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.

Upgrading pg_auto_failover, from versions 1.4 onwward

When upgrading a pg_auto_failover setup, the procedure is different on the monitor and on the Postgres nodes:

  • on the monitor, the internal pg_auto_failover database schema might have changed and needs to be upgraded to its new definition, porting the existing data over. The pg_auto_failover database contains the registration of every node in the system and their current state.

    It is not possible to trigger a failover during the monitor update. Postgres operations on the Postgres nodes continue normally.

    During the restart of the monitor, the other nodes might have trouble connecting to the monitor. The pg_autoctl command is designed to retry connecting to the monitor and handle errors gracefully.

  • on the Postgres nodes, the pg_autoctl command connects to the monitor every once in a while (every second by default), and then calls the node_active protocol, a stored procedure in the monitor databases.

    The pg_autoctl also verifies at each connection to the monitor that it’s running the expected version of the extension. When that’s not the case, the “node-active” sub-process quits, to be restarted with the possibly new version of the pg_autoctl binary found on-disk.

As a result, here is the standard upgrade plan for pg_auto_failover:

  1. Upgrade the pg_auto_failover package on the all the nodes, monitor included.

    When using a debian based OS, this looks like the following command when from 1.4 to 1.5:

    sudo apt-get remove pg-auto-failover-cli-enterprise-1.4 postgresql-11-auto-failover-enterprise-1.4
    sudo apt-get install -q -y pg-auto-failover-cli-enterprise-1.5 postgresql-11-auto-failover-enterprise-1.5
    
  2. Restart the pgautofailover service on the monitor.

    When using the systemd integration, all we need to do is:

    sudo systemctl restart pgautofailover
    

    Then we may use the following commands to make sure that the service is running as expected:

    sudo systemctl status pgautofailover
    sudo journalctl -u pgautofailover
    

    At this point it is expected that the pg_autoctl logs show that an upgrade has been performed by using the ALTER EXTENSION pgautofailover UPDATE TO ... command. The monitor is ready with the new version of pg_auto_failover.

When the Postgres nodes pg_autoctl process connects to the new monitor version, the check for version compatibility fails, and the “node-active” sub-process exits. The main pg_autoctl process supervisor then restart the “node-active” sub-process from its on-disk binary executable file, which has been upgraded to the new version. That’s why we first install the new packages for pg_auto_failover on every node, and only then restart the monitor.

Important

Before upgrading the monitor, which is a simple restart of the pg_autoctl process, it is important that the OS packages for pgautofailover be updated on all the Postgres nodes.

When that’s not the case, pg_autoctl on the Postgres nodes will still detect a version mismatch with the monitor extension, and the “node-active” sub-process will exit. And when restarted automatically, the same version of the local pg_autoctl binary executable is found on-disk, leading to the same version mismatch with the monitor extension.

After restarting the “node-active” process 5 times, pg_autoctl quits retrying and stops. This includes stopping the Postgres service too, and a service downtime might then occur.

And when the upgrade is done we can use pg_autoctl show state on the monitor to see that eveything is as expected.

Upgrading from previous pg_auto_failover versions

The new upgrade procedure described in the previous section is part of pg_auto_failover since version 1.4. When upgrading from a previous version of pg_auto_failover, up to and including version 1.3, then all the pg_autoctl processes have to be restarted fully.

To prevent triggering a failover during the upgrade, it’s best to put your secondary nodes in maintenance. The procedure then looks like the following:

  1. Enable maintenance on your secondary node(s):

    pg_autoctl enable maintenance
    
  2. Upgrade the OS packages for pg_auto_failover on every node, as per previous section.

  3. Restart the monitor to upgrade it to the new pg_auto_failover version:

    When using the systemd integration, all we need to do is:

    sudo systemctl restart pgautofailover
    

    Then we may use the following commands to make sure that the service is running as expected:

    sudo systemctl status pgautofailover
    sudo journalctl -u pgautofailover
    

    At this point it is expected that the pg_autoctl logs show that an upgrade has been performed by using the ALTER EXTENSION pgautofailover UPDATE TO ... command. The monitor is ready with the new version of pg_auto_failover.

