debian-mirror-gitlab/doc/development/database/required_stops.md

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Adding required stops

Required stops should only be added when it is deemed absolutely necessary, due to their disruptive effect on customers. Before adding a required stop, consider if any alternative approaches exist to avoid a required stop. Sometimes a required stop is unavoidable. In those cases, follow the instructions below.

Common scenarios that require stops

Long running migrations being finalized

If a migration takes a long time, it could cause a large number of customers to encounter timeouts during upgrades. The increased support volume may cause us to introduce a required stop. While any background migration may cause these issues with particularly large customers, we typically only introduce stops when the impact is widespread.

• Cause: When an upgrade takes more than an hour, omnibus times out.
• Mitigation: Schedule finalization for the first minor version after the next required stop.

Improperly finalized background migrations

You may need to introduce a required stop for mitigation when:

• A background migration is not finalized, and

• A migration is written that depends on that background migration.

• Cause: The dependent migration may fail if the background migration is incomplete.

• Mitigation: Ensure that all background migrations are finalized before authoring dependent migrations.

Remove a migration

If a migration is removed, you may need to introduce a required stop to ensure customers don't miss the required change.

• Cause: Dependent migrations may fail, or the application may not function, because a required migration was removed.
• Mitigation: Ensure migrations are only removed after they've been a part of a planned required stop.

A migration timestamp is very old

If a migration timestamp is very old (> 3 weeks, or after a before the last stop), these scenarios may cause issues:

• If the migration depends on another migration with a newer timestamp but introduced in a previous release after a required stop, then the new migration may run sequentially sooner than the prerequisite migration, and thus fail.

• If the migration timestamp ID is before the last, it may be inadvertently squashed when the team squashes other migrations from the required stop.

• Cause: The migration may fail if it depends on a migration with a later timestamp introduced in an earlier version. Or, the migration may be inadvertently squashed after a required stop.

• Mitigation: Aim for migration timestamps to fall inside the release dates and be sure that they are not dated prior to the last required stop.

Bugs in migration related tooling

In a few circumstances, bugs in migration related tooling has required us to introduce stops. While we aim to prevent these in testing, sometimes they happen.

• Cause: There have been a few different causes where we recognized these too late.
• Mitigation: Typically we try to backport fixes for migrations, but in some cases this is not possible.

Before the required stop is released

Before releasing a known required stop, complete these steps. If the required stop is identified after release, the following steps must still be completed:

1. Update upgrade paths to include the new required stop.

2. Communicate the changes with the customer Support and Release management teams.

3. File an issue with the Database group to squash migrations to that version in the next release. Use this template for your issue:

   Title: `Squash migrations to <Required stop version>`
   As a result of the required stop added for <required stop version> we should squash
   migrations up to that version, and update the minimum schema version.
   
   Deliverables:
   - [ ] Migrations are squashed up to <required stop version>
   - [ ] `Gitlab::Database::MIN_SCHEMA_VERSION` matches init_schema version
   
   /label ~"group::database" ~"section::enablement" ~"devops::data_stores" ~"Category:Database" ~"type::maintenance"
   /cc @gitlab-org/database-team/triage
   

In the release following the required stop

1. Update Gitlab::Database::MIN_SCHEMA_GITLAB_VERSION in lib/gitlab/database.rb to the new required stop versions. Do not change Gitlab::Database::MIN_SCHEMA_VERSION.