What is Postgress Streaming replication

Streaming Replication (SR) provides the capability to continuously ship and apply the WAL XLOG records to some number of standby servers in order to keep them current.

This feature was added to PostgreSQL 9.0. The discussion below is a developer oriented one that contains some out of date information.

• Log-shipping
• XLOG records generated in the primary are periodically shipped to the standby via the network.
• In the existing warm standby, only records in a filled file are shipped, what's referred to as file-based log-shipping. In SR, XLOG records in partially-filled XLOG file are shipped too, implementing record-based log-shipping. This means the window for data loss in SR is usually smaller than in warm standby, unless the warm standby was also configured for record-based shipping (which is complicated to setup).
• The content of XLOG files written to the standby are exactly the same as those on the primary. XLOG files shipped can be used for a normal recovery and PITR.
• Multiple standbys
• More than one standby can establish a connection to the primary for SR. XLOG records are concurrently shipped to all these standbys. The delay/death of a standby does not harm log-shipping to other standbys.
• The maximum number of standbys can be specified as a GUC variable.
• Continuous recovery
• The standby continuously replays XLOG records shipped without using pg_standby.
• XLOG records shipped are replayed as soon as possible without waiting until XLOG file has been filled. The combination of Hot Standby and SR would make the latest data inserted into the primary visible in the standby almost immediately.
• The standby periodically removes old XLOG files which are no longer needed for recovery, to prevent excessive disk usage.
• Setup
• The start of log-shipping does not interfere with any query processing on the primary.
• The standby can be started in various conditions.
• If there are XLOG files in archive directory and restore_command is supplied, at first those files are replayed. Then the standby requests XLOG records following the last applied one to the primary. This prevents XLOG files already present in the standby from being shipped again. Similarly, XLOG files in pg_xlog are also replayed before starting log-shipping.
• If there is no XLOG files on the standby, the standby requests XLOG records following the starting XLOG location of recovery (the redo starting location).
• Connection settings and authentication
• A user can configure the same settings as a normal connection to a connection for SR (e.g., keepalive, pg_hba.conf).
• Activation
• The standby can keep waiting for activation as long as a user likes. This prevents the standby from being automatically brought up by failure of recovery or network outage.
• Progress report
• The primary and standby report the progress of log-shipping in PS display.
• Graceful shutdown
• When smart/fast shutdown is requested, the primary waits to exit until XLOG records have been sent to the standby, up to the shutdown checkpoint record.

References:

https://wiki.postgresql.org/wiki/Streaming_Replication