When to Use Azure Caching Local Cache

Question

I want to start using the Azure Distributed Caching and came across the concept of LocalCache. But the fact that it can go out of sync with the Distributed Cache, makes me wonder, why I would want to use it and how I could use it safely.

When enabled, items retrieved from the cache cluster are locally stored in memory on the client machine. This improves performance of subsequent get requests, but it can result in inconsistency of data between the locally cached version and the actual item in the cache cluster.

Calling DataCache.GetIfNewer is one option to ensure that I get the latest version, but that requires that I still do a call to the Distributed Cache, passing in the object that I want to check, in order to see if the two versions differ.

I could use Notifications to invalidate the LocalCache object, but that is done on a polling basis, which opens up the opportunity for an update to occur within the poll period leaving me with stale data.

So,why would I ever use LocalCache, and if there is a reason to do so, how do I use it safely?

Answer 1

"There are only two hard things in Computer Science: cache invalidation and naming things" - Phil Karlton

You would use LocalCache when a) performance is critical b) you don't care that the retrieved object might be stale.

There are many cases where the object is never going to be out of date (e.g. list of public/bank holidays), or when you are not too worried about being 100% up-to-date (e.g. if item has > 1000 units in stock, use local cache, otherwise re-fetch from database).

Don't try and invalidate the local cache. If you need more up-to-date objects, get them from the cluster. If you cannot tolerate out-of-sync data, get it from the database. Caching is always a performance-inconsistency compromise — LocalCache more than the server cache, but the server cache is still a compromise.