HOME

TheInfoList



OR:

Multi-master replication is a method of database replication which allows data to be stored by a group of computers, and updated by any member of the group. All members are responsive to client data queries. The multi-master replication system is responsible for propagating the data modifications made by each member to the rest of the group and resolving any conflicts that might arise between concurrent changes made by different members. Multi-master replication can be contrasted with primary-replica replication, in which a single member of the group is designated as the "master" for a given piece of data and is the only node allowed to modify that data item. Other members wishing to modify the data item must first contact the master node. Allowing only a single master makes it easier to achieve consistency among the members of the group, but is less flexible than multi-master replication. Multi-master replication can also be contrasted with failover clustering where passive replica servers are replicating the master data in order to prepare for takeover in the event that the master stops functioning. The master is the only server active for client interaction. Often, communication and replication in Multi-master systems are handled via a type of
Consensus algorithm A fundamental problem in distributed computing and multi-agent systems is to achieve overall system reliability in the presence of a number of faulty processes. This often requires coordinating processes to reach consensus, or agree on some data va ...
, but can also be implemented via custom or proprietary algorithms specific to the software. The primary purposes of multi-master replication are increased availability and faster server response time.


Advantages

* Availability: If one master fails, other masters continue to update the
database 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 ...
. * Distributed Access: Masters can be located in several physical sites, i.e. distributed across the network.


Disadvantages

* Consistency: Most multi-master replication systems are only loosely consistent, i.e. lazy and asynchronous, violating
ACID In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
properties. * Performance: Eager replication systems are complex and increase communication latency. * Integrity: Issues such as conflict resolution can become intractable as the number of nodes involved rises and latency increases.


Implementations


Directory services

Many directory servers are based on
Lightweight Directory Access Protocol The Lightweight Directory Access Protocol (LDAP ) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network. Directory servi ...
(LDAP) and implement multi-master replication.


Active Directory

One of the more prevalent multi-master replication implementations in directory servers is
Microsoft Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washi ...
's
Active Directory Active Directory (AD) is a directory service developed by Microsoft for Windows domain networks. It is included in most Windows Server operating systems as a set of processes and services. Initially, Active Directory was used only for centralize ...
. Within Active Directory, objects that are updated on one
Domain Controller A domain controller (DC) is a server computer that responds to security authentication requests within a computer network domain. It is a network server that is responsible for allowing host access to domain resources. It authenticates users, stores ...
are then replicated to other domain controllers through multi-master replication. It is not required for all domain controllers to replicate with each other as this would cause excessive network traffic in large Active Directory deployments. Instead, domain controllers have a complex update pattern that ensures that all servers are updated in a timely fashion without excessive replication traffic. Some Active Directory needs are however better served by Flexible single master operation.


CA Directory

CA Directory supports multi-master replication.


OpenDS/OpenDJ

OpenDS (and its successor product OpenDJ) implemented multi-master since version 1.0. The OpenDS/OpenDJ multi-master replication is asynchronous, it uses a log with a publish-subscribe mechanism that allows scaling to a large number of nodes. OpenDS/OpenDJ replication does conflict resolution at the entry and attribute level. OpenDS/OpenDJ replication can be used over a
wide area network A wide area network (WAN) is a telecommunications network that extends over a large geographic area. Wide area networks are often established with leased telecommunication circuits. Businesses, as well as schools and government entities, u ...
.


OpenLDAP

OpenLDAP OpenLDAP is a free, open-source implementation of the Lightweight Directory Access Protocol (LDAP) developed by the OpenLDAP Project. It is released under its own BSD-style license called the OpenLDAP Public License. LDAP is a platform-independen ...
, the widely used
open-source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized so ...
LDAP server, implements multi-master replication since version 2.4 (October 2007


Database Management Systems


Amazon Aurora

Amazon Aurora is composed of writer nodes, which replicate redo records, and 6 storage nodes. The writer node sends change to each storage node, each of which checks for conflicts then reports confirmation or rejection of the change.


Apache CouchDB

Apache CouchDB Apache CouchDB is an open-source document-oriented NoSQL database, implemented in Erlang. CouchDB uses multiple formats and protocols to store, transfer, and process its data. It uses JSON to store data, JavaScript as its query language using M ...
uses a simple, HTTP-based multi-master replication system built from its use of an append-only data-store and use of Multiversion Concurrency Control (MVCC). Each document contains a revision ID, so every record stores the evolutionary timeline of all previous revision IDs leading up to itself—which provides the foundation of CouchDB's MVCC system. Additionally, it keeps a by-sequence index for the entire database. "The replication process only copies the last revision of a document, so all previous revisions that were only on the source database are not copied to the destination database." The CouchDB replicator acts as a simple HTTP client acting on both a and database. It compares current sequence IDs for the database, calculates revision differences, and makes the necessary changes to the based on what it found in the history of the database. Bi-directional replication is the result of merely doing another replication with the and values swapped.


