Let us assume that you have an architecture that demands that you'll have a database per client (quite common, by my experiance), but the application layers are the same. This is usually done for reasons of security and/or scalability. So there is no way a client can get information for another client.
This is fairly simple to handle with NHibernate, you just create a session factory per each client. This is actually an advantage, since you can start playing with the implementations of the classes for each client. (Yes, you are reading me correctly. In this case, it possible to turn NHibernate into an Inversion Of Control container).
The issue turns up when you have caching turned on. Assuming you have an entity of the same type with the same identifier in two of the databases, you may very will mix the data when it is cached. NHibernate maintain a single second level cache per session factory, so you are supposed to be safe from this. The problem is that the cache space is shared (and it has to be shared).
Let us take an example of an Employee object with id of #5, which exist in both IniTech and Fuzbar databases. When the employee instnace is loaded and put in the cache, the cache key that is used looks something like: "NHibernate:Entities.EmployeeManagement.Employee:5@5". Now, when we try to load an employee from the second database, we first lookup in the cache, finding that there is an instance there, because the cache keys matches.
This is happening because the cache space (so to speak) is the same space for all the caching strategies. For SysCache, it is the ASP.Net cache that is used for the cache space. For distributed caching, Memcached is used. In both cases, if the same cache key is generated, the information will be returned even if the key was actually inserted by another session factory.
In this sceanrio, I solved the issue by modifying the SysCacheProvider to be aware of the current database, and insert the current database name to the cache key. This solved all the issues in one stroke.
The JIRA issue for this is here. I was surprised to learn that I was the first to encounter this, and in the comments to the JIRA issue, I found that indeed I wasn't the first. :-)
The issue is solve by specifying cache region prefix in the session factory configuration, but the functionality wasn't ported to NHibernate yet.