repmgr node rejoin

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.

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

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`

- Execute pg_rewind.

  See repmgr-node-rejoin-pg-rewind for more details on using pg_rewind.

`--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's PostgreSQL instance must have been shut down cleanly. If this was not the case, it will need to be started up until it has reached a consistent recovery point, then shut down cleanly.

In PostgreSQL 13 and later, this will be done automatically if the `--force-rewind` is provided (even if an actual rewind is not necessary).

With PostgreSQL 12 and earlier, PostgreSQL will need to be started and shut down manually; see below for the best way to do this.

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

Note that `standby.signal` (PostgreSQL 11 and earlier: `recovery.conf`) must be removed from the data directory for PostgreSQL to be able to start in single user mode.

## 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` 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.

Additionally, `full_page_writes` must be enabled; this is the default and normally should never be disabled.

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
        NOTICE: rejoin target is node "node3" (node ID: 3)
        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
        NOTICE: rejoin target is node "node3" (node ID: 3)
        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

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 repmgr-node-rejoin-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

### `pg_rewind` and PostgreSQL 9.4

pg_rewind is available in PostgreSQL 9.5 and later as part of the core distribution. Users of PostgreSQL 9.4 will need to manually install it; the source code is available here: https://github.com/vmware/pg_rewind. If the pg_rewind binary is not installed in the PostgreSQL `bin` directory, provide its full path on the demotion candidate with `--force-rewind`.

Note that building the 9.4 version of pg_rewind requires the PostgreSQL source code.

## 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`".

In PostgreSQL 9.5 and earlier, it is not possible to use pg_rewind to attach to a target node with a lower timeline than the local node.

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.

In all PostgreSQL released before February 2021, 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 was submitted and is included in all PostgreSQL versions released in February 2021 or later.

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 propagated 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.

## See also

repmgr-standby-follow, repmgr-standby-switchover