ArangoDB

ArangoDB ArangoDB is a free and open-source native graph database system developed by ArangoDB Inc. ArangoDB is a multi-model database system since it supports three data models (graphs, JSON documents, key/value) with one database core and a unified que ...
is a native multi-model database system using multi-master replication. Clusters in ArangoDB use the CP master/master model with no single point of failure. When a cluster encounters a network partition, ArangoDB prefers to maintain its internal consistency over availability. Clients experience the same view of the database regardless of which node they connect to. And, the cluster continues to serve requests even when one machine fails.


Cloudant

Cloudant Cloudant is an IBM software product, which is primarily delivered as a cloud-based service. Cloudant is a non-relational, distributed database service of the same name. Cloudant is based on the Apache-backed CouchDB project and the open source B ...
, a distributed database system, uses largely the same HTTP API as
Apache CouchDB Apache CouchDB is an open-source document-oriented NoSQL database, implemented in Erlang. CouchDB uses multiple formats and protocols to store, transfer, and process its data. It uses JSON to store data, JavaScript as its query language using M ...
, and exposes the same ability to replicate using Multiversion Concurrency Control (MVCC). Cloudant databases can replicate between each other, but internally, nodes within Cloudant clusters use multi-master replication to stay in sync with each other and provide high availability to API consumers.


eXtremeDB Cluster

eXtremeDB Cluster is the clustering sub-system for McObject's eXtremeDB embedded database product family. It maintains database consistency across multiple hardware nodes by replicating transactions in a synchronous manner (two-phase commit). An important characteristic of eXtremeDB Cluster is transaction replication, in contrast to log file-based, SQL statement-based, or other replication schemes that may or may not guarantee the success or failure of entire transactions. Accordingly, eXtremeDB Cluster is an
ACID In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
compliant system (not BASE or eventual consistency); a query executed on any cluster node will return the same result as if executed on any other cluster node.


Oracle

Database 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 ...
clusters implement multi-master replication using one of two methods.
Asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal wit ...
multi-master replication commits data changes to a deferred transaction queue which is periodically processed on all databases in the cluster. Synchronous multi-master replication uses Oracle's two-phase commit functionality to ensure that all databases with the cluster have a consistent dataset.


Microsoft SQL

Microsoft SQL Microsoft SQL Server is a relational database management system developed by Microsoft. As a database server, it is a software product with the primary function of storing and retrieving data as requested by other software applications—which ma ...
provides multi-master replication through peer-to-peer replication. It provides a scale-out and high-availability solution by maintaining copies of data across multiple nodes. Built on the foundation of transactional replication, peer-to-peer replication propagates transactionally consistent changes in near real-time.


MySQL / MariaDB

At a basic level, it is possible to achieve a multi-master replication scheme beginning with MySQL version 3.23 with circular replication. Departing from that, MariaDB and
MySQL MySQL () is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database ...
ship with some replication support, each of them with different nuances. In terms of direct support we have: MariaDB: natively supports multi-master replication since version 10.0, but conflict resolution is not supported, so each master must contain different databases. On MySQL, this is named multi-source available since versio
5.7.6
MySQL: MySQL Group Replication, a plugin for virtual synchronous multi-master with conflict handling and distributed recovery was released wit

Cluster Projects: MySQL Cluster supports conflict detection and resolution between multiple masters since version 6.3 for true multi-master capability for the MySQL Server. There is also an external project, Galera Cluster created b
codership
, that provides true multi-master capability, based on a fork of the InnoDB storage engine and custom replication plug-ins. Replication is synchronous, so no conflict is possible.
Percona Percona is an American company based in Durham, North Carolina and the developer of a number of open source software projects for MySQL, MariaDB, PostgreSQL, MongoDB and RocksDB users. The company’s revenue of around $25 million a year is der ...
XtraDB Cluster also is a combination of Galera replication library and MySQL supporting multi-master.


