Reliable n-Tier WCF (Threading issue?)
I am working on an n-Tier application using WCF between the layers such that:
Tier 1: Silverlight application Invokes the search request
IClientBroker clientBroker = UIContext.CreateWcfInterface<IClientBroker>("Data/ClientBroker.svc");
clientBroker.BeginSearchForClients(SearchTerm, 20, (result) =>
{
SearchResult[] results = ((IClientBroker)result.AsyncState).EndSearchForClients(result).ToArray();
// do stuff, update UI, etc.
}, clientBroker);
Tier 2: is a WCF web service using basicHttp for Silverlight to call. This is acting as a proxy to the 3rd tier.
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
class ClientBroker : IClientBroker
{
[OperationContract] // as defined in IClientBroker
public SearchResult[] SearchForClients(string keywords, int? maxResults)
{
ClientBrokerClient clientBroker = CreateClientBrokerClient();
SearchResult[] searchResults=clientBroker.SearchForClients(keywords, maxResults);
return searchResults;
}
}
Tier 3: is the "server", in that this offers a net.tcp endpoint (allowing secure clients to connect without using Silverlight). This is the ultimate target of a request.
public class ClientBroker : IClientBroker // note this is different to tier 2 interface
{
public SearchResult[] SearchForClients(string keywords, int? maxResults)
{
// do stuff
if (maxResults.HasValue)
{
return results.Take(maxResults.Value).ToArray();
}
else
{
return results.ToArray();
}
}
}
So my calls go:
Silverlight -> httpBasic -> IIS-hosted Proxy WCF service --> net.tcp --> EXE-hosted WCF service
This works well. I can pass headers through the layers, and maintain sessions, etc. And it is pretty trim.
BUT it takes only a few calls through this to cause a Timeout in the communication.
The time the server-EXE takes to do its work is neglible. The problem I am seeing is that the server "freezes" in returning results to Tier 2.
I think this is to do with a thread getting locked.
I've looked around and see that the ideal way of doing this is to make my Tier 2 run asyncronously, similar to the code below:
public SearchResult[] SearchForClients(string keywords, int? maxResults)
{
ClientBrokerClient clientBroker = CreateClientBrokerClient();
clientBroker.BeginSearchForClients(keywords, maxResults, result =>
{
SearchResult[] searc开发者_开发问答hResults=((ClientBrokerClient)result.AsyncState).EndSearchForClients(result);
// how to return results from here?
}, clientBroker);
}
But how do I achieve this when my Tier 1 client is waiting for the result of this method, which will just drop straight out before the callback execute? Am I missing something on my construction of my OperationContract methods?
UPDATE:
I have since put my server (Tier 3) through its paces from a client avoiding Tier 2 by making many requests from it. Seems the net.tcp WCF channel for Tier 3 is rock solid.
UPDATE 2:
This blog post outlined using the IAsyncResult pattern, which I have eluded to here. Am I barking up the wrong tree here? http://blogs.msdn.com/wenlong/archive/2009/02/09/scale-wcf-application-better-with-asynchronous-programming.aspx
UPDATE 3:
Ok, this paragraph from that blog:
"If you are building N-tier WCF services, you would have WCF service operations invoking WCF client proxies for other backend services. In this case, you would need to make sure that the middle-tier (routing layer) has asynchronous service operation invoking asynchronous WCF proxy operations. In this way, your middle-tier won’t run out of threads when processing many slow operations."
seems to confirm my suspicions that the problem lies in the middle layer (Tier 2). How can I achieve this Begin/End asynchrony? Do I have to do this manually or can I retain VS tools to generate the proxy classes for me? (REALLY don't want to have to do this manually, the contracts have a degree of flux in them)
Well, I think I've solved it. This topic helped me:
wcf service stops after few requests
Basically, I hadn't been closing my client proxies in my Tier 2, which I realise would cause blocking. The evolution of the code was such that I ended up removing using() {} blocks to facilitate exceptions not getting extinguished with the client proxy. However, I have restructured and retested and my Tier 2 code now looks like:
public SearchResult[] SearchForClients(string keywords, int? maxResults)
{
ClientBrokerClient clientBroker = CreateClientBrokerClient();
SearchResult[] searchResults=clientBroker.SearchForClients(keywords, maxResults);
clientBroker.Close();
return searchResults;
}
... and exceptions are not extinguished.
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