DFS for WebFarm Usage - Content Replication and Failover
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by Web Team at ORCS Web
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File Change Notification in ASP.NET

There is one thing to keep in mind during a failover and failback situation.  ASP.NET and IIS uses what is called File Change Notification (FCN) to let IIS know of any changes to the files.  For example, if you add a new .dll to your /bin folder, ASP.NET will recycle the AppDomain and reload and recompile some of the site.  During a failure, although the switchover is smooth, it does take a few seconds which is abrupt enough for IIS and ASP.NET to reestablish the File Change Notification handle using the different content server.

The issue comes with the failback.  The failback is so smooth that the File Change Notification is not updated back to the restored server.  This means that if you make any changes to ASP.NET files on the restored content server, the changes are not noticed by IIS and ASP.NET.  Even deleting the entire /bin folder will not be recognized by ASP.NET if the site was visited and cached while running on the backup server.  Static pages do not have this issue, but the caching in ASP.NET makes this a problem.  At the time of this writing, I am working with Microsoft Product Support Services (PSS) to try to find a good solution for this.  To resolve it, simply recycle the app pool of the site(s) and it will start to function normally again.  So, this is not necessarily a show-stopper, but it is something to keep in mind with the failover/failback.


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