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repmgr standby clone can use 2ndQuadrant's Barman application to clone a standby (and also as a fallback source for WAL files).
Tip: Barman (aka PgBarman) should be considered as an integral part of any PostgreSQL replication cluster. For more details see: https://www.pgbarman.org/.
Barman support provides the following advantages:
the primary node does not need to perform a new backup every time a new standby is cloned
a standby node can be disconnected for longer periods without losing the ability to catch up, and without causing accumulation of WAL files on the primary node
WAL management on the primary becomes much easier as there's no need to use replication slots, and wal_keep_segments does not need to be set.
In order to enable Barman support for repmgr standby clone, following prerequisites must be met:
the barman_server setting in repmgr.conf is the same as the server configured in Barman;
the barman_host setting in repmgr.conf is set to the SSH hostname of the Barman server;
the restore_command setting in repmgr.conf is configured to use a copy of the barman-wal-restore script shipped with the barman-cli package (see section Using Barman as a WAL file source below).
the Barman catalogue includes at least one valid backup for this server.
Note: Barman support is automatically enabled if barman_server is set. Normally it is good practice to use Barman, for instance when fetching a base backup while cloning a standby; in any case, Barman mode can be disabled using the --without-barman command line option.
Tip: If you have a non-default SSH configuration on the Barman server, e.g. using a port other than 22, then you can set those parameters in a dedicated Host section in ~/.ssh/config corresponding to the value ofbarman_host in repmgr.conf. See the Host section in man 5 ssh_config for more details.
It's now possible to clone a standby from Barman, e.g.:
NOTICE: using configuration file "/etc/repmgr.conf"
NOTICE: destination directory "/var/lib/postgresql/data" provided
INFO: connecting to Barman server to verify backup for test_cluster
INFO: checking and correcting permissions on existing directory "/var/lib/postgresql/data"
INFO: creating directory "/var/lib/postgresql/data/repmgr"...
INFO: connecting to Barman server to fetch server parameters
INFO: connecting to upstream node
INFO: connected to source node, checking its state
INFO: successfully connected to source node
DETAIL: current installation size is 29 MB
NOTICE: retrieving backup from Barman...
receiving file list ...
(...)
NOTICE: standby clone (from Barman) complete
NOTICE: you can now start your PostgreSQL server
HINT: for example: pg_ctl -D /var/lib/postgresql/data start
As a fallback in case streaming replication is interrupted, PostgreSQL can optionally retrieve WAL files from an archive, such as that provided by Barman. This is done by setting restore_command in recovery.conf to a valid shell command which can retrieve a specified WAL file from the archive.
barman-wal-restore is a Python script provided as part of the barman-cli package (Barman 2.0 and later; for Barman 1.x the script is provided separately as barman-wal-restore.py) which performs this function for Barman.
To use barman-wal-restore with repmgr and assuming Barman is located on the barmansrv host and that barman-wal-restore is located as an executable at /usr/bin/barman-wal-restore, repmgr.conf should include the following lines:
barman_host=barmansrv
barman_server=somedb
restore_command=/usr/bin/barman-wal-restore barmansrv somedb %f %p
Note: barman-wal-restore supports command line switches to control parallelism (--parallel=N) and compression ( --bzip2, --gzip).
Note: To use a non-default Barman configuration file on the Barman server, specify this in repmgr.conf with barman_config:
barman_config=/path/to/barman.conf