understanding patroni

understanding_patroni.md

There are two fundamental stages in Patroni:

• bootstrap
• running the cluster

Bootstrap stage

In the bootstrap stage, Patroni will:

• generate DCS (etcd, zookeper, consul, etc.) contents
• generate a postgres configuration

Patroni detects that it has not yet bootstrapped by checking whether both:

• the /initialize key exists in the defined namespace in the DCS

• and the configured postgresql.data_dir directory exists

The most simple case is when postgresql.data_dir is empty and there is no /initialize key in the DCS.

Then Patroni will try to create the /initialize key and on success will either run initdb or the configured custom bootstrap script, both of which will create the postgresql.data_dir.

If initdb or custom bootstrap fails then Patroni will remove the /initialize key from the DCS.

If Patroni gets terminated during bootstrap then the /initialize key will be set and needs to be removed manually.

Once postgres is up and running Patroni will put the cluster system identifier into the /initialize key.

Running the cluster

When Patroni runs the cluster:

• it first determines who in the cluster is the leader via DCS

• accordingly the participants in the cluster will resync their database contents from the leader