# Cloning standbys

## Cloning a standby from Barman

can use 2ndQuadrant's Barman application to clone a standby (and also as a fallback source for WAL files).

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.

Currently &repmgr;'s support for cloning from Barman is implemented by using rsync to clone from the Barman server.

It is therefore not able to make use of Barman's parallel restore facility, which is executed on the Barman server and clones to the target server.

Barman's parallel restore facility can be used by executing it manually on the Barman server and integrating the resulting cloned standby using `repmgr standby clone --recovery-conf-only`.

### Prerequisites for cloning from Barman

In order to enable Barman support for `repmgr standby clone`, following prerequisites must be met:

- the Barman catalogue must include at least one valid backup for this server;

- the `barman_host` setting in `repmgr.conf` is set to the SSH hostname of the Barman server;

- the `barman_server` setting in `repmgr.conf` is the same as the server configured in Barman.

For example, assuming Barman is located on the host "`barmansrv`" under the "`barman`" user account, `repmgr.conf` should contain the following entries:

        barman_host='barman@barmansrv'
        barman_server='somedb'

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

We also recommend configuring the `restore_command` setting in `repmgr.conf` to use the `barman-wal-restore` script (see section below).

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 of `barman_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.:

        $ repmgr -f /etc/repmgr.conf -h node1 -U repmgr -d repmgr standby clone
        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 source node
        DETAIL: current installation size is 30 MB
        NOTICE: retrieving backup from Barman...
        (...)
        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

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.

### Using Barman as a WAL file source

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 ~ 2.7) or as part of the core Barman distribution (Barman 2.8 and later).

To use `barman-wal-restore` with &repmgr;, assuming Barman is located on the host "`barmansrv`" under the "`barman`" user account, 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='barman@barmansrv'
        barman_server='somedb'
        restore_command='/usr/bin/barman-wal-restore barmansrv somedb %f %p'

`barman-wal-restore` supports command line switches to control parallelism (`--parallel=N`) and compression (`--bzip2`, `--gzip`).

## Cloning and replication slots

Replication slots were introduced with PostgreSQL 9.4 and are designed to ensure that any standby connected to the primary using a replication slot will always be able to retrieve the required WAL files. This removes the need to manually manage WAL file retention by estimating the number of WAL files that need to be maintained on the primary using `wal_keep_segments`. Do however be aware that if a standby is disconnected, WAL will continue to accumulate on the primary until either the standby reconnects or the replication slot is dropped.

To enable &repmgr; to use replication slots, set the boolean parameter `use_replication_slots` in `repmgr.conf`:

           use_replication_slots=true

Replication slots must be enabled in `postgresql.conf` by setting the parameter `max_replication_slots` to at least the number of expected standbys (changes to this parameter require a server restart).

When cloning a standby, &repmgr; will automatically generate an appropriate slot name, which is stored in the `repmgr.nodes` table, and create the slot on the upstream node:

        repmgr=# SELECT node_id, upstream_node_id, active, node_name, type, priority, slot_name
                   FROM repmgr.nodes ORDER BY node_id;
         node_id | upstream_node_id | active | node_name |  type   | priority |   slot_name
        ---------+------------------+--------+-----------+---------+----------+---------------
               1 |                  | t      | node1     | primary |      100 | repmgr_slot_1
               2 |                1 | t      | node2     | standby |      100 | repmgr_slot_2
               3 |                1 | t      | node3     | standby |      100 | repmgr_slot_3
         (3 rows)

        repmgr=# SELECT slot_name, slot_type, active, active_pid FROM pg_replication_slots ;
           slot_name   | slot_type | active | active_pid
        ---------------+-----------+--------+------------
         repmgr_slot_2 | physical  | t      |      23658
         repmgr_slot_3 | physical  | t      |      23687
        (2 rows)

Note that a slot name will be created by default for the primary but not actually used unless the primary is converted to a standby using e.g. `repmgr standby switchover`.

Further information on replication slots in the PostgreSQL documentation: https://www.postgresql.org/docs/current/interactive/warm-standby.html#STREAMING-REPLICATION-SLOTS

While replication slots can be useful for streaming replication, it's recommended to monitor for inactive slots as these will cause WAL files to build up indefinitely, possibly leading to server failure.

As an alternative we recommend using 2ndQuadrant's Barman, which offloads WAL management to a separate server, removing the requirement to use a replication slot for each individual standby to reserve WAL. See section for more details on using &repmgr; together with Barman.

## Cloning and cascading replication

Cascading replication, introduced with PostgreSQL 9.2, enables a standby server to replicate from another standby server rather than directly from the primary, meaning replication changes "cascade" down through a hierarchy of servers. This can be used to reduce load on the primary and minimize bandwith usage between sites. For more details, see the PostgreSQL cascading replication documentation.

&repmgr; supports cascading replication. When cloning a standby, set the command-line parameter `--upstream-node-id` to the `node_id` of the server the standby should connect to, and &repmgr; will create `recovery.conf` to point to it. Note that if `--upstream-node-id` is not explicitly provided, &repmgr; will set the standby's `recovery.conf` to point to the primary node.

