repmgr node rejoin

repmgr node rejoin
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repmgr node rejoin — rejoin a dormant (stopped) node to the replication cluster

Description

Enables a dormant (stopped) node to be rejoined to the replication cluster.

This can optionally use pg_rewind to re-integrate a node which has diverged from the rest of the cluster, typically a failed primary.

Tip

If the node is running and needs to be attached to the current primary, use repmgr standby follow.

Note repmgr standby follow can only be used for standbys which have not diverged from the rest of the cluster.

Usage

      repmgr node rejoin -d '$conninfo'

where $conninfo is the PostgreSQL conninfo string of the current primary node (or that of any reachable node in the cluster, but not the local node). This is so that repmgr can fetch up-to-date information about the current state of the cluster.

repmgr.conf for the stopped node *must* be supplied explicitly if not otherwise available.

Options

--dry-run

Check prerequisites but don't actually execute the rejoin.

--force-rewind[=/path/to/pg_rewind]

Execute pg_rewind.

It is only necessary to provide the pg_rewind path if using PostgreSQL 9.3 or 9.4, and pg_rewind is not installed in the PostgreSQL bin directory.

--config-files

comma-separated list of configuration files to retain after executing pg_rewind.

Currently pg_rewind will overwrite the local node's configuration files with the files from the source node, so it's advisable to use this option to ensure they are kept.

--config-archive-dir

Directory to temporarily store configuration files specified with --config-files; default: /tmp.

-W/--no-wait

Don't wait for the node to rejoin cluster.

If this option is supplied, repmgr will restart the node but not wait for it to connect to the primary.

Configuration file settings

node_rejoin_timeout: the maximum length of time (in seconds) to wait for the node to reconnect to the replication cluster (defaults to the value set in standby_reconnect_timeout, 60 seconds).
Note that standby_reconnect_timeout must be set to a value equal to or greater than node_rejoin_timeout.

Event notifications

A node_rejoin event notification will be generated.

Exit codes

One of the following exit codes will be emitted by repmgr node rejoin:

SUCCESS (0): The node rejoin succeeded; or if --dry-run was provided, no issues were detected which would prevent the node rejoin.
ERR_BAD_CONFIG (1): A configuration issue was detected which prevented repmgr from continuing with the node rejoin.
ERR_NO_RESTART (4): The node could not be restarted.
ERR_REJOIN_FAIL (24): The node rejoin operation failed.

Notes

Currently repmgr node rejoin can only be used to attach a standby to the current primary, not another standby.

The node must have been shut down cleanly; if this was not the case, it will need to be manually started (remove any existing recovery.conf file first) until it has reached a consistent recovery point, then shut down cleanly.

Tip

If PostgreSQL is started in single-user mode and input is directed from /dev/null/, it will perform recovery then immediately quit, and will then be in a state suitable for use by pg_rewind.

          rm -f /var/lib/pgsql/data/recovery.conf
          postgres --single -D /var/lib/pgsql/data/ < /dev/null

repmgr will attempt to verify whether the node can rejoin as-is, or whether pg_rewind must be used (see following section).

Using `pg_rewind`

repmgr node rejoin can optionally use pg_rewind to re-integrate a node which has diverged from the rest of the cluster, typically a failed primary. pg_rewind is available in PostgreSQL 9.5 and later as part of the core distribution, and can be installed from external sources for PostgreSQL 9.3 and 9.4.

Note

pg_rewind requires that either wal_log_hints is enabled, or that data checksums were enabled when the cluster was initialized. See the pg_rewind documentation for details.

We strongly recommend familiarizing yourself with pg_rewind before attempting to use it with repmgr, as while it is an extremely useful tool, it is not a "magic bullet" which can resolve all problematic replication situations.

A typical use-case for pg_rewind is when a scenario like the following is encountered:

    $ repmgr node rejoin -f /etc/repmgr.conf -d 'host=node3 dbname=repmgr user=repmgr' \
        --force-rewind --config-files=postgresql.local.conf,postgresql.conf --verbose --dry-run
    INFO: replication connection to the rejoin target node was successful
    INFO: local and rejoin target system identifiers match
    DETAIL: system identifier is 6652184002263212600
    ERROR: this node cannot attach to rejoin target node 3
    DETAIL: rejoin target server's timeline 2 forked off current database system timeline 1 before current recovery point 0/610D710
    HINT: use --force-rewind to execute pg_rewind

Here, node3 was promoted to a primary while the local node was still attached to the previous primary; this can potentially happen during e.g. a network split. pg_rewind can re-sync the local node with node3, removing the need for a full reclone.

To have repmgr node rejoin use pg_rewind, pass the command line option --force-rewind, which will tell repmgr to execute pg_rewind to ensure the node can be rejoined successfully.

`pg_rewind` and configuration file retention

Be aware that if pg_rewind is executed and actually performs a rewind operation, any configuration files in the PostgreSQL data directory will be overwritten with those from the source server.

To prevent this happening, provide a comma-separated list of files to retain using the --config-file command line option; the specified files will be archived in a temporary directory (whose parent directory can be specified with --config-archive-dir, default: /tmp) and restored once the rewind operation is complete.

Example using `repmgr node rejoin` and `pg_rewind`

Example, first using --dry-run, then actually executing the node rejoin command.

