Word "Replication" means a process of sharing information so as to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility.
Replication is very interesting subject in any databases. In database competition world, PostgreSQL has its own uniqueness in RDBMS Open source for High availability. Latest PostgreSQL 9.1 has in-built support of Synchronous and Asynchronous replication. In-built Asynchronous replications are Warm Standby, Hot Standby and Streaming Replication and with third party tools Slony,Londiste,Mammoth etc. Below chart will help you to understand about available Synchronous and Asynchronouse replication.
WAL Shipping (Hot Standby and Warm Standby):
PostgreSQL has the ability to ship WAL's to another Server i.e, Standby. The Standby server will be running in recovery mode with the pg_standby utility applying the WAL's. Primary Server generates archives (a copy of WAL, usually 16 MB file) and sends them to multiple slaves, later it will be applied by pg_standby utility.
- Warm Standby: Primary generates archives and feed them to Slave. Its a WAL Shipping to slave. Slave will be in continous recovery and not accessible for reads.
- Hot Standby: Hot Standby is the name for the capability to run queries on a database that is currently performing archive recovery. In Hot Standby slaves can be used for read-only access.
- Slave applie's WAL's periodically not continously, means only completed XLOG's will be available to slave as WAL archives and those will be applied. So, lag will be the unfilled or uncompleted WAL which has not generated archive. Data loss will be minimum of one WAL(16 MB).
Trigger Based Replication :
In trigger based replication, tools like Slony,Londiste, Mammoth uses ON INSERT, ON UPDATE, ON DELETE triggers on tables to maintain replication between Master and Slave. Slave will hold consistent Snapshots.
Streaming Replication :
Its also called as Binary replication. PostgreSQL, XLOG's records generated at primary will be shipped to Standby via network. Lag in streaming replication is very minimum like single transaction depending on Network Speed and Hot Standby Settings. Multiple Slave can be configured. Streaming replication comes with additional process 'WAL SENDER' at Primary and 'WAL RECEIVER' at Standby.
- On Primary Crash, standby can be recovered in very less time.
- Standby can be opened and it will be in READ ONLY mode.
- It can be used for Reporting Server.
- Load balancing can be configured using pgpool-II between Primary and Standby.
- Standby Server should hold same amount of Memory/Disk/CPU etc., because, in case of Primary crashes the Slave acts as Primary.
- Minimal Lag i.e. (one transaction behind Primary)
Slony is a asynchronous trigger-based replication. Its a single master to multiple slave replication system for PostgreSQL. Every table or sequence on Master will be replicated via remote triggers to Slave. Updates are committed to one database and are applied to Slave later as EVENTs. Using Slony Switchover and switchback is possible.
Limitations of Slony-I
- Tables must have a primary key or a unique.
- Only Tables and sequeces are allowed for replication.
- Slave databases cannot be modified.
- Slony-I supports switchback.
- Using Slony-I, we can upgrade PG from one version to another version without any downtime.
- Slony cannot detect the network failuer, hence causing all the EVENT's created at primary will be queued and are released once Network catch ups.
- NO DDL changes allowed on the replication Tables while Slony Daemons running.
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