What is meant by persistence?

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Question: What is meant by persistence in Java?

Answer: Persistence indicates that a collection of data remains intact even if its source is no longer attached to the network or even more - Java program is not running.

The objects, in JavaSpaces for example, remain available to other users even if the source has temporarily disconnected from the network. This feature is critical to keep distributed systems highly available.

In Java by persistence is meant that an object’s lifetime is not determined by whether a program is executing. In between invocations of the program the object lives.

The simple example of persistance: serialize object and write it to disk, then restore the object when the program is reinvoked. If you need a more serious persistence mechanism, consider Java Data Objects (JDO) or a tool like Hibernate (http://hibernate.sourceforge.net). This example is taken from "Thinking in Java" book.

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How is JavaSpaces technology different from databases?

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Question: How is JavaSpaces technology different from a database?

Answer: JavaSpaces technology  provides a more loosely coupled repository of information than databases. The identity of a client or a server is no longer relevant; data packets are treated just like any other object posted to the space as an anonymous service. Databases require a tremendous amount of overhead in structured programming, screen design, type matching, and indexing to achieve their results. Clients post specific questions to the server, and receive a specific answer.

More importantly, JavaSpaces technology can host more than just data. A space can find, match and reference objects by both type and value, meaning they can store objects as information or behavior. This means any object-based program, device, or user profile can join a JavaSpace system. This capacity greatly enhances the ability to coordinate different functions and processes in a network environment.

Java Space technology is ready for usage product although it is still under development. One example of Java Space based product here:

This newsletter is based on:
"JAVASPACES: A CRITIICAL ANALYSIS OFF CONCEPTS & TECHNIQUES"
http://www.dcs.shef.ac.uk/teaching/eproj/ug2001/pdf/u8sm.pdf

"JavaSpacesTM FAQ"
http://java.sun.com/products/javaspaces/faqs/jsfaq.html

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