System prevalence
is a simple software
architectural pattern that combines
system images (snapshots) and
transaction
Transaction or transactional may refer to:
Commerce
* Financial transaction, an agreement, communication, or movement carried out between a buyer and a seller to exchange an asset for payment
*Debits and credits in a Double-entry bookkeeping sys ...
journaling to provide speed, performance scalability,
transparent persistence and transparent
live mirroring of
computer system
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
.
In a prevalent system,
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
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 remembered ...
in native format, all transactions are
journaled and
System images are regularly saved to disk.
System images and transaction journals can be stored in language-specific
serialization
In computing, serialization (or serialisation) is the process of translating a data structure or object state into a format that can be stored (e.g. files in secondary storage devices, data buffers in primary storage devices) or transmitted (e ...
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. ...
format for cross-language portability.
The first usage of the term and generic, publicly available implementation of a system prevalence
layer was
Prevayler
Prevayler is an open-source (BSD) system-prevalence layer for Java: it transparently persists plain old Java objects. It is an in-RAM database backed by snapshots of the system via object serialization, which are loaded after a system crash t ...
, written for Java by Klaus Wuestefeld in 2001.
[
]
Advantages
Simply keeping system
state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
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.
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 remembered ...
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