Support PostgreSQL DB migration and initialization for Auto DevOps

Problem to solve

Apps need to have databases configured before they work, and migrations run when they're updated. Auto DevOps doesn't currently support methods to initialize or migrate databases.

Further details

When apps are first deployed, they sometimes need a database to be initialized before they'll even start running.

When apps are updated and migrations are needed, they'll sometimes fail until the migrations are performed

Proposal

Option 1
Implement support for db initilization and migration for auto devops.

On initialize:

• Create post install hook
• initialize
• restart app

On migration:

• Create pre-upgrade hook
• run migration

Pseudocode:

• if $DB_INITIALIZE

  • Create post-install hook:
    • run $DB_INITIALIZE
    • restart app pods

• if $DB_MIGRATE

  • Create pre-upgrade hook:
    • run $DB_MIGRATE

Option 2
Split up the helm chart into a db chart and an app chart. This allows for separate deployment of db with a post-install hook to initialize. Application can then be started after db in configured.

Option 3
User kubernetes custom controllers (operators) to do this outside of helm. There seem to be already some existing postgres operators that could be leveraged.

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Notes/Questions

• How can we know how to run database commands for every language? Some buildpacks have this built in, but some don't (notably Ruby/Rails buildpack). Ideally we'd just rely on Herokuish to configure this for us, but it doesn't work. Even if we have good defaults, some teams may need to customize their database initialization and migration commands.

• Keep env variables generic (ie $initialize, $pre_deploy, $post_deploy

Links / references

• Helm hooks
• Heroku Release Phase
• https://gitlab.com/gitlab-org/gitlab-ce/issues/33520

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