Making Backward-Compatible WCF Services

Question

TLDR: How do I create WCF services that are backward compatible -- that is, when I deploy a new version of the service on the server-side, all the clients on the older versions can still use the service.

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I'm creating a web service that will allow the client applications to fetch a listing of plugins. I will at least have one operation like FindPlugins(string nameOrDescription) which will, on the server, do a search and return a list of objects.

Unfortunately, I cannot guarantee that my clients will all be updated with each new release of my service; nay, I am sure that many of them will be trailing the latest version, and will have old versions -- how old, I cannot be sure, but I know they will be old :)

If I create a new service operation, change the schema, or make some sort of breaking operation on the server side, I'm done. I need to engineer backward compatibility at all times.

Here's one example. Say I return a list of Plugins, each which has a name and description, and I deploy v0.1 of my service. Then, I add a download link, and deploy that as v0.2 of my service.

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Some options which I see are:

• Force clients to update to the latest service (not feasible)
• Break the service for old clients (not feasible)
• Append a version number to each operation and only consume the version-specific operations (eg. FindPluginsV1, FindPluginsV2) -- doesn't seem practical with multiple operations
• Provide a new service with each new version -- doesn't seem practical

Answer 1

WCF is backwards-compatible by default.

The following MSDN link contains a list of all the possible changes of a WCF contract and describes their effect on old clients:

• WCF Essentials: Versioning Strategies

Most importantly, the following operations will not cause old clients to break:

Service contracts (methods)

• Adding method parameters: The default value will be used when called from old clients.
• Removing methods parameters: The values sent by old clients will be ignored silently.
• Adding new methods: Obviously, old clients won't call them, since they don't know them.

Data contracts (custom classes for passing data)

• Adding non-required properties.
• Removing non-required properties.

Thus, unless you mark the new DownloadLink field as IsRequired (default is false), your change should be fine.

Answer 2

If you look at this article http://blogs.msdn.com/b/craigmcmurtry/archive/2006/07/23/676104.aspx

The first example the guy gives will satisfy your requirements. It has the benefit that existing clients will not break, and you can add as many new service operations as you want this way.

[ServiceContract] 
public interface IMyServiceContract 
{    
    [OperationContract(IsOneWay=true)]    
    public void MyMethod(MyDataContract input); 
}

[ServiceContract] 
public interface IMyAugmentedServiceContract: IMyServiceContract 
{    
    [OperationContract(IsOneWay=true)]    
    public void MyNewMethod(MyOtherDataContract input); 
}  

The change your service implementation:

public class MyOriginalServiceType: IAugmentedServiceContract { }