  4. Restart pg_autoctl on all Postgres nodes on the cluster.

    When using the systemd integration, all we need to do is:

    sudo systemctl restart pgautofailover
    

    As in the previous point in this list, make sure the service is now running as expected.

  5. Disable maintenance on your secondary nodes(s):

    pg_autoctl disable maintenance
    

Cluster Management and 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 command line tool pg_autoctl exposes an API to schedule maintenance operations on the current node, which must be a secondary node at the moment when maintenance is requested.

Here’s an example of using the maintenance commands on a secondary node, including the output. Of course, when you try that on your own nodes, dates and PID information might differ:

$ pg_autoctl enable maintenance
17:49:19 14377 INFO  Listening monitor notifications about state changes in formation "default" and group 0
17:49:19 14377 INFO  Following table displays times when notifications are received
    Time |  ID |      Host |   Port |       Current State |      Assigned State
---------+-----+-----------+--------+---------------------+--------------------
17:49:19 |   1 | localhost |   5001 |             primary |        wait_primary
17:49:19 |   2 | localhost |   5002 |           secondary |    wait_maintenance
17:49:19 |   2 | localhost |   5002 |    wait_maintenance |    wait_maintenance
17:49:20 |   1 | localhost |   5001 |        wait_primary |        wait_primary
17:49:20 |   2 | localhost |   5002 |    wait_maintenance |         maintenance
17:49:20 |   2 | localhost |   5002 |         maintenance |         maintenance

The command listens to the state changes in the current node’s formation and group on the monitor and displays those changes as it receives them. The operation is done when the node has reached the maintenance state.

It is now possible to disable maintenance to allow pg_autoctl to manage this standby node again:

$ pg_autoctl disable maintenance
17:49:26 14437 INFO  Listening monitor notifications about state changes in formation "default" and group 0
17:49:26 14437 INFO  Following table displays times when notifications are received
    Time |  ID |      Host |   Port |       Current State |      Assigned State
---------+-----+-----------+--------+---------------------+--------------------
17:49:27 |   2 | localhost |   5002 |         maintenance |          catchingup
17:49:27 |   2 | localhost |   5002 |          catchingup |          catchingup
17:49:28 |   2 | localhost |   5002 |          catchingup |           secondary
17:49:28 |   1 | localhost |   5001 |        wait_primary |             primary
17:49:28 |   2 | localhost |   5002 |           secondary |           secondary
17:49:29 |   1 | localhost |   5001 |             primary |             primary

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.

Maintenance of a primary node

A primary node must be available at all times in any formation and group in pg_auto_failover, that is the invariant provided by the whole solution. With that in mind, the only way to allow a primary node to go to a maintenance mode is to first failover and promote the secondary node.

The same command pg_autoctl enable maintenance implements that operation when run on a primary node with the option --allow-failover. Here is an example of such an operation:

$ pg_autoctl enable maintenance
11:53:03 50526 WARN  Enabling maintenance on a primary causes a failover
11:53:03 50526 FATAL Please use --allow-failover to allow the command proceed

As we can see the option allow-maintenance is mandatory. In the next example we use it:

$ pg_autoctl enable maintenance --allow-failover
13:13:42 1614 INFO  Listening monitor notifications about state changes in formation "default" and group 0
13:13:42 1614 INFO  Following table displays times when notifications are received
    Time |  ID |      Host |   Port |       Current State |      Assigned State
---------+-----+-----------+--------+---------------------+--------------------
13:13:43 |   2 | localhost |   5002 |             primary | prepare_maintenance
13:13:43 |   1 | localhost |   5001 |           secondary |   prepare_promotion
13:13:43 |   1 | localhost |   5001 |   prepare_promotion |   prepare_promotion
13:13:43 |   2 | localhost |   5002 | prepare_maintenance | prepare_maintenance
13:13:44 |   1 | localhost |   5001 |   prepare_promotion |    stop_replication
13:13:45 |   1 | localhost |   5001 |    stop_replication |    stop_replication
13:13:46 |   1 | localhost |   5001 |    stop_replication |        wait_primary
13:13:46 |   2 | localhost |   5002 | prepare_maintenance |         maintenance
13:13:46 |   1 | localhost |   5001 |        wait_primary |        wait_primary
13:13:47 |   2 | localhost |   5002 |         maintenance |         maintenance

