# Patroni configuration

Patroni configuration is stored in the DCS (Distributed Configuration Store). There are 3 types of configuration:

- Dynamic configuration.
    These options can be set in DCS at any time. If the options changed are not part of the startup configuration, they are applied asynchronously (upon the next wake up cycle) to every node, which gets subsequently reloaded. If the node requires a restart to apply the configuration (for options with context postmaster, if their values have changed), a special flag, `pending_restart` indicating this, is set in the members.data JSON. Additionally, the node status also indicates this, by showing `"restart_pending": true`.
- Local configuration (patroni.yml).
    These options are defined in the configuration file and take precedence over dynamic configuration. patroni.yml could be changed and reloaded in runtime (without restart of Patroni) by sending SIGHUP to the Patroni process, performing `POST /reload` REST-API request or executing `patronictl reload`.
- Environment configuration.
    It is possible to set/override some of the “Local” configuration parameters with environment variables. Environment configuration is very useful when you are running in a dynamic environment and you don’t know some of the parameters in advance (for example it’s not possible to know your external IP address when you are running inside `docker`).

Some of the PostgreSQL parameters must hold the same values on the master 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:

- max_connections: 100
- max_locks_per_transaction: 64
- max_worker_processes: 8
- max_prepared_transactions: 0
- wal_level: hot_standby
- wal_log_hints: on
- track_commit_timestamp: off

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

- max_wal_senders: 5
- max_replication_slots: 5
- wal_keep_segments: 8

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

There are some other Postgres parameters controlled by Patroni:

- listen_addresses - is set either from `postgresql.listen` or from `PATRONI_POSTGRESQL_LISTEN` environment variable
- port - is set either from `postgresql.listen` or from `PATRONI_POSTGRESQL_LISTEN` environment variable
- cluster_name - is set either from `scope` or from `PATRONI_SCOPE` environment variable
- hot_standby: on

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

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

- The node first checks if there is a postgresql.base.conf or if the `custom_conf` parameter is set.
- If the custom_conf parameter is set, it will take the file specified on it as a base configuration, ignoring postgresql.base.conf and postgresql.conf.
- If the custom_conf parameter is not set and postgresql.base.conf exists, it contains the renamed “original” configuration and it will be used as a base configuration.
- If there is no custom_conf nor postgresql.base.conf, the original postgresql.conf is taken and renamed to postgresql.base.conf.
- The dynamic options (with the exceptions above) are dumped into the postgresql.conf and an include is set in postgresql.conf to the used base configuration (either postgresql.base.conf or what is on `custom_conf`). Therefore, we would be able to apply new options without re-reading the configuration file to check if the include is present not.
- Some parameters that are essential for Patroni to manage the cluster are overridden using the command line.
- If some of the options that require restart are changed (we should look at the context in pg_settings and at the actual values of those options), a pending_restart flag of a given node is set. This flag is reset on any restart.

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

1. load parameters from file postgresql.base.conf (or from a custom_conf file, if set)
2. load parameters from file postgresql.conf
3. load parameters from file postgresql.auto.conf
4. run-time parameter using -o –name=value

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).

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

- ttl: 30
- loop_wait: 10
- retry_timeouts: 10
- maximum_lag_on_failover: 1048576
- check_timeline: false
- postgresql.use_slots: true

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 master is allowed to restore these options from the on-disk dump if these are completely absent from the DCS or if they are invalid.