The client sends a lot of messages to the server from a single thread, over a single WCF channel.
The client sends the message with BeginMyMethod(x, b) as it does not wish to block while they get processed.
We have reliable messaging turned on, as we don’t wish to lose any messages, or have them get out of order.
However the messages are being despatched on multiple threads on the server, so are being process out of order.
We can’t have the server being single threaded, as we don’t wish a long running request from one client to block other clients.
So I just wish to process all the messages that come from a single client (over a single channel) in order with only one message from each cleint being processed at a time.
This would be easy for raw socket programming, however how to I get WCF to work as I wish?
I am now thinking that ConcurrencyMode.Reentrant does not behave well when used with InstanceContextMode.Single If I set use ConcurrencyMode.Single the messages are kept in order, but my call-backs deadlock.
(The test that gets the messages out of order has no callbacks and does not make any outgoing WCF calls, so I would expect ConcurrencyMode.Reentrant to behave the same as ConcurrencyMode.Single in that given test, but it does not)
I a not using any WCF config files, the code is:
serviceHost = new ServiceHost(this);
serviceHost.AddServiceEndpoint(
typeof(IAllEngineManagersAsyncCallbacks),
new NetTcpBinding(SecurityMode.None, true),
endPointAddress);
I have now worked round this problem by:
Changing all my call-backs from the server to the client to be OneWay
Using a dispatcher in the client before passing on any callback from the server, so client code never calls the sever from within a call-back
The client call-back object is marked with CallbackBehavior(UseSynchronizationContext=false, ConcurrencyMode=ConcurrencyMode.Single)
When running in Winform or WPF I use SynchronizationContext.Post to depatch the callbacks
When the cleint is a Console or a Windows server I use a custom depatcher.
So letting me use ConcurrencyMode.Single on both the server and the client.
It is now working as expected.
(BeginMyMethod(x, b) is still being used to send messaged from the client to the server)
(ConcurrencyMode.Reentrant seems to sometimes release the lock even when the WCF call is not made on the some thread that is processing the incoming message, it is just not a useful as Reentrant was in DCOM)
If you are using the generated BeginXXX async-methods, these are executed on a ThreadPool thread.
So although you've send the messages in a defined order, nobody guarantees you in which order the ThreadPool executes the requests.
I think that Reentrant mode implies that you allow messages be processed out of order. Normal behavior of such service would be: get message, put in queue for internal threads to process, and when it's done notify client about result. So maybe your service get messages in proper order but some of them are quiker to process and return earlier than others?
Related
Setting up a CMS consumer with a listener involves two separate calls: first, acquiring a consumer:
cms::MessageConsumer* cms::Session::createConsumer( const cms::Destination* );
and then, setting a listener on the consumer:
void cms::MessageConsumer::setMessageListener( cms::MessageListener* );
Could messages be lost if the implementation subscribes to the destination (and receives messages from the broker/router) before the listener is activated? Or are such messages queued internally and delivered to the listener upon activation?
Why isn't there an API call to create the consumer with a listener as a construction argument? (Is it because the JMS spec doesn't have it?)
(Addendum: this is probably a flaw in the API itself. A more logical order would be to instantiate a consumer from a session, and have a cms::Consumer::subscribe( cms::Destination*, cms::MessageListener* ) method in the API.)
I don't think the API is flawed necessarily. Obviously it could have been designed a different way, but I believe the solution to your alleged problem comes from the start method on the Connection object (inherited via Startable). The documentation for Connection states:
A CMS client typically creates a connection, one or more sessions, and a number of message producers and consumers. When a connection is created, it is in stopped mode. That means that no messages are being delivered.
It is typical to leave the connection in stopped mode until setup is complete (that is, until all message consumers have been created). At that point, the client calls the connection's start method, and messages begin arriving at the connection's consumers. This setup convention minimizes any client confusion that may result from asynchronous message delivery while the client is still in the process of setting itself up.
