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Although object-oriented databases are a topic of curiosity for many developers, few enterprise developers deploy their applications using an OO database. The initial period of over-excitement about OO databases has long abated, and gave way to a strong dose of skepticism about OO database technology.
In this interview from JavaOne 2008, Anat Gafni, CTO of db4objects, explains that a second generation of object databases addresses important problems in application development and maintenance. She also talks about some of the features of db4o, the open-source object-oriented database for Java and C#:
db4o is an object database, which is not what developers typically turn to when they think about persisting information. Although most developers program in objects and object-oriented languages, they tend to use relational databases, and may not look beyond relational database technology.
Object databases have been around for a long time. And, in fact, we are now at a stage when a second-generation of object-oriented databases have started to spring up that actually provide very useful properties for developers as well as for enterprises. The way object-oriented databases work, and the way in which they differ from relational databases, requires that developers think of persistence a bit differently.
With the first generation of OO databases, people got overly excited, and thought that object databases would replace relational ones. OO databases, however, are instead complementary to relational databases, a realization that was not understood until relatively recently. OO databases have strengths and weaknesses, and they can live side-by-side with relational databases.
How do you know when an OO database is more useful than a relational one? You want to identify areas where the data and the data schema is modified frequently, or areas where you have complex relationships, if you need to store your in-memory objects very fast, or if you need those objects to be retrieved in the same form your program uses. If you have multimedia data, then an OO database is useful also.
On the other hand, areas that are pretty standard, where fields and your schema are static and don't change, and where you need a lot of ad-hoc reporting with standard queries, those could use a relational database.
You also need to consider who is in charge of the data schema. If the developer is in charge, and if there is no central administrator, then an OO database is a good fit. On the other hand, if the company has a central data repository, and in fact a central administrator owns the data, and if that data is outside the developer's purview, then you'd want to go with a relational database.
db4o goes beyond what even the second-generation object-oriented databases provide. db4o set it as a goal to maintain and preserve the object-oriented nature of an environment by using a natural interface—natural to the programming language—and by allowing as much flexibility and transparency in the schema and the object model as possible.
Although object-relational technology has come a long way, it still does not solve the object/relational impedance mismatch. Some developers have come to believe that the latest O/R mapping technologies bridge the gap of objects and relational databases seamlessly. On the surface, that may appear so. But you need to look a bit deeper, and realize that object systems and relational databases have different purposes and foundations.
Object-oriented languages became popular because they facilitate change well. All the object-oriented constructs, such as inheritance, encapsulation, abstraction, polymorphism, or interfaces, and many of the OO patterns, shine in comparison with other approaches to program design when it comes time to change your data model or object model and, in general, when you need to make changes to your application. And changes happen all the time to many types of applications. The more complex your objects and your data model, the more changes are likely to occur.
By definition, relational databases have a schema. The schema is something that's supposed to be set, and is something that doesn't change very often. As soon as you break your objects into tables, those tables are constrained by the schema, and you loose many advantages OO programming languages and systems give you. That's true even if you can relatively easily map your objects to tables with an O/R mapping technology: object systems and relational schema differ in their notions of change and flexibility.
In db4o, you can modify your objects, and on the fly db4o will adjust to those changes. You don't need to incur any overhead for that. There are no special steps to take, no schema migration.
When you want to add a field to an object, or change the type of a field, for instance, in db4o you can just make those changes to your objects and classes. db4o will identify the changes and manage them behind the scenes, automatically. When you retrieve an instance of your old object, whose property type is different, for example, the new object's property value will either be a default value, or you can even specify to db4o a method that the database will use to assign a new field value. All of that is done transparently. db4o upgrades your schema dynamically, as you go. That's a great value when you're making changes to you program. You can even work with the old and the new version of an object together. When the new version has a field that the old one didn't, that field in the new object will be null.
db4o provides a simple, programming language-based query system, and you can persist your objects with a single line of code. We support Java and C# currently. Objects you persist in Java can be accessed from C#, for example.
db4o also goes beyond many other OO databases in making it easy to use OO and a relational databases side-by-side. To that effect, we have replication capabilities that can replicate the object data store to a relational store and vice versa. The way that works now is that you provide Hibernate-based mappings between a db4o data model and a relational model. We also have something called the db4o Replication System, or DRS, that facilitates db4o-to-db4o replication. It queries one database to determine the differences from another database, and makes a list of objects that need to be synchronized and performs the synchronization in both directions. So that allows db4o to live happily with relational databases, and makes it possible for the developer to use each technology as it fits best.
What do you think of db4o and the second generation of OO databases?Post your opinion in the discussion forum.
The db4o home page: http://www.db4o.com
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Frank Sommers is Editor-in-Chief of Artima Developer. He also serves as chief editor of the IEEE Technical Committee on Scalable Computing's newsletter, and is an elected member of the Jini Community's Technical Advisory Committee. Prior to joining Artima, Frank wrote the Jiniology and Web services columns for JavaWorld.