Question

Although i have specified a unique key, it seems the following code will return one value for 5 requests, then another for the next couple, then revert back to the value saved in the original request and just continue until there are 10’s of different objects all stored under the same key.
It then seems almost random which of these values it will return from the cache.

string strDateTime = string.Empty;
string cachename = "datetimeexample";
object cachedobject = HttpRuntime.Cache.Get(cachename);
if (cachedobject != null)
    strDateTime = (string)cachedobject;
else
{
    strDateTime = DateTime.Now.ToString();
    HttpRuntime.Cache.Insert(cachename, strDateTime, null, DateTime.MaxValue, TimeSpan.FromDays(10), CacheItemPriority.NotRemovable, null);
}
Response.Write(strDateTime +"        keys:"+ HttpRuntime.Cache.Count);

Very confused, is this because of threading or something?

Answer 1

Ignoring the possibility of a server farm and load balancing, this behaviour can be caused by the application pool running as a web-garden. To quote the relevant section from MSDN:

Because Web gardens enable the use of
multiple processes, each process will
have its own copy of application
state, in-process session state,
caches, and static data. Web gardens
should not be used for all
applications, especially if they need
to maintain state. Be sure to
benchmark the performance of the
application before deciding whether
Web garden mode is appropriate.

This will cause it to appear as if caching is storing multiple values for the same key, effectively having duplicate entries in the cache.

To resolve this in IIS 7, open the application pool’s Advanced Settings and set Maximum Worker Processes to 1. For IIS 6, see the MSDN article (With pretty screenshots).

Albeit 8 months late, I’m answering this question because I found it long before I found this decent article on web-garden gotchas. Hopefully this answer will save future searchers a chunk of time. 🙂