To demonstrate cascading replication, first ensure you have a primary and standby set up as shown in the . Then create an additional standby server with `repmgr.conf` looking like this:

        node_id=3
        node_name=node3
        conninfo='host=node3 user=repmgr dbname=repmgr'
        data_directory='/var/lib/postgresql/data'

Clone this standby (using the connection parameters for the existing standby), ensuring `--upstream-node-id` is provide with the `node_id` of the previously created standby (if following the example, this will be `2`):

        $ repmgr -h node2 -U repmgr -d repmgr -f /etc/repmgr.conf standby clone --upstream-node-id=2
        NOTICE: using configuration file "/etc/repmgr.conf"
        NOTICE: destination directory "/var/lib/postgresql/data" provided
        INFO: connecting to upstream node
        INFO: connected to source node, checking its state
        NOTICE: checking for available walsenders on upstream node (2 required)
        INFO: sufficient walsenders available on upstream node (2 required)
        INFO: successfully connected to source node
        DETAIL: current installation size is 29 MB
        INFO: creating directory "/var/lib/postgresql/data"...
        NOTICE: starting backup (using pg_basebackup)...
        HINT: this may take some time; consider using the -c/--fast-checkpoint option
        INFO: executing: 'pg_basebackup -l "repmgr base backup" -D /var/lib/postgresql/data -h node2 -U repmgr -X stream '
        NOTICE: standby clone (using pg_basebackup) complete
        NOTICE: you can now start your PostgreSQL server
        HINT: for example: pg_ctl -D /var/lib/postgresql/data start

then register it (note that `--upstream-node-id` must be provided here too):

         $ repmgr -f /etc/repmgr.conf standby register --upstream-node-id=2
         NOTICE: standby node "node2" (ID: 2) successfully registered

After starting the standby, the cluster will look like this, showing that `node3` is attached to `node2`, not the primary (`node1`).

        $ repmgr -f /etc/repmgr.conf cluster show
         ID | Name  | Role    | Status    | Upstream | Location | Connection string
        ----+-------+---------+-----------+----------+----------+--------------------------------------
         1  | node1 | primary | * running |          | default  | host=node1 dbname=repmgr user=repmgr
         2  | node2 | standby |   running | node1    | default  | host=node2 dbname=repmgr user=repmgr
         3  | node3 | standby |   running | node2    | default  | host=node3 dbname=repmgr user=repmgr

Under some circumstances when setting up a cascading replication cluster, you may wish to clone a downstream standby whose upstream node does not yet exist. In this case you can clone from the primary (or another upstream node); provide the parameter `--upstream-conninfo` to explictly set the upstream's `primary_conninfo` string in `recovery.conf`.

## Advanced cloning options

### pg_basebackup options when cloning a standby

As &repmgr; uses `pg_basebackup` to clone a standby, it's possible to provide additional parameters for `pg_basebackup` to customise the cloning process.

By default, `pg_basebackup` performs a checkpoint before beginning the backup process. However, a normal checkpoint may take some time to complete; a fast checkpoint can be forced with `repmgr standby clone`'s `-c/--fast-checkpoint` option. Note that this may impact performance of the server being cloned from (typically the primary) so should be used with care.

If Barman is set up for the cluster, it's possible to clone the standby directly from Barman, without any impact on the server the standby is being cloned from. For more details see .

Other options can be passed to `pg_basebackup` by including them in the `repmgr.conf` setting `pg_basebackup_options`.

Not that by default, &repmgr; executes `pg_basebackup` with `-X/--wal-method` (PostgreSQL 9.6 and earlier: `-X/--xlog-method`) set to `stream`. From PostgreSQL 9.6, if replication slots are in use, it will also create a replication slot before running the base backup, and execute `pg_basebackup` with the `-S/--slot` option set to the name of the previously created replication slot.

These parameters can set by the user in `pg_basebackup_options`, in which case they will override the &repmgr; default values. However normally there's no reason to do this.

If using a separate directory to store WAL files, provide the option `--waldir` (`--xlogdir` in PostgreSQL 9.6 and earlier) with the absolute path to the WAL directory. Any WALs generated during the cloning process will be copied here, and a symlink will automatically be created from the main data directory.

See the PostgreSQL pg_basebackup documentation for more details of available options.

### Managing passwords

If replication connections to a standby's upstream server are password-protected, the standby must be able to provide the password so it can begin streaming replication.

The recommended way to do this is to store the password in the `postgres` system user's `~/.pgpass` file. It's also possible to store the password in the environment variable `PGPASSWORD`, however this is not recommended for security reasons. For more details see the PostgreSQL password file documentation.

If using a `pgpass` file, an entry for the replication user (by default the user who connects to the `repmgr` database) must be provided, with database name set to `replication`, e.g.:

              node1:5432:replication:repmgr:12345

If, for whatever reason, you wish to include the password in `recovery.conf`, set `use_primary_conninfo_password` to `true` in `repmgr.conf`. This will read a password set in `PGPASSWORD` (but not `~/.pgpass`) and place it into the `primary_conninfo` string in `recovery.conf`. Note that `PGPASSWORD` will need to be set during any action which causes `recovery.conf` to be rewritten, e.g. .

It is of course also possible to include the password value in the `conninfo` string for each node, but this is obviously a security risk and should be avoided.

From PostgreSQL 9.6, libpq supports the `passfile` parameter in connection strings, which can be used to specify a password file other than the default `~/.pgpass`.

To have &repmgr; write a custom password file in `primary_conninfo`, specify its location in `passfile` in `repmgr.conf`.

### Separate replication user

In some circumstances it might be desirable to create a dedicated replication-only user (in addition to the user who manages the &repmgr; metadata). In this case, the replication user should be set in `repmgr.conf` via the parameter `replication_user`; &repmgr; will use this value when making replication connections and generating `recovery.conf`. This value will also be stored in the parameter `repmgr.nodes` table for each node; it no longer needs to be explicitly specified when cloning a node or executing .