I have a WCF service with duplex binding hosted within a windows service. The service has to continuously poll for changes in a datasource (Azure service bus to be specific, but type of datasource isn't really important). If there is any new change in the datasource, the service should immediately notify one or more clients.
I have gone through many different potential approaches of achieving this such as Broadcasting, Publish/Subscribe with WCF, and List based publish subscribe
But in all these approaches, the chain of events is initiated explicitly by a client/publisher. For example in case of Pub-Sub, the whole chain of events is initiated when a publisher publishes something.
But in my scenario, there is no publisher. Service must itself poll onto a datasource and look for any new changes.
How do I achieve this in my scenario? Where should I have the polling logic?
One option is to have the polling logic in hosting environment (windows service) and call the publish of service whenever there is new data.
I am not sure how we can have the polling logic within the WCF service itself! Any leads/thoughts regarding this would be very helpful.
Related
I have an application that places messages on MSMQ, than I have a WCF service that I want to automatically read these messages from MSMQ.
I do not want to trigger my WCF service but I want it to be notified whenever a message is pushed on MSMQ. I have found some stuff related to WAS and netMSMQBinding, but the details seems a little unclear about automatic notifications from MSMQ to WCF.
Any help/direction in this regard? Thanks.
What is it that you're trying to achieve?
There is really no such thing as "be notified when a message is pushed on MSMQ". MSMQ uses a pull model to retrieve messages from a queue, so there always needs to exist some kind of "listener" that reads messages from the queue and processes them.
In this case, this "listener" is provided by the WCF NetMsmq or MsmqIntegration bindings automatically, so the programming model for a WCF service that is exposed over MSMQ is just like that of a regular one-way service.
As you state, you can either host that WCF service using your own custom host (such as a Windows service), or, in many cases, the preferred method will be to host that service in IIS 7.X/8.X through WAS.
The following sample shows how to host an MSMQ service in WAS: http://msdn.microsoft.com/en-us/library/ms752246(v=vs.110).aspx
Notice that you do need some specific configuration in IIS to be able to host non-HTTP services, which includes enabling the "Non-HTTP Service Activation" feature. The following articles will be useful to get this done:
http://blogs.msdn.com/b/tomholl/archive/2008/07/12/msmq-wcf-and-iis-getting-them-to-play-nice-part-1.aspx
http://blogs.msdn.com/b/tomholl/archive/2008/07/13/msmq-wcf-and-iis-getting-them-to-play-nice-part-2.aspx
http://blogs.msdn.com/b/tomholl/archive/2008/07/14/msmq-wcf-and-iis-getting-them-to-play-nice-part-3.aspx
http://blogs.msdn.com/b/tomholl/archive/2008/05/17/building-a-pub-sub-message-bus-with-wcf-and-msmq.aspx
I have a good understanding SignalR Hubs in a client/server scenario, where both the client and server are tightly coupled.
Let's say I have a WCF service that receives an update from some external resource. That service could update the database with a new value. However the client would need to be notified that an update has occurred. This could be handled through a service proxy that notifies the client (sounds a bit like polling) or some cache resource.
I could create C#-based clients and connect all the nodes via SignalR hubs, but this creates a closed, non-distributed system.
A SignaR hub that attaches to a WCF service could use the .Net 4.5 could implement a WCF asynchronous service operation, where a hub client would be notified with any service data changes.
I saw something similar in Push Notifications with NServiceBus and SignaR, but not sure if this is an optimal production-level solution.
What other methods could be used in this scenario and how would they be implemented?
If you are not using push notifications directly to the client or some kind of long polling then it is pretty typical to communicate with clients on another channel altogether. Not knowing the business case, it is hard to tell what would be feasible. Usually this manifests itself in the form of SMS, push notifications to mobile, email, etc. This does not answer your question directly, but you may find that there is another way to achieve your goal.
So I'm looking into implementing NServiceBus in our current setup and just trying to get a better understanding of how things should be setup.
Our current setup consists of multiple clients (websites, scheduled tasks, etc..) calling a WCF service we have set up for handling the sending of emails. Of course, if the service goes down then our clients start getting errors and all of those messages are then lost (one of the reasons we want an ESB).
I've seen how you can configure your WCF service to handle nservicebus messages in a pub/sub setup. What I'm not sure on is what is the best way to set it up.
