# repmgrd overview

&repmgrd; ("`replication manager daemon`") is a management and monitoring daemon which runs on each node in a replication cluster. It can automate actions such as failover and updating standbys to follow the new primary, as well as providing monitoring information about the state of each standby.

&repmgrd; is designed to be straightforward to set up and does not require additional external infrastructure.

Functionality provided by &repmgrd; includes:

- wide range of configuration options

- option to execute custom scripts ("event notifications") at different points in the failover sequence

- ability to pause repmgrd operation on all nodes with a `single command`

- optional witness server

- "location" configuration option to restrict potential promotion candidates to a single location (e.g. when nodes are spread over multiple data centres)

- choice of method to determine node availability (PostgreSQL ping, query execution or new connection)

- retention of monitoring statistics (optional)

## repmgrd demonstration

To demonstrate automatic failover, set up a 3-node replication cluster (one primary and two standbys streaming directly from the primary) so that the cluster looks something like this:

        $ repmgr -f /etc/repmgr.conf cluster show --compact
         ID | Name  | Role    | Status    | Upstream | Location | Prio.
        ----+-------+---------+-----------+----------+----------+-------
         1  | node1 | primary | * running |          | default  | 100
         2  | node2 | standby |   running | node1    | default  | 100
         3  | node3 | standby |   running | node1    | default  | 100

See section Required configuration for automatic failover for an example of minimal `repmgr.conf` file settings suitable for use with &repmgrd;.

Start &repmgrd; on each standby and verify that it's running by examining the log output, which at log level `INFO` will look like this:

        [2019-08-15 07:14:42] [NOTICE] repmgrd (repmgrd 5.0) starting up
        [2019-08-15 07:14:42] [INFO] connecting to database "host=node2 dbname=repmgr user=repmgr connect_timeout=2"
        INFO:  set_repmgrd_pid(): provided pidfile is /var/run/repmgr/repmgrd-12.pid
        [2019-08-15 07:14:42] [NOTICE] starting monitoring of node "node2" (ID: 2)
        [2019-08-15 07:14:42] [INFO] monitoring connection to upstream node "node1" (ID: 1)

Each &repmgrd; should also have recorded its successful startup as an event:

        $ repmgr -f /etc/repmgr.conf cluster event --event=repmgrd_start
         Node ID | Name  | Event         | OK | Timestamp           | Details
        ---------+-------+---------------+----+---------------------+--------------------------------------------------------
         3       | node3 | repmgrd_start | t  | 2019-08-15 07:14:42 | monitoring connection to upstream node "node1" (ID: 1)
         2       | node2 | repmgrd_start | t  | 2019-08-15 07:14:41 | monitoring connection to upstream node "node1" (ID: 1)
         1       | node1 | repmgrd_start | t  | 2019-08-15 07:14:39 | monitoring cluster primary "node1" (ID: 1)

Now stop the current primary server with e.g.:

        pg_ctl -D /var/lib/postgresql/data -m immediate stop

This will force the primary to shut down straight away, aborting all processes and transactions. This will cause a flurry of activity in the &repmgrd; log files as each &repmgrd; detects the failure of the primary and a failover decision is made. This is an extract from the log of a standby server (`node2`) which has promoted to new primary after failure of the original primary (`node1`).

        [2019-08-15 07:27:50] [WARNING] unable to connect to upstream node "node1" (ID: 1)
        [2019-08-15 07:27:50] [INFO] checking state of node 1, 1 of 3 attempts
        [2019-08-15 07:27:50] [INFO] sleeping 5 seconds until next reconnection attempt
        [2019-08-15 07:27:55] [INFO] checking state of node 1, 2 of 3 attempts
        [2019-08-15 07:27:55] [INFO] sleeping 5 seconds until next reconnection attempt
        [2019-08-15 07:28:00] [INFO] checking state of node 1, 3 of 3 attempts
        [2019-08-15 07:28:00] [WARNING] unable to reconnect to node 1 after 3 attempts
        [2019-08-15 07:28:00] [INFO] primary and this node have the same location ("default")
        [2019-08-15 07:28:00] [INFO] local node's last receive lsn: 0/900CBF8
        [2019-08-15 07:28:00] [INFO] node 3 last saw primary node 12 second(s) ago
        [2019-08-15 07:28:00] [INFO] last receive LSN for sibling node "node3" (ID: 3) is: 0/900CBF8
        [2019-08-15 07:28:00] [INFO] node "node3" (ID: 3) has same LSN as current candidate "node2" (ID: 2)
        [2019-08-15 07:28:00] [INFO] visible nodes: 2; total nodes: 2; no nodes have seen the primary within the last 4 seconds
        [2019-08-15 07:28:00] [NOTICE] promotion candidate is "node2" (ID: 2)
        [2019-08-15 07:28:00] [NOTICE] this node is the winner, will now promote itself and inform other nodes
        [2019-08-15 07:28:00] [INFO] promote_command is:
          "/usr/pgsql-12/bin/repmgr -f /etc/repmgr/12/repmgr.conf standby promote"
        NOTICE: promoting standby to primary
        DETAIL: promoting server "node2" (ID: 2) using "/usr/pgsql-12/bin/pg_ctl  -w -D '/var/lib/pgsql/12/data' promote"
        NOTICE: waiting up to 60 seconds (parameter "promote_check_timeout") for promotion to complete
        NOTICE: STANDBY PROMOTE successful
        DETAIL: server "node2" (ID: 2) was successfully promoted to primary
        [2019-08-15 07:28:01] [INFO] 3 followers to notify
        [2019-08-15 07:28:01] [NOTICE] notifying node "node3" (ID: 3) to follow node 2
        INFO:  node 3 received notification to follow node 2
        [2019-08-15 07:28:01] [INFO] switching to primary monitoring mode
        [2019-08-15 07:28:01] [NOTICE] monitoring cluster primary "node2" (ID: 2)

The cluster status will now look like this, with the original primary (`node1`) marked as inactive, and standby `node3` now following the new primary (`node2`):

        $ repmgr -f /etc/repmgr.conf cluster show --compact
         ID | Name  | Role    | Status    | Upstream | Location | Prio.
        ----+-------+---------+-----------+----------+----------+-------
         1  | node1 | primary | - failed  |          | default  | 100
         2  | node2 | primary | * running |          | default  | 100
         3  | node3 | standby |   running | node2    | default  | 100

`repmgr cluster event` will display a summary of what happened to each server during the failover:

        $ repmgr -f /etc/repmgr.conf cluster event
         Node ID | Name  | Event                      | OK | Timestamp           | Details
        ---------+-------+----------------------------+----+---------------------+-------------------------------------------------------------
         3       | node3 | repmgrd_failover_follow    | t  | 2019-08-15 07:38:03 | node 3 now following new upstream node 2
         3       | node3 | standby_follow             | t  | 2019-08-15 07:38:02 | standby attached to upstream node "node2" (ID: 2)
         2       | node2 | repmgrd_reload             | t  | 2019-08-15 07:38:01 | monitoring cluster primary "node2" (ID: 2)
         2       | node2 | repmgrd_failover_promote   | t  | 2019-08-15 07:38:01 | node 2 promoted to primary; old primary 1 marked as failed
         2       | node2 | standby_promote            | t  | 2019-08-15 07:38:01 | server "node2" (ID: 2) was successfully promoted to primary