PostgreSQL

Various options for synchronous multi-master replication exist. Postgres-XL which is available under the Mozilla Public License, an
PostgresXC
(now known a
Postgres-X2
which is available under the same license as PostgreSQL itself are examples. Note that th
PgCluster
() project was abandoned in 2007. The replication documentation for
PostgreSQL PostgreSQL (, ), also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. It was originally named POSTGRES, referring to its origins as a successor to the ...
categorises the different types of replication available. Various options exist for distributed multi-master, includin
Bucardo

rubyrep
and BD
Bi-Directional Replication


=PostgreSQL BDR

= BDR is aimed at eventual inclusion in PostgreSQL core and has been benchmarked as demonstrating significantly enhanced performanceBDR Performance
Petr Jelinek, 2ndQuadrant. Retrieved 2014-07-10 over earlier options. BDR includes replication of data writes (DML), as well as changes to data definition (DDL) and global sequences. BDR nodes may be upgraded online from version 0.9 onwards. 2ndQuadrant has developed BDR continuously since 2012, with the system used in production since 2014. The latest version BDR 3.6 provides column-level conflict detection, CRDTs, eager replication, multi-node query consistency, and many other features.


Ingres

Within
Ingres Jean-Auguste-Dominique Ingres ( , ; 29 August 1780 – 14 January 1867) was a French Neoclassical painter. Ingres was profoundly influenced by past artistic traditions and aspired to become the guardian of academic orthodoxy against the a ...
Replicator, objects that are updated on one Ingres server can then be replicated to other servers whether local or remote through multi-master replication. If one server fails, client connections can be re-directed to another server. It is not required for all Ingres servers in an environment to replicate with each other as this could cause excessive network traffic in large implementations. Instead, Ingres Replicator allows the appropriate data to be replicated to the appropriate servers without excessive replication traffic. This means that some servers in the environment can serve as failover candidates while other servers can meet other requirements such as managing a subset of columns or tables for a departmental solution, a subset of rows for a geographical region or one-way replication for a reporting server. In the event of a source, target, or network failure, data integrity is enforced through this two-phase commit protocol by ensuring that either the whole transaction is replicated, or none of it is. In addition, Ingres Replicator can operate over RDBMS’s from multiple vendors to connect them.


See also

* Flexible single master operation *
Active Directory Active Directory (AD) is a directory service developed by Microsoft for Windows domain networks. It is included in most Windows Server operating systems as a set of processes and services. Initially, Active Directory was used only for centralize ...
* Distributed database management system * DNS zone transfer * Optimistic Replication


References

{{reflist


External links


Active Directory Replication Model

Terms and Definitions for Database Replication

SymmetricDS
is database independent, data synchronization software. It uses web and database technologies to replicate tables between relational databases in near real time. The software was designed to scale for a large number of databases, work across low-bandwidth connections, and withstand periods of network outage. It supports
MySQL MySQL () is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database ...
,
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The word ...
, SQL Server,
PostgreSQL PostgreSQL (, ), also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. It was originally named POSTGRES, referring to its origins as a successor to the ...
,
IBM Db2 Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON a ...
, Firebird, Interbase,
HSQLDB HSQLDB (''Hyper SQL Database'') is a relational database management system written in Java. It has a JDBC driver and supports a large subset of SQL-92, SQL:2008, SQL:2011, and SQL:2016 standards. It offers a fast, small (around 1300 kilobyte ...
, H2, Apache Derby,
Informix IBM Informix is a product family within IBM's Information Management division that is centered on several relational database management system (RDBMS) offerings. The Informix products were originally developed by Informix Corporation, whose ...
, Greenplum,
SQLite SQLite (, ) is a database engine written in the C programming language. It is not a standalone app; rather, it is a library that software developers embed in their apps. As such, it belongs to the family of embedded databases. It is the mo ...
,
Sybase ASE SAP ASE (Adaptive Server Enterprise), originally known as Sybase SQL Server, and also commonly known as Sybase DB or Sybase ASE, is a relational model database server developed by Sybase Corporation, which later became part of SAP AG. ASE was ...
, and Sybase ASA. Licensed under both open source ( GPL) and commercial licenses.
Daffodil Replicator
is a
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
tool for data synchronization, data migration, and data backup between various database servers. Daffodil Replicator works over standard JDBC driver and supports replication across heterogeneous databases. At present, it supports following databases:
Microsoft SQL Server Microsoft SQL Server is a relational database management system developed by Microsoft. As a database server, it is a software product with the primary function of storing and retrieving data as requested by other software applications—which ...
,
Oracle An oracle is a person or agency considered to provide wise and insightful counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. As such, it is a form of divination. Description The word ...
, Daffodil database,
IBM Db2 Db2 is a family of data management products, including database servers, developed by IBM. It initially supported the relational model, but was extended to support object–relational features and non-relational structures like JSON a ...
, Apache Derby,
MySQL MySQL () is an open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A relational database ...
, and
PostgreSQL PostgreSQL (, ), also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. It was originally named POSTGRES, referring to its origins as a successor to the ...
. Daffodil Replicator is available in both enterprise (commercial) and
open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized so ...
( GPL-licensed) versions.
DMOZ Open Directory Project - Database Replication Page
Identity management Distributed computing architecture