Slony-I

Last updated February 22, 2019

Slony-I is an asynchronous master-slave replication system for the PostgreSQL DBMS, providing support for cascading and failover. Asynchronous means that when a database transaction has been committed to the master server, it is not yet guaranteed to be available in slaves. Cascading means that replicas can be created (and updated) via other replicas, i.e. they needn't directly connect to the master.

PostgreSQL, often simply Postgres, is an open source object-relational database management system (ORDBMS) with an emphasis on extensibility and standards compliance. It can handle workloads ranging from small single-machine applications to large Internet-facing applications with many concurrent users. It is the default database on macOS Server and is available for Microsoft Windows and Linux.

In computing and related technologies such as networking, failover is switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application, server, system, hardware component, or network. Failover and switchover are essentially the same operation, except that failover is automatic and usually operates without warning, while switchover requires human intervention.

A transaction symbolizes 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. A transaction generally represents any change in a database. Transactions in a database environment have two main purposes:

To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure, when execution stops and many operations upon a database remain uncompleted, with unclear status.
To provide isolation between programs accessing a database concurrently. If this isolation is not provided, the programs' outcomes are possibly erroneous.

Terminology

The name "slony" comes from the Russian word "слоны" which means "elephants".^[1] This is a reference to the PostgreSQL elephant logo, as well as being a "tip of the hat" to Vadim Mikheev, who came up with some of the core ideas Slony-I uses to work. Note that there is a whole set of related terminology:

Russian is an East Slavic language, which is official in the Russian Federation, Belarus, Kazakhstan and Kyrgyzstan, as well as being widely used throughout Eastern Europe, the Baltic states, the Caucasus and Central Asia. It was the de facto language of the Soviet Union until its dissolution on 25 December 1991. Although, nowadays, over two decades after the breakup of the Soviet Union, Russian is used in official capacity or in public life in all the post-Soviet nation-states, as well as in Israel and Mongolia, the rise of state-specific varieties of this language tends to be strongly denied in Russia, in line with the Russian World ideology.

The elephants are the large mammals forming the family Elephantidae in the order Proboscidea. Three species are currently recognised: the African bush elephant, the African forest elephant, and the Asian elephant. Elephants are scattered throughout sub-Saharan Africa, South Asia, and Southeast Asia. Elephantidae is the only surviving family of the order Proboscidea; other, now extinct, members of the order include deinotheres, gomphotheres, mammoths, and mastodons.

slony: is the plural word for elephants, and indicates that a cluster consists of multiple databases
slon: is the singular word for elephant; each replication node is managed by a program named "slon", which aggregate together into the aforementioned "cluster of elephants"
slonik: is the word for a "little elephant," and is the name of the program used to configure the cluster. In effect, the "little elephant" tells the cluster, "here's what you need to do!"

Unique features

Unlike many other replication solutions for PostgreSQL, Slony-I is not tied to any particular version of the database, which makes it possible to upgrade a database cluster one node at a time, without material downtime. (Version 2.0.0 requires PostgreSQL 8.3+, though)

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References

↑ "Slony-I 2.2.5 Documentation: Slony-I Concepts". 2016-05-31. Retrieved 2017-02-02.

Ludovic Marcotte (2005-04-28). "Database Replication with Slony-I". Linux Journal . Retrieved 2017-02-02.

Linux Journal is a monthly technology magazine published by Linux Journal, LLC.. It focuses specifically on Linux, allowing the content to be a highly specialized source of information for open source enthusiasts.

A. Elein Mustain (2004-11-18). "Introducing Slony". O'Reilly OnLamp. Retrieved 2017-02-02.

O'Reilly Media is an American media company established by Tim O'Reilly that publishes books and Web sites and produces conferences on computer technology topics. Their distinctive brand features a woodcut of an animal on many of their book covers.

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[1] "Slony-I 2.2.5 Documentation: Slony-I Concepts". 2016-05-31. Retrieved 2017-02-02.