Setup 1:
Client (Publisher) -> NServiceBus handler (Subscriber) -> WCF Service
In this case, to scale you'd increase the number of handlers (hosted nservicebus services?), keeping just the one WCF service.
Setup 2:
Client (Publisher) -> WCF Service (Subscriber)
This one you just increase the number of WCF services to scale (updates would be a nightmare).
I just started looking into the ESB architecture in general so if I'm completely off let me know. I'm essentially just wanting to know what is working for you, and what the "best practice" tends to be.
Thanks!
I'm not completely clear on what you need WCF for anymore if you implement this via NServiceBus. Is the WCF component required for anything besides receiving messages (to send an email) from the multiple clients? If not, you could remove WCF from the equation.
From the sound of it, you will also want the Service to act as a single logical endpoint that handle requests to send emails. If that's the case, you will want to use Send (a command) instead of Publish (an event). Publish is used to broadcast an event, which means that something happened already; Send is used to instruct another component to do something. It sounds like you want the latter.
Scaling of an endpoint can be done via the Distributor. This may or may not be useful depending on where you expect the bottleneck to be.
Edit: Based on your comment, I would simply go with the second setup, and just add the handler to the WCF service. If you are hosting WCF in IIS, make sure you have something that wakes the process up if the app pool recycles (the incoming message won't wake it up the same way an incoming request to WCF will).
We do something similar internally where one NSB endpoint handles all the sending of email. The clients can either use NSB directly to Bus.Send() the command to send a message to the email endpoint or you can expose that endpoint via WCF as well (only to get the commands over to the endpoint). Once the endpoint has the commands, they would just call your existing service to maintain compatibility with your existing clients.
I'm new to WCF and need some assistance with architecture for the following solution:
I want to create a WCF Service that hosts a FileSystemWatcher. This service must watch a series of folders on the machine and when a file is placed in a folder or is renamed I need this WCF service to 'Publish' an event.
Another WCF service must then 'Subscribe' to this event and do some processing.
The basic idea is that there is one service to monitor a predetermined set of directories, whilst an appropriate service gets notified that there are files available for processing.
So my questions are :-
1. Regarding the FileSystemWatcher WCF Service, I want to host it in AppFabric but am not sure exactly how this is going to work. I.E. When this service is deployed to the server I want the appropriate FileSystemWatcher object to be called and stay 'on' and monitor folders indefinately. Can WCF do this in this manner or is this a better candidate for a Windows Service
2. Can one WCF service publish events and another WCF service subscribe to these events and how? All WCF services hosted inside AppFabric?
WCF is a communication framework that can be used to expose or consume SOAP/REST services. A Windows Service is one way of hosting such applications. WAS/IIS and self-hosting are other methods. A Windows Service provides greater control of service startup/shutdown, which seems relevant to your scenario.Windows Server AppFabric improves some of the control over applications lifecycle hosted in WAS/IIS, but it is still a fundamentally different hosting choice to a Windows Service.
As far as I know WCF has no inbuilt event publishing capability (I assume you're referring to the publish/subscribe pattern). However, you can implement such a pattern using WCF duplex channels. There is no restriction on an application concurrently acting as 'server' and 'client'.
I am looking at various options for a WCF based publish subscribe framework. Say I have one WCF web service that will be the publisher and 1000 clients registered as subscriber. For some published messages all clients will be interested but at the same time I wish the ability to notify a single client with a specific message. On receiving notification the client will call other web service methods on the web service.
Is NServiceBus suitable for this kind of scenario ?
If I use MSMQ for transport does it mean that every PC where the client is installed requires a queue to be created ?
Some of the challenges include how you want the publisher to behave when a given subscribing client is down - do you want that message to be available when the subscriber comes back up? If so, then some kind of durable messaging is needed between them - like MSMQ.
Your question about notifying a single client, is that as a result of a request sent by that client? If so, then standard NServiceBus calls in the form of Bus.Reply will do it for you. When using WCF, if the response is to be asynchronous you'll need to use callback contracts.
NServiceBus can do all the things you described, and has the ability to automatically install MSMQ and create queues so that greatly simplifies client-side deployments.
You also have the ability with NServiceBus to expose messages over WCF so you can support non-NServiceBus clients if you need to as well. It also has its own http gateway and XSD schemas which can allow clients on non-Windows platforms to interoperate even without using WCF.
Hope that answers your questions.