IObjectInstanceCache.ReadThrough

I thought I should share a small tip about a feature that we use internally but that perhaps not all are aware of that it exists. It is probably nothing you will use everyday but it can be really handy if you have "expensive" outbound calls to e.g. the database or a Web API. That is the extension method ReadThrough on interface IObjectInstanceCache. 

The signature look like (there are some overloads for example to specify CacheEvictionPolicy)

public static T ReadThrough<T>(
	this IObjectInstanceCache cache,
	string key,
	Func<T> readValue,
	ReadStrategy readStrategy
)
where T : class

The generic type parameter T is the expected type of the item to read/cache. The parameter key is the cachekey for the item. readValue parameter is a delegate that will be called to load the item (and putting it in cache) when it does not exist in cache. Now the interesting parameter here is the readStrategy. That is an enum that can have two values Immediate which will call the readValue delegate immediately to load the item in case it is not present in cache. The other readStrategy option is Wait. 

A strategy Wait means that when the first request comes in and find that the item queried for is not present in cache then it will put a marker in cache and then call the readValue delegate to load the item. Now if a second request comes in and asks for the same item before the first readValue delegate has finished then the cache will notice that there is a marker for that item (meaning it is being loaded) and then the readValue delegate will not be called for the second request. Instead will it be put to wait for the first request to finish and then it gets the result from the first request.

So by using Wait strategy you can avoid that several threads simultaneously makes the same expensive call. Instead one call is made and the result is then shared. So if you have some code where you expect that several parallell request might hit at the same time then this could be useful. A typical such location could be resources loaded from startpage, menus etc that could be hit simultaneously by several threads when site is restarted (and hence the cache is empty).