Object persistence
Encyclopedia
System prevalence is a simple software architectural pattern that combines system image
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
in native format, rather than being written to an RDBMS or other data storage system. All transactions are journaled
and System image
s are regularly saved to disk.
System images and transaction journals can be stored in language-specific serialization
format for speed or in XML
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.
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.
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
.
Java
.NET
Python
Common Lisp
System image
A system image in computing is a 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 the same state...
s (snapshots) and transaction
Database transaction
A transaction comprises a unit of work performed within a database management system against a database, and treated in a coherent and reliable way independent of other transactions...
journaling
Journal (computing)
In computer storage, a journal is a chronological record of data processing operations that may be used to construct or reinstate an historical or alternative version of a computer system or computer file....
to provide speed, performance scalability, transparent persistence and transparent live mirroring
Mirror (computing)
In computing, a mirror is an exact copy of a data set. On the Internet, a mirror site is an exact copy of another Internet site.Mirror sites are most commonly used to provide multiple sources of the same information, and are of particular value as a way of providing reliable access to large downloads...
of computer system state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
.
In a prevalent system, state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
is kept in memory
Computer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
in native format, rather than being written to an RDBMS or other data storage system. All transactions are journaled
Journal (computing)
In computer storage, a journal is a chronological record of data processing operations that may be used to construct or reinstate an historical or alternative version of a computer system or computer file....
and System image
System image
A system image in computing is a 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 the same state...
s are regularly saved to disk.
System images and transaction journals can be stored in language-specific serialization
Serialization
In computer science, in the context of data storage and transmission, serialization is the process of converting a data structure or object state into a format that can be stored and "resurrected" later in the same or another computer environment...
format for speed or in XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
format for cross-language portability.
The first usage of the term and generic, publicly-available implementation of a system prevalence layer
Layer (object-oriented design)
In object-oriented design, a layer is a group of classes that have the same set of link-time module dependencies to other modules. In other words, a layer is a group of reusable components that are reusable in similar circumstances...
was Prevayler
Prevayler
Prevayler is an open source system prevalence layer for Java: it transparently persists Plain Old Java Objects. Read operations are three to four orders of magnitude faster with Prevayler when compared to traditional database systems since all objects are always in RAM and in-process.The fact that...
, written for Java by Klaus Wuestefeld in 2001.
Advantages
Simply keeping system stateState (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
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
Martin Fowler
-Online presentations:* at RailsConf 2006* at JAOO 2006* at QCon London 2007 * at QCon London 2008 * at ThoughtWorks Quarterly Technology Briefing, October 2008...
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 memoryComputer memory
In computing, memory refers to the physical devices used to store programs or data on a temporary or permanent basis for use in a computer or other digital electronic device. The term primary memory is used for the information in physical systems which are fast In computing, memory refers to the...
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
Journal (computing)
In computer storage, a journal is a chronological record of data processing operations that may be used to construct or reinstate an historical or alternative version of a computer system or computer file....
.
JavaJava (programming language)Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
.NET.NET FrameworkThe .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
- Bamboo.Prevalence
- #liveDB
- DevMagicFake
PythonPython (programming language)Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
Common LispCommon LispCommon Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
External links
- "An Introduction to Object Prevalence", by Carlos Villela for IBM Developerworks. http://www-128.ibm.com/developerworks/library/wa-objprev/
- "Prevalence: Transparent, Fault-Tolerant Object Persistence", by Jim Paterson for O'Reilly's OnJava.com http://www.onjava.com/pub/a/onjava/2005/06/08/prevayler.html
- List of Prevalent System layer implementations for various languages http://www.prevayler.org/wiki.jsp?topic=PrevaylerPortsToOtherLanguages
- "Object Prevalence": Original Article by Klaus Wuestefeld published in 2001 on Advogato. http://www.advogato.org/article/398.html