Frequently Asked Questions

Those questions have been asked in GitHub issues for the project by several people. If you have more questions, feel free to open a new issue, and your question and its answer might make it to this FAQ.

The secondary is blocked in the CATCHING_UP state, what should I do?

In the pg_auto_failover design, the following two things are needed for the monitor to be able to orchestrate nodes integration completely:

  1. Health Checks must be successful

    The monitor runs periodic health checks with all the nodes registered in the system. Those health checks are Postgres connections from the monitor to the registered Postgres nodes, and use the hostname and port as registered.

    The pg_autoctl show state commands column Reachable contains “yes” when the monitor could connect to a specific node, “no” when this connection failed, and “unknown” when no connection has been attempted yet, since the last startup time of the monitor.

    The Reachable column from pg_autoctl show state command output must show a “yes” entry before a new standby node can be orchestrated up to the “secondary” goal state.

  2. pg_autoctl service must be running

    The pg_auto_failover monitor works by assigning goal states to individual Postgres nodes. The monitor will not assign a new goal state until the current one has been reached.

    To implement a transition from the current state to the goal state assigned by the monitor, the pg_autoctl service must be running on every node.

When your new standby node stays in the “catchingup” state for a long time, please check that the node is reachable from the monitor given its hostname and port known on the monitor, and check that the pg_autoctl run command is running for this node.

When things are not obvious, the next step is to go read the logs. Both the output of the pg_autoctl command and the Postgres logs are relevant. See the Should I read the logs? Where are the logs? question for details.

Should I read the logs? Where are the logs?

Yes. If anything seems strange to you, please do read the logs.

As maintainers of the pg_autoctl tool, we can’t foresee everything that may happen to your production environment. Still, a lot of efforts is spent on having a meaningful output. So when you’re in a situation that’s hard to understand, please make sure to read the pg_autoctl logs and the Postgres logs.

When using systemd integration, the pg_autoctl logs are then handled entirely by the journal facility of systemd. Please then refer to journalctl for viewing the logs.

The Postgres logs are to be found in the $PGDATA/log directory with the default configuration deployed by pg_autoctl create .... When a custom Postgres setup is used, please refer to your actual setup to find Postgres logs.

The state of the system is blocked, what should I do?

This question is a general case situation that is similar in nature to the previous situation, reached when adding a new standby to a group of Postgres nodes. Please check the same two elements: the monitor health checks are successful, and the pg_autoctl run command is running.

When things are not obvious, the next step is to go read the logs. Both the output of the pg_autoctl command and the Postgres logs are relevant. See the Should I read the logs? Where are the logs? question for details.

The monitor is a SPOF in pg_auto_failover design, how should we handle that?

When using pg_auto_failover, the monitor is needed to make decisions and orchestrate changes in all the registered Postgres groups. Decisions are transmitted to the Postgres nodes by the monitor assigning nodes a goal state which is different from their current state.

Consequences of the monitor being unavailable

Nodes contact the monitor each second and call the node_active stored procedure, which returns a goal state that is possibly different from the current state.

The monitor only assigns Postgres nodes with a new goal state when a cluster wide operation is needed. In practice, only the following operations require the monitor to assign a new goal state to a Postgres node:

  • a new node is registered

  • a failover needs to happen, either triggered automatically or manually

  • a node is being put to maintenance

  • a node replication setting is being changed.

When the monitor node is not available, the pg_autoctl processes on the Postgres nodes will fail to contact the monitor every second, and log about this failure. Adding to that, no orchestration is possible.

The Postgres streaming replication does not need the monitor to be available in order to deliver its service guarantees to your application, so your Postgres service is still available when the monitor is not available.

To repair your installation after having lost a monitor, the following scenarios are to be considered.

The monitor node can be brought up again without data having been lost

This is typically the case in Cloud Native environments such as Kubernetes, where you could have a service migrated to another pod and re-attached to its disk volume. This scenario is well supported by pg_auto_failover, and no intervention is needed.

It is also possible to use synchronous archiving with the monitor so that it’s possible to recover from the current archives and continue operating without intervention on the Postgres nodes, except for updating their monitor URI. This requires an archiving setup that uses synchronous replication so that any transaction committed on the monitor is known to have been replicated in your WAL archive.

At the moment, you have to take care of that setup yourself. Here’s a quick summary of what needs to be done:

  1. Schedule base backups

    Use pg_basebackup every once in a while to have a full copy of the monitor Postgres database available.

  2. Archive WAL files in a synchronous fashion

    Use pg_receivewal --sync ... as a service to keep a WAL archive in sync with the monitor Postgres instance at all time.

  3. Prepare a recovery tool on top of your archiving strategy

    Write a utility that knows how to create a new monitor node from your most recent pg_basebackup copy and the WAL files copy.

    Bonus points if that tool/script is tested at least once a day, so that you avoid surprises on the unfortunate day that you actually need to use it in production.

A future version of pg_auto_failover will include this facility, but the current versions don’t.

The monitor node can only be built from scratch again

If you don’t have synchronous archiving for the monitor set-up, then you might not be able to restore a monitor database with the expected up-to-date node metadata. Specifically we need the nodes state to be in sync with what each pg_autoctl process has received the last time they could contact the monitor, before it has been unavailable.

It is possible to register nodes that are currently running to a new monitor without restarting Postgres on the primary. For that, the procedure mentionned in Replacing the monitor online must be followed, using the following commands:

$ pg_autoctl disable monitor
$ pg_autoctl enable monitor