# Convert a Standalone to a Patroni Cluster

This section describes the process for converting a standalone PostgreSQL instance into a Patroni cluster.

To deploy a Patroni cluster without using a pre-existing PostgreSQL instance, see Running and Configuring instead.

## Procedure

A Patroni cluster can be started with a data directory from a single-node PostgreSQL database. This is achieved by following closely these steps:

1. Manually start PostgreSQL daemon

2. Create Patroni superuser and replication users as defined in the authentication section of the Patroni configuration. If this user is created in SQL, the following queries achieve this:

    CREATE USER $PATRONI_SUPERUSER_USERNAME WITH SUPERUSER ENCRYPTED PASSWORD '$PATRONI_SUPERUSER_PASSWORD';
    CREATE USER $PATRONI_REPLICATION_USERNAME WITH REPLICATION ENCRYPTED PASSWORD '$PATRONI_REPLICATION_PASSWORD';

3. Start Patroni (e.g. `patroni /etc/patroni/patroni.yml`). It automatically detects that PostgreSQL daemon is already running but its configuration might be out-of-date.

4. Ask Patroni to restart the node with `patronictl restart cluster-name node-name`. This step is only required if PostgreSQL configuration is out-of-date.

# Major Upgrade of PostgreSQL Version

The only possible way to do a major upgrade currently is:

1. Stop Patroni

2. Upgrade PostgreSQL binaries and perform pg_upgrade on the master node

3. Update patroni.yml

4. Remove the initialize key from DCS or wipe complete cluster state from DCS. The second one could be achieved by running `patronictl remove <cluster-name>`. It is necessary because pg_upgrade runs initdb which actually creates a new database with a new PostgreSQL system identifier.

5. If you wiped the cluster state in the previous step, you may wish to copy patroni.dynamic.json from old data dir to the new one. It will help you to retain some PostgreSQL parameters you had set before.

6. Start Patroni on the master node.

7. Upgrade PostgreSQL binaries, update patroni.yml and wipe the data_dir on standby nodes.

8. Start Patroni on the standby nodes and wait for the replication to complete.

Running pg_upgrade on standby nodes is not supported by PostgreSQL. If you know what you are doing, you can try the rsync procedure described in https://www.postgresql.org/docs/current/pgupgrade.html instead of wiping data_dir on standby nodes. The safest way is however to let Patroni replicate the data for you.

## FAQ

- During Patroni startup, Patroni complains that it cannot bind to the PostgreSQL port.

  You need to verify `listen_addresses` and `port` in `postgresql.conf` and `postgresql.listen` in `patroni.yml`. Don’t forget that `pg_hba.conf` should allow such access.

- After asking Patroni to restart the node, PostgreSQL displays the error message `could not open configuration file "/etc/postgresql/10/main/pg_hba.conf": No such file or directory`

  It can mean various things depending on how you manage PostgreSQL configuration. If you specified postgresql.config_dir, Patroni generates the `pg_hba.conf` based on the settings in the bootstrap section only when it bootstraps a new cluster. In this scenario the `PGDATA` was not empty, therefore no bootstrap happened. This file must exist beforehand.