When does ASP.NET’s Session_End event get raised by the StateServer service?

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As the title implies, under what conditions does the ASP .NET Session_End() event fire?

 

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The event will fire for one of two reasons:

• A session times out (in accordance with the timeout attribute in the web.config's <sessionState> element).
• The SOSS service is running low on memory and needs to evict the least recently used sessions. (This will only occur if the lru_threshold setting in the soss_params.txt file is set to a value less than 100.)