Migrate database data using pg_dump / pg_restore

This guide describes database data migration only. To migrate charms on new juju interfaces, refer to the guide How to integrate a database with my charm.

Do you need to migrate?

A database migration is only required if the output of the following command is latest/stable:

juju show-application postgresql | yq '.[] | .channel'

Migration is not necessary if the output above is 14/stable!

This guide can be used to copy data between different installations of the same (modern) charm postgresql, but the backup/restore is more recommended for migrations between modern charms.

Summary

The legacy VM charm archived in the latest/stable channel, read more here. A minor difference in commands might be necessary for different revisions and/or Juju versions, but the general logic remains:

• Deploy the modern charm nearby
• Request credentials from legacy charm
• Remove relation to legacy charm (to stop data changes)
• Perform legacy DB dump (using the credentials above)
• Upload the legacy charm dump into the modern charm
• Add relation to modern charm
• Validate results and remove legacy charm

Prerequisites

• Your application is compatible with Charmed PostgreSQL VM
• A client machine with access to the deployed legacy charm
• juju v.2.9 or later (check Juju 3.0 Release Notes for more information about key differences)
• Enough storage in the cluster to support backup/restore of the databases.

Obtain existing database credentials

To obtain credentials for existing databases, execute the following commands for each database that will be migrated. Take note of these credentials for future steps.

First, define and tune your application and db (database) names. For example:

CLIENT_APP=< my-application/0 >
OLD_DB_APP=< legacy-postgresql/leader | postgresql/0 >
NEW_DB_APP=< new-postgresql/leader | postgresql/0 >
DB_NAME=< your_db_name_to_migrate >

Then, obtain the username from the existing legacy database via its relation info:

OLD_DB_USER=$(juju show-unit ${CLIENT_APP} | yq '.[] | .relation-info | select(.[].endpoint == "db") | .[0].application-data.user')

Deploy new PostgreSQL databases and obtain credentials

Deploy new PostgreSQL database charm:

juju deploy postgresql ${NEW_DB_APP} --channel 14/stable

Obtain operator user password of new PostgreSQL database from PostgreSQL charm:

NEW_DB_USER=operator
NEW_DB_PASS=$(juju run ${NEW_DB_APP} get-password | yq '.password')

Migrate database

Use the credentials and information obtained in previous steps to perform the database migration with the following procedure.

Create dump from legacy charm

Remove the relation between application charm and legacy charm:

juju remove-relation  ${CLIENT_APP}  ${OLD_DB_APP}

Connect to the database VM of a legacy charm:

juju ssh ${OLD_DB_APP} bash

Create a dump via Unix socket using credentials from the relation:

mkdir -p /srv/dump/
OLD_DB_DUMP="legacy-postgresql-${DB_NAME}.sql"
pg_dump -Fc -h /var/run/postgresql/ -U ${OLD_DB_USER} -d ${DB_NAME} > "/srv/dump/${OLD_DB_DUMP}"

Exit the database VM:

exit

Upload dump to new charm

Fetch dump locally and upload it to the new Charmed PostgreSQL charm:

juju scp ${OLD_DB_APP}:/srv/dump/${OLD_DB_DUMP}  ./${OLD_DB_DUMP}
juju scp ./${OLD_DB_DUMP}  ${NEW_DB_APP}:.

ssh into new Charmed PostgreSQL charm and create a new database (using ${NEW_DB_PASS}):

juju ssh ${NEW_DB_APP} bash
createdb -h localhost -U ${NEW_DB_USER} --password ${DB_NAME}

Restore the dump (using ${NEW_DB_PASS}):

pg_restore -h localhost -U ${NEW_DB_USER} --password -d ${DB_NAME} --no-owner --clean --if-exists ${OLD_DB_DUMP}

Integrate with modern charm

Integrate (formerly “relate” in juju v.2.9) your application and new PostgreSQL database charm (using the modern database endpoint)

juju integrate ${CLIENT_APP}  ${NEW_DB_APP}:database

If the database endpoint (from the postgresql_client interface) is not yet supported, use instead the db endpoint from the legacy pgsql interface:

juju integrate ${CLIENT_APP}  ${NEW_DB_APP}:db

Verify database migration

Test your application to make sure the data is available and in a good condition.

Remove old databases

Test your application and if you are happy with a data migration, do not forget to remove legacy charms to keep the house clean:

juju remove-application --destroy-storage <legacy_postgresql>