Best practice for calling web service from Sharepoint List Item Event Receiver

Question

What is the recommended way to introduce the invocation of an ASP.NET web service from a SharePoint list item event receiver (specifically, a MOSS 2007 document library)? I don't want to tightly couple the web service URL to the event receiver; but can I still add a service reference (or web reference) to the event receiver project, make the URL Behavior dynamic, so I can update the reference's URL in the event receiver code using SharePoint's own web.config or a feature property of the event receiver?

Answer 1

As you said, you could store the url of the service endpoint somewhere and then use that info from your event receiver. Two main options come to mind.

1. Web.Config for the associated web application. By using the SPWebConfigModification class, either from a web application scoped feature or a PowerShell script you can apply modifications to the web.config file in a standard and automated way. SPWebConfigModification based changes are automatically propagated to all the servers in a farm, so they are the preferred way to handle changes in production environments. Just remember that SharePoint does not really like programmatically changes to be mixed with manual ones - so if you go for this road stick with it. Also, some customers do not really like to have changes applied to the web config from code developed by third parties/external companies.
2. Property Bags - many object in SharePoint have an associated property bag - think of it as a key-value store. The SPFolder is one of those objects, so an option is to save the url in the property bag associated to the root folder of the list you are using. Another option would be to use the web or site collection levels bags. This option is often preferred if the client don't want to risk having the web.config modified. This comes at a cost: there is no "UI" in SharePoint to change the value of a property bag key, or see which keys are defined, so you would need to build the interface to manage the settings on your own or do that directly via code.

As a related side note, I would also point out that unless you have some valid reason to do so, I wouldn't have the item receiver directly depending on a web service call, as it may require some time or even cause a timeout. If it is possible, I would try to move the core of the operation to some "batch" process. One idea could be using a SPLongOperation, but this would probably be worth another question by itself. Just ask yourself if you really need the item receiver to wait for the service call to complete