Database - Replication
About
The fundamental motivation behind replication is to make data highly available to applications with minimal performance impact.
In addition to its role in failure recovery, replication is also useful for distributing application workloads across multiple databases for maximum performance and for facilitating online upgrades and maintenance.
Replication is the process of copying data from a master database to a subscriber database. Replication at each master and subscriber database is controlled by replication agents that communicate through TCP/IP stream sockets. The replication agent on the master database reads the records from the transaction log for the master database. It forwards changes to replicated elements to the replication agent on the subscriber database. The replication agent on the subscriber then applies the updates to its database. If the subscriber agent is not running when the updates are forwarded by the master, the master retains the updates in its transaction log until they can be applied at the subscriber.
Replication of databases often relates closely to transactions. If a database can log its individual actions, one can create a duplicate of the data in real time. DBAs can use the duplicate to improve performance and/or the availability of the whole database system.
Articles Related
Replication concepts
Common replication concepts include:
- master/slave Replication: All write-requests are performed on the master and then replicated to the slave(s)
- quorum: The result of Read and Write requests are calculated by querying a “majority” of replicas
- multimaster: Two or more replicas sync each other via a transaction identifier
Database clustering
Parallel synchronous replication of databases enables the replication of transactions on multiple servers simultaneously, which provides a method for backup and security as well as data availability. This is commonly referred to as “database clustering”.