A connection can be started immediately, and the setup can be done afterwards. Clients that do this must be prepared to handle asynchronous message delivery while they are still in the process of setting up.
This is the same pattern that JMS follows.
In any case I don't think there's any risk of message loss regardless of when you invoke start(). If the consumer is using an auto-acknowledge mode then messages should only be automatically acknowledged once they are delivered synchronously via one of the receive methods or asynchronously through the listener's onMessage. To do otherwise would be a bug in my estimation. I've worked with JMS for the last 10 years on various implementations and I've never seen any kind of condition where messages were lost related to this.
If you want to add consumers after you've already invoked start() you could certainly call stop() first, but I don't see any problem with simply adding them on the fly.
In my client/server applications, I want a single duplex WCF channel to be available for communication with its server - sort of a background connection that isn't strictly necessary for the client application to run, but is desirable for reporting status to the server. I have a Ping() call and Echo() callback in the IServerContract and IClientContract respectively.
I implement class ServerProxy : System.ServiceModel.DuplexClientBase<IServerContract> with the pass-thru methods to base.InnerChannel.
If I create var proxy = new ServerProxy(...) in my client, I can just begin calling proxy.Ping() and WCF will automatically open the connection for the first call and perform the operation call immediately after. However, the first call always takes ~10 seconds because of channel initialization and authentication. (I'm using windows authentication, Message-based security, EncryptAndSign.) Subsequent calls are quicker.
I believe this 10 seconds is unavoidable, but there is usually time prior to the first call by the client to the server during which this initialization could happen. So rather than wait for the auto-open feature of DuplexClientBase, I open the channel early with a call to proxy.InnerDuplexChannel.Open(). (proxy.Open() throws an exception and this indirection seems to avoid it.)
Unfortunately, authenticating the client-to-server channel does not also authenticate the server-to-client callback channel. Instead, the first call by the server to the client ALSO requires ~10 seconds. Since I'm using netTcp binding I'm surprised by this, but I'll assume it is to be expected for now.
How can I open the callback channel preemptively as well?
I could require the client to call some Login() method instead, but I don't believe that WCF should strictly require an operation before user-code can be aware of a connected client!
Hint(?): I imagine that this code would have to go in a place in the WCF pipeline/lifecycle where the server has the opportunity to perform custom actions upon a client-connected event (and BEFORE any operation message is transmitted). This integration point has so far eluded me.
Assume a WCF service with ServiceBehavior.ConcurrencyMode = Single.
When exactly does the service start blocking for concurrent calls?
For example, say we have two clients: Slow and Fast.
At time 0 Slow starts a slow service call that includes a huge chunk of data.
At time 1 Fast makes a fast service call.
At time 2 the slow data finally arrives and the service code is executed on the server.
Assuming buffers configured in WCF to be larger than the huge chunk, which call will get executed first?
In other words, does blocking start when all call data has been received at the server side or when the client initiates the call?
Is the service blocked during the data transfer or only during code execution?
Unless you configure InstanceContextMode to Single as well both calls will be executed concurrently. So suppose that you have InstanceContextMode set to Single.
I didn't test it but I would expect such behavior. Concurrency mode is service behavior so it takes place once the service instance / instance context is resolved. In buffered mode that happens after whole message is received in streaming mode it should happen after message headers are received. So in case of buffered transport I would expect that fast client will be processed first and in case of streamed transport it depends if message headers from slow client was already received.
But as I wrote before this is only my expectation.
Using techniques as hinted at in:
http://msdn.microsoft.com/en-us/library/system.servicemodel.servicecontractattribute.callbackcontract.aspx
I am implementing a ServerPush setup for my API to get realtime notifications from a server of events (no polling). Basically, the Server has a RegisterMe() and UnregisterMe() method and the client has a callback method called Announcement(string message) that, through the CallbackContract mechanisms in WCF, the server can call. This seems to work well.
Unfortunately, in this setup, if the Server were to crash or is otherwise unavailable, the Client won't know since it is only listening for messages. Silence on the line could mean no Announcements or it could mean that the server is not available.
