Rory Primrose

Learn from my mistakes, you don't have time to make them yourself

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Calling a workflow service operation multiple times

Ron Jacobs has just answered an interesting question over on his blog. The question is about whether a workflow service operation can be invoked multiple times. Ron does not provide the details of the question but the example he provides implies that the implementation of the two invocations of the same service operation may be different as the same operation name is implemented twice in the workflow. This seems like a design issue as far as the service goes but the question itself is still interesting.

If we assume that the implementation of the service operation is the same (as it should be), how do we keep the service alive so that we can invoke the same method multiple times?

The answer is by using some kind of service session using WF correlation. Content correlation is my preferred option because it is independent of infrastructure concerns and does not restrict the WCF bindings available to you. I have previously posted about how to get a workflow service to create a session using content correlation.

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Custom DisposalScope activity

The previous post outlined the issues with working with unmanaged and IDisposable resources in WF. To recap, the issues with these resources in WF are:

  • Persistence
  • Restoration after persistence
  • Error handling
    • Messy WF experience to handle this correctly

A custom activity can handle these issues in a much more elegant way than the partially successful implementation in the previous post.

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Working with unmanaged and IDisposable resources in WF

Generally speaking you will want to steer clear of unmanaged or IDisposable resources when authoring WF workflows. The primary reasons for this are:

  • Persistence
  • Restoration after persistence
  • Error handling

A workflow may be persisted at any time (with the exception of a No Persist Zone). There are two problems for unmanaged or IDisposable resources with regard to persistence. Firstly, the resource may not be released when the workflow is persisted. Secondly the state of the resource when the workflow is restored is unlikely to be the same as when the workflow was persisted. There is no native support in WF for dealing with these two issues.

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Custom IfThen activity

The two most common WF activities to use when implementing decision branching are the If activity and the Flowchart activity. Using a Flowchart is overkill for a simple/singular decision branch so the If is often chosen. Unfortunately the If activity that comes with WF4 forces an Else branch to be visible on the designer even if you are not using it. This can make workflows unnecessarily cluttered.image

The solution is to author a custom activity that only executes an If-Then branch rather than an If-Then-Else branch and provide appropriate designer support for this. A simplistic example of such an activity was provided in the January MSDNMag by Leon Welicki in his Authoring Control Flow Activities in WF 4 article.

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WF Retry activity

One of the tools missing out of the WF toolbox is the ability to run some retry logic. Applications often have known scenarios where something can go wrong such that a retry of the last action could get a successful outcome. One such example is a connection timeout to a database. You may want to try a couple of times before throwing the exception in order to get more success over time.

The specific scenario I am addressing is a little different. I have created some custom MSF providers that will allow me to run a MSF sync session behind a WCF service. The main issue with this design is that it is possible that multiple clients may want to sync the same data set at the same time. MSF stores the metadata (Replica) that describes the items to sync in a SQLCE file. It will throw an exception if the replica is already open and in use. This doesn’t work well in a services environment. The way I will attempt to manage this is to limit the amount of time the replica is in use and then implement some retry logic around opening the replica in order to run a sync.

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Extract WCF identity into a WorkflowServiceHost activity

I came across an issue with the combination of WCF, WF, WIF and AppFabric in December that had me a little worried. The issue was how to get the identity of the user calling a WCF service inside a WorkflowServiceHost workflow when using WIF to manage security and AppFabric for WF persistence.

The WIF documentation says the following:

As a note, when WIF is enabled inside WCF, the WCF ServiceSecurityContext does not work for obtaining the caller’s identity and claims; the application code must use the IClaimsPrincipal to access the caller’s information. You can obtain the IClaimsPrincipal instance using Thread.CurrentPrincipal. This is available for every authentication type as soon as WIF is enabled for the WCF application.

I had been developing my service application using Thread.CurrentPrincipal based on this information. Everything was fine until I started using WF persistence. Unfortunately enabling persistence (via AppFabric) had the affect of wiping out Thread.CurrentPrincipal.

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Restricting the types available in TypePresenter in WF designers

The TypePresenter control is the UI that the WF designer displays for selecting a type. It is often seen on generics based activities and the InvokeMethod activity.image

This drop down list provides some common types and includes some of the types already found in the current activity context. Selecting “Browse for Types …” allows for all referenced types to be searched in a dialog.image

Sometimes you don’t want the TypePresenter to provide every available type. The TypePresenter has a great feature that allows you to restrict the types it displays in this list and the associated “Browse for Types …” dialog. This is done by providing a Func<Type, Boolean> reference on the TypePresenter’s Filter property.

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WorkflowApplication throws ObjectDisposedException on ActivityContext with LINQ queries

I have written a simple WF4 activity that validates a WIF IClaimsIdentity has certain claims attached to it for a specific claim value. I wrote several unit tests for this activity to validate the implementation. The basic logic is to select a subset of claims that have a specified claim value using Where(). I then fall into a flow chart that validates whether there are any claims (Any() == false) and then whether there is only one claim (First() with a Count()) of a specified claim type that will not be accepted.

Almost all of the tests failed with the following error:

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CodeCampOz - Not a WIF of federation

My presentation of Not a WIF of federation is done and dusted for CodeCampOz 2010. I got a little cut short by the early arrival of pizza’s so there are a couple of things I had to skip.

One of the important points I was not able to make is that the demo code displayed is not production ready. It’s not that the code itself is flawed rather that the certificates used were development certificates. It is important to secure your WIF implementation with production quality certificates in order to ensure that system security is maintained.

Identity delegation was the last demo that I presented. I didn’t get the opportunity to show the code that was changed in the STS application to make this work. The code in the STS project had not be changed up until this point. The GetOutputClaimsIdentity method of the CustomSecurityTokenService class simply returned a new ClaimsIdentity created from the provided principal. The delegated identity scenario needs to explicitly cater for the ActAs scenario by attaching the delegating identity to the Actor property on the delegated identity being returned.

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Managing content correlation failures

I posted yesterday about how to get content correlation of multiple WCF service operations to work with hosted WF services. The request and reply data of the WCF service operations is the basis of content correlation. It is therefore quite easy for a client application to send data to the service that does not satisfy business validation.

If the invalid data sent to the service operation participates in content correlation then the WorkflowServiceHost will not identify a WF instance. This unfortunately means that business validation of this data will not run in the workflow as no workflow instance is executing. The result is that the client will receive the following very unfriendly FaultException (assuming no exception shielding is in place).

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