Object persistence

Object persistence is an object-oriented, language-specific, transparent data storage and retrieval model used in computer programming. It is based on the techniques of system snapshotting and transaction journalling. The first usage of the term and generic, publicly-available implementation of object prevalence was Prevayler, written for Java by Klaus Wuestefeld in 2001.^[1]

In the prevalent model, the object data is kept in memory in native object format, rather than being marshalled to an RDBMS or other data storage system. A snapshot of data is regularly saved to disk, and in addition to this, all changes are serialised and a log of transactions is also stored on disk.

Snapshots and transaction logs can be stored in language-specific serialization format for speed or in XStream (XML) format for cross-language portability.

Advantages:

• Simply keeping objects in memory in their normal, natural, language-specific format is both orders of magnitude faster and more programmer-friendly than the multiple conversions that are needed when the objects are stored and retrieved from an RDBMS.

Requirement:

• The application needs enough memory to hold the entire database in RAM (the "prevalent hypothesis"). Prevalence advocates claim this is continuously alleviated by decreasing RAM prices, and the fact that many business databases are small enough to fit in memory anyway.
• Programmers need skill in working with business objects natively in RAM, rather than using explicit API calls to store them and queries to retrieve them.

Implementations

• Prevayler (Java)
• Bamboo.Prevalence (.NET)
• Pypersyst (Python)
• Madeleine (Ruby)
• cl-prevalence (Common Lisp)
• Perlvayler (Perl)
• SPrevayler (Smalltalk)
• EntityORM (.NET)
• Habanero.NET (Habanero) is an Open Source Enterprise Application framework for creating Enterprise applications using the principles of Domain-driven design and implemented in .NET.

External links

• "An Introduction to Object Prevalence", by a Prevayler user. [1]
• "Prevalence: Transparent, Fault-Tolerant Object Persistence", by Jim Paterson for O'Reilly's OnJava.com [2]
• 3rd-party, criticism about Prevayler. [3]
• List of Prevalent System layer implementations for various languages [4]
• "Object Prevalence": Original Article by Klaus Wuestefeld published in 2001 on Advogato. [5]

References

See also

• Persistence (computer science)
• Object-relational mapping

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