# PostgreSQL High Availability
# Replication in PostgreSQL
- Replication User for replication activities
- Directory to store the WAL archives
- **Create a archive directory in data directory** `mkdir $PGDATA/archive`
**Edit the pg_hba.conf file**
This is host base authentication file, contains the setting for client autherntication. Add below entry:
#hosttype database_name user_name hostname/IP method host replication replication <slave-IP>/32 md5
**Edit the postgresql.conf file**
This is the configuration file of PostgreSQL.
`wal_level = hot_standby`
This parameter decides the behavior of slave server.
`hot_standby` logs what is required to accept read only queries on slave server. `streaming` logs what is required to just apply the WAL's on slave. `archive` which logs what is required for archiving.
archive_mode=onThis parameters allows to send WAL segments to archive location using
archive_command = 'test ! -f /path/to/archivedir/%f && cp %p /path/to/archivedir/%f'Basically what above
archive_commanddoes is it copies the WAL segments to archive directory.
Now restart the primary server.
wal_senders = 5This is maximum number of WAL sender processes.
- **Backing up the primay server to the slave server** Before making changes on the server stop the primary server.
**Now run the pg_basebackup utility**
`pg_basebackup` utility copies the data from primary server data directory to slave data directory.
`$ pg_basebackup -h
-D /var/lib/postgresql/ /main -U replication -v -P --xlog-method=stream`
**Configuring the standby server**
To configure the standby server, you'll edit postgresql.conf and create a new configuration file named recovery.conf.
`hot_standby = on`
This specifies whether you are allowed to run queries while recovering
- **Creating recovery.conf file** `standby_mode = on` Set the connection string to the primary server. Replace with the external IP address of the primary server. Replace with the password for the user named replication `primary_conninfo = 'host= port=5432 user=replication password=' (Optional) Set the trigger file location: `trigger_file = '/tmp/postgresql.trigger.5432'` The `trigger_file` path that you specify is the location where you can add a file when you want the system to fail over to the standby server. The presence of the file "triggers" the failover. Alternatively, you can use the pg_ctl promote command to trigger failover.
Start the standby server
You now have everything in place and are ready to bring up the standby server
This article is substantially derived from and attributed to How to Set Up PostgreSQL for High Availability and Replication with Hot Standby, with minor changes in formatting and examples and some text deleted. The source was published under the Creative Commons Public License 3.0, which is maintained here.
Create Replication user
createuser -U postgres replication -P -c 5 --replication
+ option -P will prompt you for new password + option -c is for maximum connections. 5 connections are enough for replication + -replication will grant replication privileges to the user
Important: Don't start the service again until all configuration and backup steps are complete. You must bring up the standby server in a state where it is ready to be a backup server. This means that all configuration settings must be in place and the databases must be already synchronized. Otherwise, streaming replication will fail to start`
-D: This is tells pg_basebackup where to the initial backup -h: Specifies the system where to look for the primary server -xlog-method=stream: This will force the pg_basebackup to open another connection and stream enough xlog while backup is running. It also ensures that fresh backup can be started without failing back to using an archive.