Monday, August 12, 2013

Multiple Session Factories and the Second Level Cache

In a previous post, we discussed our approach to delaying the delete operation such that the user does not have to pay the price by waiting for the operation to finish. Instead, we set the IsDeleted flag to be true and queue up a deletion task. It has worked well for us; although, we have run into a few issues. Let's look at how multiple session factories interact with the second level cache.

Before we start, let's have a quick look at the NHibernate caching system. NHibernate uses the following caches:
  • Entity Cache
  • Query Cache
  • Collections Cache
  • Timestamp Cache
The issue we are running into is that, by default, NHibernate will clear out the proper cache based on what entities are being inserted and deleted. Let's look at this query.

// Queries DB, inserts into cache
session.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();
// Pulls result from cache
session.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();

When NHibernate receives the result from the database, it will store the entities in the entity cache, while storing the set of returned IDs in the query cache. When you perform the query again, It pulls the list of IDs from the query cache and then hydrates each entity from the entity cache.

Now suppose we delete a user in between performing both of these queries, or perhaps create a new one.

// Queries DB, insert into cache
session.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();
// Marks as stale in cache
// Queries DB again
session.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();
NHibernate would take notice and not pull the second query from the query cache but instead return to the database for the latest information. In this way, NHibernate seems to do a rather great job at taking care of the cache. For a bit more information, have a look at this post by Ayende.

So, if instead, we create the two sessions (in the example above) from different session factories but with identical configurations, then the second level cache will be shared and still be used. But if the delete is performed between, then the second query will still hit the cache.

// Queries DB, insert into cache
session1.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();
// Marks query and entities as stale in cache
// Does not notice that session1 marked it as stale, pulls from cache
session2.QueryOver<Users>().Where(u => u.FirstName = "John").Cachable();

It would seem that the sharing of the timestamp cache should take care of this. Perhaps, the timestamp cache is not shared between the factories.

The cache is not designed to be shared between session factories. Normally, chances of key collisions are low due to the use of the entity GUID in the key. But since we create multiple session factories to access the same database, or if you used an incrementing int as the key, key collisions are possible. Most of the time, you could use the region prefix as shown in the blog post or at the bug report.

Where does this leave us? Due to the fact that the DeleteVisibleSessionFactory is only used to access entities about to be deleted, we decided that caching these entities is pointless and disabled caching on it. This prevents it from retrieving any stale data. The last issue is that an entity deleted in the DeleteVisibleSession will not be removed from the second level entity cache. Now we are clearing the entity cache manually after any delete in the event listeners.

NHibernateHelper.EvictEntity(@event.Entity as ModelBase2);

Due to the granular nature of our query cache, we decided to manage them on a per case basis. They often contain the ID of a parent object and need to be cleared individually. This provides us with the best compromise of complexity and performance. Letting us know that the entity cache will be managed properly by NHibernate and that the query cache is our responsibility.

About NexPort Solutions Group
NexPort Solutions Group is a division of Darwin Global, LLC, a systems and software engineering company that provides innovative, cost-effective training solutions and support for federal, state and local government, as well as the private sector.


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