title: Strategies for Cache Access During Refresh?
body: <p>I’m looking for some strategies regarding accessing some cached data that resides in a internal company web service.  Actually, preventing access of the cached data while the cache is being refreshed. </p>
  
  <p>We have a .Net 3.5 C# web service running on a web farm that maintains a cache of a half-dozen or so datasets.  This data is configuration associated items that are referenced by the ‘real’ business logic domain that is also running in this web service as well as being returned for any client uses.  Probably talking a total of dozen or so tables with a few thousand records in them.</p>
  
  <p>We implemented a caching mechanism using the MS Enterprise Library 4.1.  No huge reason for using this over the ASP.Net cache except that we were already using Enterprise Library for some other things and we liked the cache expiration handling.  This is the first time that we have implemented some caching here so maybe I’m missing something fundamental…</p>
  
  <p>This configuration data doesn’t get changed too often – probably a couple of times a day.  When this configuration data does change we update the cache on the particular server the update request went to with the new data (the update process goes through the web service).  For those other servers in the web farm (currently a total of 3 servers), we have the cache expiration set to 15 minutes upon which the data is re-loaded from the single database that all servers in the farm hit.  For our particular purposes, this delay between servers is acceptable (although I guess not ideal).</p>
  
  <p>During this refresh process, other requests could come in that require access to the data.  Since the request could come during an expiration/refresh process, there is no data currently in the cache, which obviously causes issues.</p>
  
  <p>What are some strategies to resolve this?  If this was going in a single domain sort of WinForm type of application we could hack something up that would prevent access during the refresh by the use of class variables/loops, threading/mutex, or some other singleton-like structure.  But I’m leery on implementing something like that running on a web farm.  Should I be?  Is a distributed server caching mechanism the way to go instead of each server having its own cache?  I would like to avoid doing that for now if I could and come up with some coding to get around this problem.  Am I missing something?</p>
  
  <p>Thanks for any input.</p>
  
  <p>UPDATE: I was going to use the Lock keyword functionality around the expiration action that subsequently refreshes the data, but I was worried about doing this on a web server.  I think that would have worked although it seems to me that there still would be a possibility (although a lesser one) that we could have grabbed data from the empty cache between the time it expired and the time the lock was entered (the expiration action occurs on another thread I think).  So what we did was if there was no data in the cache during a regular request for data we assume that it is in the process of being refreshed and just grab the data from the source instead.  I think this will work since we can assume that the cache should be filled at all times since the initial cache filling process will occur when the singleton class that holds the cache is created when a web service request is first made.  So if the cache is empty it truly means that it is currently being filled, which normally only takes a few seconds so any requests for data from the cache during that time will be the only ones that aren't hitting the cache.</p>
  
  <p>If anyone with experience would like to shed any more light on this, it would be appreciated.</p>