPool
.QueueUserWorkItem(o =>
{
for (int i = 0; i < 5; i++)
{
callback.Update("Step " + i);
Thread.Sleep(1000);
}
});
}
}
Then you need a svc file that has this in it (adjust the service attribute to be your service implementation class):
<%# ServiceHost Service="SilverlightApplication.Web.LongRunningService" %>
Finally you will need this configuration inside web.config (this goes inside the configuration root element):
<system.serviceModel>
<extensions>
<bindingExtensions>
<add name=
"pollingDuplexHttpBinding"
type="System.ServiceModel.Configuration.PollingDuplexHttpBindingCollectionElement,System.ServiceModel.PollingDuplex, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
</bindingExtensions>
</extensions>
<services>
<service name="SilverlightApplication.Web.LongRunningService">
<endpoint
address=""
binding="pollingDuplexHttpBinding"
bindingConfiguration="multipleMessagesPerPollPollingDuplexHttpBinding"
contract="SilverlightApplication.Web.ILongRunningService">
</endpoint>
<endpoint
address="mex"
binding="mexHttpBinding"
contract="IMetadataExchange"/>
</service>
</services>
<bindings>
<pollingDuplexHttpBinding>
<binding name="multipleMessagesPerPollPollingDuplexHttpBinding"
duplexMode="MultipleMessagesPerPoll"
maxOutputDelay="00:00:07"/>
</pollingDuplexHttpBinding>
</bindings>
<behaviors>
<serviceBehaviors>
<behavior name="">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="false" />
</behavior>
</serviceBehaviors>
</behaviors>
<serviceHostingEnvironment multipleSiteBindingsEnabled="true" />
</system.serviceModel>
The important parts are the name attribute on the service element and the contract attribute on the endpoint element. They should be the class and interface you defined (with the namespace).
IMPORTANT You need to add a reference to the C:\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\ Server\System.ServiceModel.PollingDuplex.dll assembly (remove the x86 if not 64 bit OS) to the web application project.
Silverlight Application
You need to firstly add a service reference to the service you created and then, let's say you want to call the service from a button you would have the following code:
private void button1_Click(object sender, RoutedEventArgs e)
{
// Create the client proxy with the URL of the service
var proxy = new LongRunningServiceClient(
new PollingDuplexHttpBinding(
PollingDuplexMode.MultipleMessagesPerPoll),
new EndpointAddress(
"http://localhost/WebApplication/LongRunningService.svc"));
// Attach the handler to be called periodically and start the process
proxy.UpdateReceived += Update;
proxy.StartLongRunningProcessAsync("blah");
}
private void Update(object sender, UpdateReceivedEventArgs e)
{
// Use the result from the e parameter here
}
IMPORTANT You need to add a reference to the C:\Program Files (x86)\Microsoft SDKs\Silverlight\v4.0\Libraries\ Client\System.ServiceModel.PollingDuplex.dll assembly (remove the x86 if not 64 bit OS) to the silverlight client project.
And that is it - the Update method will get called, in this case once a second five times. but you can do whatever you like.

Answer 2

I would NOT recommend using a polling duplex to solve this business requirement, due to the load it places in the server. What I read was that you have a need to return several files back to the client, and provide feedback to the client as the files are being downloaded.
As downloading a file does not seem to be an issue (which should be a stream), I think you are asking is how to update the UI, while the files are being downloaded, perhaps capturing how much of the file is actually being downloaded. At the least, you should be updating the UI as each file arrives.
The latter is easy, simply download the download links to each file on the client, do them one by one, and update the UI between file downloads. That would be a single threaded approach.
Update the UI while a background thread is executing, is a more complicated approach, but a much better implementation, as the client "feels" that they remain in control while the files are being downloaded. Here is an approach for downloading a file async.
Remember in SilverLight, you really only have HTTP to transfer information to the application, which cannot do duplex. HTTP long pooling achieves the same result, but connections are left open, and greatly minimize the ability to scale.