    $ repmgr node rejoin -f /etc/repmgr.conf -d 'host=node3 dbname=repmgr user=repmgr' \
        --config-files=postgresql.local.conf,postgresql.conf --verbose --force-rewind --dry-run
    INFO: replication connection to the rejoin target node was successful
    INFO: local and rejoin target system identifiers match
    DETAIL: system identifier is 6652460429293670710
    NOTICE: pg_rewind execution required for this node to attach to rejoin target node 3
    DETAIL: rejoin target server's timeline 2 forked off current database system timeline 1 before current recovery point 0/610D710
    INFO: prerequisites for using pg_rewind are met
    INFO: file "postgresql.local.conf" would be copied to "/tmp/repmgr-config-archive-node2/postgresql.local.conf"
    INFO: file "postgresql.replication-setup.conf" would be copied to "/tmp/repmgr-config-archive-node2/postgresql.replication-setup.conf"
    INFO: pg_rewind would now be executed
    DETAIL: pg_rewind command is:
      pg_rewind -D '/var/lib/postgresql/data' --source-server='host=node3 dbname=repmgr user=repmgr'
    INFO: prerequisites for executing NODE REJOIN are met

Note

If --force-rewind is used with the --dry-run option, this checks the prerequisites for using pg_rewind, but is not an absolute guarantee that actually executing pg_rewind will succeed. See also section Caveats below.

    $ repmgr node rejoin -f /etc/repmgr.conf -d 'host=node3 dbname=repmgr user=repmgr' \
        --config-files=postgresql.local.conf,postgresql.conf --verbose --force-rewind
    NOTICE: pg_rewind execution required for this node to attach to rejoin target node 3
    DETAIL: rejoin target server's timeline 2 forked off current database system timeline 1 before current recovery point 0/610D710
    NOTICE: executing pg_rewind
    DETAIL: pg_rewind command is "pg_rewind -D '/var/lib/postgresql/data' --source-server='host=node3 dbname=repmgr user=repmgr'"
    NOTICE: 2 files copied to /var/lib/postgresql/data
    NOTICE: setting node 2's upstream to node 3
    NOTICE: starting server using "pg_ctl -l /var/log/postgres/startup.log -w -D '/var/lib/pgsql/data' start"
    NOTICE: NODE REJOIN successful
    DETAIL: node 2 is now attached to node 3

Caveats when using `repmgr node rejoin`

repmgr node rejoin attempts to determine whether it will succeed by comparing the timelines and relative WAL positions of the local node (rejoin candidate) and primary (rejoin target). This is particularly important if planning to use pg_rewind, which currently (as of PostgreSQL 12) may appear to succeed (or indicate there is no action needed) but potentially allow an impossible action, such as trying to rejoin a standby to a primary which is behind the standby. repmgr will prevent this situation from occurring.

Currently it is not possible to detect a situation where the rejoin target is a standby which has been "promoted" by removing recovery.conf (PostgreSQL 12 and later: standby.signal) and restarting it. In this case there will be no information about the point the rejoin target diverged from the current standby; the rejoin operation will fail and the current standby's PostgreSQL log will contain entries with the text "record with incorrect prev-link".

We strongly recommend running repmgr node rejoin with the --dry-run option first. Additionally it might be a good idea to execute the pg_rewind command displayed by repmgr with the pg_rewind --dry-run option. Note that pg_rewind does not indicate that it is running in --dry-run mode.

Warning

In all current PostgreSQL versions (as of September 2020), pg_rewind contains a corner-case bug which affects standbys in a very specific situation.

This situation occurs when a standby was shut down before its primary node, and an attempt is made to attach this standby to another primary in the same cluster (following a "split brain" situation where the standby was connected to the wrong primary). In this case, repmgr will correctly determine that pg_rewind should be executed, however pg_rewind incorrectly decides that no action is necessary.

In this situation, repmgr will report something like:

    NOTICE: pg_rewind execution required for this node to attach to rejoin target node 1
    DETAIL: rejoin target server's timeline 3 forked off current database system timeline 2 before current recovery point 0/7019C10

but when executed, pg_rewind will report:

    pg_rewind: servers diverged at WAL location 0/7015540 on timeline 2
    pg_rewind: no rewind required

and if an attempt is made to attach the standby to the new primary, PostgreSQL logs on the standby will contain errors like:

    [2020-09-07 15:01:41 UTC]    LOG:  00000: replication terminated by primary server
    [2020-09-07 15:01:41 UTC]    DETAIL:  End of WAL reached on timeline 2 at 0/7015540.
    [2020-09-07 15:01:41 UTC]    LOG:  00000: new timeline 3 forked off current database system timeline 2 before current recovery point 0/7019C10

Currently it is not possible to resolve this situation using pg_rewind. A patch has been submitted and will hopefully be included in a forthcoming PostgreSQL minor release.

As a workaround, start the primary server the standby was previously attached to, and ensure the standby can be attached to it. If pg_rewind was actually executed, it will have copied in the .history file from the target primary server; this must be removed. repmgr node rejoin can then be used to attach the standby to the original primary. Ensure any changes pending on the primary have propogated to the standby. Then shut down the primary server first, before shutting down the standby. It should then be possible to use repmgr node rejoin to attach the standby to the new primary.