There is an ASP.NET application using VirtualPathProvider to return web pages from the database. The VPP uses SQL Server Query Notification (SQLCacheDependency) to get notified if pages change in the database. Hence for every web page request, an entry goes into the database registering w3wp.exe, which handles the page request, as a subsriber to page change notifications.
Lets assume that the application has handled 1 million requests in a day. So there are 1 million subscriptions in the DB registered against the process id of w3wp.exe (pid:1018). Now lets take a look at the list of scenarios which must be considered by the development team.
Case 1: Application pool recycled and then Page is changed in the DB
The subscriber process w3 with pid 1018 disappears. SQL Server will attempt to send a change notification but will fail and log the error message. User is very likely to see an older version of the page. Disk space utilization will increase.
Case 2: SQL Server recycled.
SQL Server will attempt to send out 1 million change notifications to the w3wp.exe process with pid 1018. Memory utilization of SQL Server will shoot up. Users may experience slow applications response.
Case 3: SQL Server and App pool recycled.
SQL Server will error out when trying to send out change notifications to the w3wp.exe process. Memory utilization of SQL Server & disk space utilization will shoot up. Users may experience slow applications response.
All 3 cases present an interesting engineering problem to solve! Will blog more about the possible solutions. Keep watching this space.
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