Patroni

Some of the PostgreSQL parameters must hold the same values on the primary and the replicas. For those, values set either in the local patroni configuration files or via the environment variables take no effect. To alter or set their values one must change the shared configuration in the DCS. Below is the actual list of such parameters together with the default values:

For the parameters below, PostgreSQL does not require equal values among the primary and all the replicas. However, considering the possibility of a replica to become the primary at any time, it doesn’t really make sense to set them differently; therefore, Patroni restricts setting their values to the dynamic configuration.

These parameters are validated to ensure they are sane, or meet a minimum value.

There are some other Postgres parameters controlled by Patroni:

To be on the safe side parameters from the above lists are not written into postgresql.conf, but passed as a list of arguments to the pg_ctl`+ ``+start` which gives them the highest precedence, even above ALTER SYSTEM

There also are some parameters like postgresql.listen, postgresql.data_dir that can be set only locally, i.e. in the Patroni config file or via configuration variable. In most cases the local configuration will override the dynamic configuration.

When applying the local or dynamic configuration options, the following actions are taken:

The parameters would be applied in the following order (run-time are given the highest priority):

This allows configuration for all the nodes (2), configuration for a specific node using ALTER`+ ``+SYSTEM` (3) and ensures that parameters essential to the running of Patroni are enforced (4), as well as leaves room for configuration tools that manage postgresql.conf directly without involving Patroni (1).

PostgreSQL has some parameters that determine the size of the shared memory used by them:

Changing these parameters require a PostgreSQL restart to take effect, and their shared memory structures cannot be smaller on the standby nodes than on the primary node.

As explained before, Patroni restrict changing their values through dynamic configuration, which usually consists of:

Note: please keep in mind that you should perform a restart of the PostgreSQL nodes through patronictl restart command, or via REST API /restart endpoint. An attempt to restart PostgreSQL by restarting the Patroni daemon, e.g. by executing systemctl+ ``+restart+ +`patroni`, can cause a failover to occur in the cluster, if you are restarting the primary node.

However, as those settings manage shared memory, some extra care should be taken when restarting the nodes:

Note: if you attempt to restart all nodes in one go after decreasing the value of any of those settings, Patroni will ignore the change and restart the standby with the original setting value, thus requiring that you restart the standbys again later. Patroni does that to prevent the standby to enter in an infinite crash loop, because PostgreSQL quits with a FATAL message if you attempt to set any of those parameters to a value lower than what is visible in pg_controldata on the Standby node. In other words, we can only decrease the setting on the standby once its pg_controldata is up-to-date with the primary in regards to these changes on the primary.

More information about that can be found at PostgreSQL Administrator’s Overview.

Also the following Patroni configuration options can be changed only dynamically:

Upon changing these options, Patroni will read the relevant section of the configuration stored in DCS and change its run-time values.

Patroni nodes are dumping the state of the DCS options to disk upon for every change of the configuration into the file patroni.dynamic.json located in the Postgres data directory. Only the leader is allowed to restore these options from the on-disk dump if these are completely absent from the DCS or if they are invalid.