In a previous post I talked about my first impressions of NHibernate. Not a whole lot has changed since then, except I have learned a lot more. But, I said I would discuss what I didn’t like about NHibernate so here it goes.
Session Management
The biggest thing that I don’t like is the session management aspect. Although I have come to understand why NHibernate sessions are so valuable and critical to its functionality, it doesn’t stop the fact that persistent objects are tied to a single database. For a project that follows the patterns and practices set forth in ivory towers, this works great. All business entities are stored in a database in 6th normal form and everyone is happy. Well, when working with legacy and enterprise systems, that usually isn’t the case. There are many different databases that are used and objects are stored across a variety of them.
In order to get my system to use NHibernate, the first challenge was incorporating multiple sessions. A session is basically a connection to a single database that manages persistent objects. This article describes a really complex way to accomplish it, but all you really need to pay attention to is the NHibernateSessionModule and NHibernateSessionManager. Using the outline described in the article, I successfully got multiple sessions to work fairly painlessly.
The second challenge was storing a persistent object in multiple database. Since persistent objects in NHibernate can’t span multiple sessions, some data massaging has to be performed. As I see it, there are two possibilities for storing an object in multiple database.
- Different objects for each database with type conversion operators
- The same object for all databases with extra session logic The latter is obviously easier from a domain model perspective, but not from an implementation perspective. For a persistent object from one session to be stored in another session, all references must be removed from the session. This means unproxying the object and unpersisting any collections contained within the object. Looking through the NHibernate source code, I was able to accomplish this with only a few lines of code.
Setup & Documentation
Most of the getting started guides for NHibernate are no where near complete or up-to-date. It took a couple days to get my environment up and running. This isn’t really a problem with the library itself, but is still a problem for first time users. In all reality, there seems to be a lack of documentation in general. Three example mappings in the documentation is no where near enough for what is marketed as an enterprise ORM solution.
Conclusion
NHibernate is awesome. Even with these minor issues, it is one of the greatest tools for web development I have ever used. I hope that the development of this library continues to improve, and hopefully some of these issues will be resolved.