When the operation is done we can have the old primary re-join the group, this time as a secondary:

$ pg_autoctl disable maintenance
13:14:46 1985 INFO  Listening monitor notifications about state changes in formation "default" and group 0
13:14:46 1985 INFO  Following table displays times when notifications are received
    Time |  ID |      Host |   Port |       Current State |      Assigned State
---------+-----+-----------+--------+---------------------+--------------------
13:14:47 |   2 | localhost |   5002 |         maintenance |          catchingup
13:14:47 |   2 | localhost |   5002 |          catchingup |          catchingup
13:14:52 |   2 | localhost |   5002 |          catchingup |           secondary
13:14:52 |   1 | localhost |   5001 |        wait_primary |             primary
13:14:52 |   2 | localhost |   5002 |           secondary |           secondary
13:14:53 |   1 | localhost |   5001 |             primary |             primary

Triggering a failover

It is possible to trigger a manual failover, or a switchover, using the command pg_autoctl perform failover. Here’s an example of what happens when running the command:

$ pg_autoctl perform failover
11:58:00 53224 INFO  Listening monitor notifications about state changes in formation "default" and group 0
11:58:00 53224 INFO  Following table displays times when notifications are received
    Time |  ID |      Host |   Port |      Current State |     Assigned State
---------+-----+-----------+--------+--------------------+-------------------
11:58:01 |   1 | localhost |   5001 |            primary |           draining
11:58:01 |   2 | localhost |   5002 |          secondary |  prepare_promotion
11:58:01 |   1 | localhost |   5001 |           draining |           draining
11:58:01 |   2 | localhost |   5002 |  prepare_promotion |  prepare_promotion
11:58:02 |   2 | localhost |   5002 |  prepare_promotion |   stop_replication
11:58:02 |   1 | localhost |   5001 |           draining |     demote_timeout
11:58:03 |   1 | localhost |   5001 |     demote_timeout |     demote_timeout
11:58:04 |   2 | localhost |   5002 |   stop_replication |   stop_replication
11:58:05 |   2 | localhost |   5002 |   stop_replication |       wait_primary
11:58:05 |   1 | localhost |   5001 |     demote_timeout |            demoted
11:58:05 |   2 | localhost |   5002 |       wait_primary |       wait_primary
11:58:05 |   1 | localhost |   5001 |            demoted |            demoted
11:58:06 |   1 | localhost |   5001 |            demoted |         catchingup
11:58:06 |   1 | localhost |   5001 |         catchingup |         catchingup
11:58:08 |   1 | localhost |   5001 |         catchingup |          secondary
11:58:08 |   2 | localhost |   5002 |       wait_primary |            primary
11:58:08 |   1 | localhost |   5001 |          secondary |          secondary
11:58:08 |   2 | localhost |   5002 |            primary |            primary

Again, timings and PID numbers are not expected to be the same when you run the command on your own setup.

Also note in the output that the command shows the whole set of transitions including when the old primary is now a secondary node. The database is available for read-write traffic as soon as we reach the state wait_primary.

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 triggered, 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:

$ pg_autoctl perform switchover

Because the subtelties of orchestrating either a controlled switchover or an unplanned failover are all handled by the monitor, rather than the client side command line, at the client level the two command pg_autoctl perform failover and pg_autoctl perform switchover are synonyms, or aliases.

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.