asp.net - Is MemoryCache scope session or application wide?

Question

I'm using MemoryCache in ASP.NET and it is working well. I have an object that is cached for an hour to prevent fresh pulls of data from the repository.

I can see the caching working in debug, but also once deployed to the server, after the 1st call is made and the object is cached subsequent calls are about 1/5 of the time.

However I'm noticing that each new client call (still inside that 1 hour window - in fact just a minute or 2 later) seems to have the 1st call to my service (that is doing the caching) taking almost as long as the original call before the data was cached.

This made me start to wonder - is MemoryCache session specific, and each new client making the call is storing it's own cache, or is something else going on to cause the 1st call to take so long even after I know the data has been cached?

Answer 1

From MSDN:

The main differences between the Cache and MemoryCache classes are that the MemoryCache class has been changed to make it usable by .NET Framework applications that are not ASP.NET applications. For example, the MemoryCache class has no dependencies on the System.Web assembly. Another difference is that you can create multiple instances of the MemoryCache class for use in the same application and in the same AppDomain instance.

Reading that and doing some investigation in reflected code it is obvious that MemoryCache is just a simple class. You can use MemoryCache.Default property to (re)use same instance or you can construct as many instances as you want (though recommended is as few as possible).

So basically the answer lies in your code.
If you use MemoryCache.Default then your cache lives as long as your application pool lives. (Just to remind you that default application pool idle time-out is 20 minutes which is less than 1 hour.)

If you create it using new MemoryCache(string, NameValueCollection) then the above mentioned considerations apply plus the context you create your instance in, that is if you create your instance inside controller (which I hope is not the case) then your cache lives for one request

It's a pity I can't find any references, but ... MemoryCache does not guarantee to hold data according to a cache policy you specify. In particular if machine you're running your app on gets stressed on memory your cache might be discarded.

If you still have no luck figuring out what's the reason for early cache item invalidation you could take advantage of RemoveCallback and investigate what is the reason of item invalidation.