Modifying an application's web.config file in NLB cluster

Question

Hello,

I would like to know what would be the impact of modifying web.config file while an ASP.NET application is running.

Here's my exact scenario. I have two hosts running the same application on IIS6. Both hosts have the exact same machine.config files, meaning they share same machine keys.

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[HOST A]----[NLB]---[HOST B]
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[SQL server for ASP.NET sessions]

The application must be accessible to users every day from 7:00 to 18:00. At 19h00 Host A and B are rebooted and we can make some updates of the application on both hosts. But sometimes we would like to be able to do the updates during the uptime of the application. The updates often consist of adding some values to web.config which as far as I know causes an application restart.

The application relies heavily on sessions but they are persisted in SQL server and only cache data is stored in the server memory and it is not critical if this data is lost. But losing user sessions is absolutely unacceptable. My question is what will users experience if we modify web.config file on one of the hosts in the web farm if the server is currently executing a user request? And what will happen if a new request comes in while the application is restarting?

Answer 1

Well, you've given the answer yourself already. Normally, sessions are disrupted while restarting, but you're using SQL Server to persist them, so they won't be lost.

New visitors however, will receive an error page while restarting. But they won't trigger a new session start, so the visitor can just keep F5'ing until the application is running.