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    <p>As for your requirements:</p> <blockquote> <p>It must be possible to run multiple workflows, and multiple instances of the same workflows with a different context (different data/business objects).</p> </blockquote> <p>This is no problem with WF. </p> <blockquote> <p>Some workflows will be long-running, involve multiple users/client session and waiting for external user input. So the workflows must be able to be persisted and respond to some signal from a client app. And it also means that the execution of workflows must be done on a server app (right?).</p> </blockquote> <p>WF is designed for long running tasks that can interact with outside influences. However, that doesn't say its easy to accomplish; there is no universal solution which you can hook into. You will probably have to design custom Activities that interact with <a href="http://xhinker.com/post/WF4Using-WF4-Customized-Extension.aspx" rel="nofollow noreferrer">Workflow Extensions</a> which handle moving user input into the workflow. Same with exposing the workflow to the outside, although WF4 does come with a host of WCF activities which could be used to accomplish this.</p> <blockquote> <p>I want to be able to run all kinds of workflows on the server app, and I do not want to have to re-deploy the server app when a workflow changes.</p> </blockquote> <p>This is harder to accomplish. You must, at a minimum, separate the workflows from the server code. The simplest route is to store your workflow as xaml and <a href="https://stackoverflow.com/questions/1654747/loading-a-workflow-stored-as-xaml-in-net-4-0">load it at runtime</a> from, say, a database.</p> <p>Other options are to use some kind of dependency injection framework (such as Structure Map or Unity) which loads the workflow assembly at runtime. If the workflows change, you can drop the new assembly on the server, change your config and restart. Alternatively, you can isolate your workflow assemblies within their own AppDomain, load them at runtime and throw away the domain when you must reload a new version. Which one you do depends on your requirements; I'm actually doing the third option as I have to load many different versions of workflow assemblies at runtime, run them concurrently, and they often have embedded resources thus preventing me from going the XAML route.</p> <blockquote> <p>My first thought was Workflow Services. After a lot of research I concluded that this is not the right path since Workflow Services basically gives the possibility to execute activities at a remote location from a workflow started in a client app. Is this correct?</p> </blockquote> <p>I'm hosting my workflows within a standard Windows service application. I have to manage and maintain the WCF frontend which the client uses to interact with my workflows. As far as I can tell, Workflow Services seems like a viable option for stable workflows, if I understand it correctly. Hosting an application in AppFabric is also a good option, and I believe simpler than using a Windows Service. But no matter what the host is, you have two options--your workflows define your service contract, or you must define the service contract and handle all execution and communication with workflows you are managing.</p> <p>The first is a good option for stable workflows with a simple facade. It doesn't seem like a good option for you, as you must load workflows dynamically as they change; that requires logic outside of the workflow to handle not only communications from the client ("here's a new version of workflow X!") but also managing the workflow lifespan.</p> <p>It seems like you will have to find some kind of host application (IIS WebService application, Windows Service, AppFabric, Azure), define your WCF service and bring it online, handle calls from clients, communicate these calls to your workflow running code, which then must load and execute these calls and return the result up the chain.</p> <p>I can't help but notice that you seem slightly ill-prepared for the journey that awaits you. I would strongly suggest creating a prototype that slices neatly through the centermost meat of your requirements. A hosted application (I'd suggest AppFabric) with a WCF frontend that loads a XAML-based workflow that processes client calls. Once you have the simple version nailed down, you can widen the scope to encompass all your requirements.</p>
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    1. COThank you very much for this answer. It confirms my suspicions :-)
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    2. COMy apologies, I should have tagged your answer as Accepted a long long time ago.
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