Operating pg_auto_failover

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Deployment

pg_auto_failover is a general purpose tool for setting up PostgreSQL replication in order to implement High Availability of the PostgreSQL service.

Provisioning

It is also possible to register pre-existing PostgreSQL instances with a pg_auto_failover monitor. The pg_autoctl create command honors the PGDATA environment variable, and checks whether PostgreSQL is already running. If Postgres is detected, the new node is registered in SINGLE mode, bypassing the monitor’s role assignment policy.

Security

Connections between monitor and data nodes use trust authentication by default. This lets accounts used by pg_auto_failover to connect to nodes without needing a password. Default behaviour could be changed using --auth parameter when creating monitor or data Node. Any auth method supported by PostgreSQL could be used here. Please refer to PostgreSQL pg_hba documentation for available options.

Security for following connections should be considered when setting up .pgpass file.

  1. health check connection from monitor for pgautofailover_monitor user
  2. connections for pg_autoctl command from data nodes to monitor for autoctl_node user
  3. replication connections from secondary to primary data nodes for replication user. Notice that primary and secondary nodes change during failover. Thus this setting should be done on both primary and secondary nodes.
  4. settings need to be updated after a new node is added.

See PostgreSQL documentation on setting up .pgpass file.

Operations

It is possible to operate pg_auto_failover formations and groups directly from the monitor. All that is needed is an access to the monitor Postgres database as a client, such as psql. It’s also possible to add those management SQL function calls in your own ops application if you have one.

For security reasons, the autoctl_node is not allowed to perform maintenance operations. This user is limited to what pg_autoctl needs. You can either create a specific user and authentication rule to expose for management, or edit the default HBA rules for the autoctl user. In the following examples we’re directly connecting as the autoctl role.

The main operations with pg_auto_failover are node maintenance and manual failover, also known as a controlled switchover.

Maintenance of a secondary node

It is possible to put a secondary node in any group in a MAINTENANCE state, so that the Postgres server is not doing synchronous replication anymore and can be taken down for maintenance purposes, such as security kernel upgrades or the like.

The monitor exposes the following API to schedule maintenance operations on a secondary node:

$ psql postgres://autoctl@monitor/pg_auto_failover
> select pgautofailover.start_maintenance('nodename', 5432);
> select pgautofailover.stop_maintenance('nodename', 5432);

The command line tool pg_autoctl also exposes an API to schedule maintenance operations on the current node, which must be a secondary node at the moment when maintenance is requested:

$ pg_autoctl enable maintenance
...
$ pg_autoctl disable maintenance

When a standby node is in maintenance, the monitor sets the primary node replication to WAIT_PRIMARY: in this role, the PostgreSQL streaming replication is now asynchronous and the standby PostgreSQL server may be stopped, rebooted, etc.

pg_auto_failover does not provide support for primary server maintenance.

Triggering a failover

It is possible to trigger a failover manually with pg_auto_failover, by using the SQL API provided by the monitor:

$ psql postgres://autoctl@monitor/pg_auto_failover
> select pgautofailover.perform_failover(formation_id => 'default', group_id => 0);

To call the function, you need to figure out the formation and group of the group where the failover happens. The following commands when run on a pg_auto_failover keeper node provide for the necessary information:

$ export PGDATA=...
$ pg_autoctl config get pg_autoctl.formation
$ pg_autoctl config get pg_autoctl.group

Implementing a controlled switchover

It is generally useful to distinguish a controlled switchover to a failover. In a controlled switchover situation it is possible to organise the sequence of events in a way to avoid data loss and lower downtime to a minimum.

In the case of pg_auto_failover, because we use synchronous replication, we don’t face data loss risks when triggering a manual failover. Moreover, our monitor knows the current primary health at the time when the failover is triggerred, and drives the failover accordingly.

So to trigger a controlled switchover with pg_auto_failover you can use the same API as for a manual failover:

$ psql postgres://autoctl@monitor/pg_auto_failover
> select pgautofailover.perform_failover(formation_id => 'default', group_id => 0);

Current state, last events

The following commands display information from the pg_auto_failover monitor tables pgautofailover.node and pgautofailover.event:

$ pg_autoctl show state
$ pg_autoctl show events

When run on the monitor, the commands outputs all the known states and events for the whole set of formations handled by the monitor. When run on a PostgreSQL node, the command connects to the monitor and outputs the information relevant to the service group of the local node only.

For interactive debugging it is helpful to run the following command from the monitor node while e.g. initializing a formation from scratch, or performing a manual failover:

$ watch pg_autoctl show state

Monitoring pg_auto_failover in Production

The monitor reports every state change decision to a LISTEN/NOTIFY channel named state. PostgreSQL logs on the monitor are also stored in a table, pgautofailover.event, and broadcast by NOTIFY in the channel log.

Trouble-Shooting Guide

pg_auto_failover commands can be run repeatedly. If initialization fails the first time – for instance because a firewall rule hasn’t yet activated – it’s possible to try pg_autoctl create again. pg_auto_failover will review its previous progress and repeat idempotent operations (create database, create extension etc), gracefully handling errors.