Since my goal is to reduce polling rather than immediacy, I don't mind adding a void Ping() method on the Server alongside RegisterMe() and UnregisterMe() that merely exists to test connectivity of to the server. Periodically testing this method would, I believe, ensure that we're still connected (and also that no Announcements have been dropped by the transport, since this is TCP)
But is the Ping() method necessary or is this connectivity test otherwise available as part of WCF by default - like serverProxy.IsStillConnected() or something. As I understand it, the channel's State would only return Faulted or Closed AFTER a failed Ping(), but not instead of it.
2) From a broader perspective, is this callback approach solid? This is not for http or ajax - the number of connected clients will be few (tens of clients, max). Are there serious problems with this approach? As this seems to be a mild risk, how can I limit a slow/malicious client from blocking the server by not processing it's callback queue fast enough? Is there a kind of timeout specific to the callback that I can set without affecting other operations?
Your approach sounds reasonable, here are some links that may or may not help (they are not quite exactly related):
Detecting Client Death in WCF Duplex Contracts
http://tomasz.janczuk.org/2009/08/performance-of-http-polling-duplex.html
Having some health check built into your application protocol makes sense.
If you are worried about malicious clients, then add authorization.
The second link I shared above has a sample pub/sub server, you might be able to use this code. A couple things to watch out for -- consider pushing notifications via async calls or on a separate thread. And set the sendTimeout on the tcp binding.
HTH
I wrote a WCF application and encountered a similar problem. My server checked clients had not 'plug pulled' by periodically sending a ping to them. The actual send method (it was asynchronous being a server) had a timeout of 30 seconds. The client simply checked it received the data every 30 seconds, while the server would catch an exception if the timeout was reached.
Authorisation was required to connect to the server (by using the built-in feature of WCF that force the connecting person to call a particular method first) so from a malicious client perspective you could easily add code to check and ban their account if they do something suspicious, while disconnecting users who do not authenticate.
As the server I wrote was asynchronous, there wasn't any way to really block it. I guess that addresses your last point, as the asynchronous send method fires off the ping (and any other sending of data) and returns immediately. In the SendEnd method it would catch the timeout exception (sometimes multiple for the client) and disconnect them, without any blocking or freezing of the server.
Hope that helps.
You could use a publisher / subscriber service similar to the one suggested by Juval:
http://msdn.microsoft.com/en-us/magazine/cc163537.aspx
This would allow you to persist the subscribers if losing the server is a typical scenario. The publish method in this example also calls each subscribers on a separate thread, so a few dead subscribers will not block others...
Few methods in my WCF service are quite time taking - Generating Reports and Sending E-mails.
According to current requirement, it is required so that Client application just submits the request and then do not wait for the whole process to complete. It will allow user to continue doing other operations in client applications instead of waiting for the whole process to finish.
I am in a doubt over which way to go:
AsyncPattern = true OR
IsOneWay=true
Please guide.
It can be both.
Generally I see no reason for WCF operation to not be asynchronous, other than developer being lazy.
You should not compare them, because they are not comparable.
In short, AsyncPattern=True performs asynchronous invocation, regardless of whether you're returning a value or not.
OneWay works only with void methods, and puts a lock on your thread waiting for the receiver to ack it received the message.
I know this is an old post, but IMO in your scenario you should be using IsOneWay on the basis that you don't care what the server result is. Depending on whether you need to eventually notify the client (e.g. of completion or failure of the server job) then you might also need to look at changing the interface to use SessionMode=required and then using a Duplex binding.
Even if you did want to use asynchronous 2-way communication because your client DID care about the result, there are different concepts:
AsyncPattern=true on the Server - you would do this in order to free up server resources, e.g. if the underlying resource (?SSRS for reporting, Mail API etc) supported asynchronous operations. But this would benefit the server, not the client.
On the client, you can always generate your service reference proxy with "Generate Asynchronous Operations" ticked - in which case your client won't block and the callback will be used when the operation is complete.