System prevalence
is a simple software
architectural pattern that combines
system image
In computing, a system image is a serialized copy of the entire state of a computer system stored in some non-volatile form such as a file. A system is said to be capable of using system images if it can be shut down and later restored to exactly ...
s (snapshots) and
transaction journaling to provide speed, performance scalability,
transparent persistence and transparent
live mirroring of
computer system state.
In a prevalent system,
state is kept in
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
in native format, all transactions are
journaled and
System image
In computing, a system image is a serialized copy of the entire state of a computer system stored in some non-volatile form such as a file. A system is said to be capable of using system images if it can be shut down and later restored to exactly ...
s are regularly saved to disk.
System images and transaction journals can be stored in language-specific
serialization format for speed or in
XML
Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
format for cross-language portability.
The first usage of the term and generic, publicly available implementation of a system prevalence
layer was
Prevayler, written for Java by Klaus Wuestefeld in 2001.
[
]
Advantages
Simply keeping system
state in RAM in its normal, natural, language-specific format is orders of magnitude faster and more programmer-friendly than the multiple conversions that are needed when it is stored and retrieved from a
DBMS
In computing, a database is an organized collection of data stored and accessed electronically. Small databases can be stored on a file system, while large databases are hosted on computer clusters or cloud storage. The design of databases ...
.
As an example,
Martin Fowler describes "The
LMAX Architecture"
[
] with a transaction-journal and system-image (snapshot) based business system at its core, which can process 6 million transactions per second on a single thread.
Requirement
A prevalent system needs enough
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
to hold its entire state 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 already to fit in memory.
Programmers need skill in working with business state natively in RAM, rather than using explicit
API calls for storage and queries for retrieval.
The system's events must be capturable for
journaling.
See also
*
Object-relational mapping
References
{{reflist
External links
* "An Introduction to Object Prevalence", by Carlos Villela for IBM Developerworks
* "Prevalence: Transparent, Fault-Tolerant Object Persistence", by Jim Paterson for O'Reilly's OnJava.co
* "Object Prevalence": Original Article by Klaus Wuestefeld published in 2001 on Advogato
* Madeleine: a Ruby implementatio
Persistence