You only have to update an application on the server

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you only have to update an application on the server, and all your clients will be using the new version the next time they request a new page. Out-of-browser Internet applications could well blur this distinction. Both Silverlight and Flash make it possible for Internet applications to have parts that are installed on the user’s machine and run like normal applications outside the web browser. So the considerations in this sec- tion could apply if that’s the sort of web application you’re building. To update a classic Windows application, you need to somehow get a new version of the program onto the end users’ machines. Since it’s rarely practical to install a new version on every single user’s machine simultaneously, you need to handle the possi- bility of having several different versions of the client software all trying to talk to your server. The extent to which this can cause problems will depend on how much control you have over the client computers. Tightly controlled deployment Some applications are deployed in tightly controlled environments. For example, sup- pose you’re writing a line-of-business application in WPF that will only ever be de- ployed to machines owned by your business. If your IT department has an iron grip on the company’s computers, you might be able to exert considerable control over what versions of your application are out there. Network administrators could forcibly up- grade users to the latest version. So new versions might overlap with old versions for only a day or so. You could even go further and arrange for your application to check for updates and refuse to continue running when a newer version is available. This is a happy situation for a developer, because it makes it much easier to introduce changes to your server. Chances are that at some point you’ll want to add new services to support new features in your application. You might also want to modify existing services, which is usually more problematic than completely new features—if you’re using WCF, it’s not easy to modify the way an existing service works without breaking that service for older clients. It’s possible, but it’s hard, and it’s often easier to run multiple versions of the service simultaneously during the transition period. The nice thing about having sufficient control to remove old versions of the application is that you can know when you’ve reached the end of a transition period and can shut down the older version of the service. This won’t be the case if you can’t force that sort of change on the client. 478 | Chapter 13: Networking
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Weakly controlled deployment If your application’s customers don’t all work for your company, life becomes more complex, because it’s harder to force upgrades on your customers. It’s not impossible— for example, Microsoft’s Windows Live Messenger program occasionally tells you that if you don’t upgrade you won’t be able to carry on using the service. Mind you, it’s a
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