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Object persistence and Java

Get an in-depth look at the issues surrounding object persistence in object-oriented languages

By Arsalan Saljoughy, JavaWorld.com, 05/01/97

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Object durability, or persistence, is the term you often hear used in conjunction with the issue of storing objects in databases. Persistence is expected to operate with transactional integrity, and as such it is subject to strict conditions. (See the Resources section of this article for more information on transaction processing.) In contrast, language services offered through standard language libraries and packages are often free from transactional constraints.

As we'll see in this article, evidence suggests that simple Java persistence will likely stem from the language itself, while sophisticated database functionality will be offered by database vendors.

No object is an island

In the real world, you rarely find an object that lacks relations to other objects. Objects are components of object models. The issue of object durability transcends the issue of object model durability and distribution once we make the observation that objects are interconnected by virtue of their relations to one another.

The relational approach to data storage tends to aggregate data by type. Rows in a table represent the physical aggregate of objects of the same type on disk. The relationships among objects are then represented by keys that are shared across many tables. Although through database organization, relational databases sometimes allow tables that are likely to be used together to be co-located (or clustered) in the same logical partition, such as a database segment, they have no mechanism to store object relationships in the database. Hence, in order to construct an object model, these relationships are constructed from the existing keys at run time in a process referred to as table joins. This is the same well-known property of the relational databases called data independence. Nearly all variants of object databases offer some mechanism to enhance the performance of a system that involves complex object relationships over traditional relational databases.

To query or to navigate?

In storing objects on disk, we are faced with the choice of co-locating related objects to better accommodate navigational access, or to store objects in table-like collections that aggregate objects by type to facilitate predicate-based access (queries), or both. The co-location of objects in persistent storage is an area where relational and object-oriented databases widely differ. The choice of the query language is another area of consideration. Structured Query Language (SQL) and extensions of it have provided relational systems with a predicate-based access mechanism. Object Query Language (OQL) is an object variant of SQL, standardized by ODMG, but support for this language is currently scant. Polymorphic methods offer unprecedented elegance in constructing a semantic query for a collection of objects. For example, imagine a polymorphic behavior for acccount called isInGoodStanding. It may return the Boolean true for all accounts in good standing, and false otherwise. Now imagine the elegance of querying the collection of accounts, where inGoodStanding is implemented differently based on business rules, for all accounts in good standing. It may look something like:

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Resources
  • Download a free copy of ObjectStore PSE for Java version 1.0.1:
    http://www.odi.com/products/pse/psej.html
  • C.J. Date, Introduction to Database Systems, 1990, Fifth Edition Addison-Wesley Publishing Company, New York.
  • This is a classic premier for anyone interested in database technology.
  • Rick Cattel, Object Data Management, 1994, Addison-Wesley Publishing Company, New York.
  • Rick wrote this book way before Java was en vogue and has an excellent treatment of various styles of object storage, from API to distribution to actual disk layout. Rick has gone on to play a important role in the definition of JDBC API.
  • Won Kim (Editor), Modern Database Systems, 1995, ACM Press New York, New York.
  • This book is a collection of papers written on data storage, but has a bias toward demonstrating that object/relational systems will win out over pure object systems. Nonetheless, most anyone who is someone in this business has contributed to this book, so it's worth a look.