Support for post deployment migrationsbackground-migrations

These are regular Rails migrations that are executed by default. A user
can opt-out of these migrations by setting an environment variable
during the deployment process.

Fixes gitlab-org/gitlab-ce#22133

2 files changed, 76 insertions, 0 deletions

diff --git a/doc/development/README.md b/doc/development/README.md
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@@ -41,6 +41,7 @@
 
 - [What requires downtime?](what_requires_downtime.md)
 - [Adding database indexes](adding_database_indexes.md)
+- [Post Deployment Migrations](post_deployment_migrations.md)
 
 ## Compliance
 
diff --git a/doc/development/post_deployment_migrations.md b/doc/development/post_deployment_migrations.md
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+# Post Deployment Migrations
+
+Post deployment migrations are regular Rails migrations that can optionally be
+executed after a deployment. By default these migrations are executed alongside
+the other migrations. To skip these migrations you will have to set the
+environment variable `SKIP_POST_DEPLOYMENT_MIGRATIONS` to a non-empty value
+when running `rake db:migrate`.
+
+For example, this would run all migrations including any post deployment
+migrations:
+
+```bash
+bundle exec rake db:migrate
+```
+
+This however will skip post deployment migrations:
+
+```bash
+SKIP_POST_DEPLOYMENT_MIGRATIONS=true bundle exec rake db:migrate
+```
+
+## Deployment Integration
+
+Say you're using Chef for deploying new versions of GitLab and you'd like to run
+post deployment migrations after deploying a new version. Let's assume you
+normally use the command `chef-client` to do so. To make use of this feature
+you'd have to run this command as follows:
+
+```bash
+SKIP_POST_DEPLOYMENT_MIGRATIONS=true sudo chef-client
+```
+
+Once all servers have been updated you can run `chef-client` again on a single
+server _without_ the environment variable.
+
+The process is similar for other deployment techniques: first you would deploy
+with the environment variable set, then you'll essentially re-deploy a single
+server but with the variable _unset_.
+
+## Creating Migrations
+
+To create a post deployment migration you can use the following Rails generator:
+
+```bash
+bundle exec rails g post_deployment_migration migration_name_here
+```
+
+This will generate the migration file in `db/post_migrate`. These migrations
+behave exactly like regular Rails migrations.
+
+## Use Cases
+
+Post deployment migrations can be used to perform migrations that mutate state
+that an existing version of GitLab depends on. For example, say you want to
+remove a column from a table. This requires downtime as a GitLab instance
+depends on this column being present while it's running. Normally you'd follow
+these steps in such a case:
+
+1. Stop the GitLab instance
+2. Run the migration removing the column
+3. Start the GitLab instance again
+
+Using post deployment migrations we can instead follow these steps:
+
+1. Deploy a new version of GitLab while ignoring post deployment migrations
+2. Re-run `rake db:migrate` but without the environment variable set
+
+Here we don't need any downtime as the migration takes place _after_ a new
+version (which doesn't depend on the column anymore) has been deployed.
+
+Some other examples where these migrations are useful:
+
+* Cleaning up data generated due to a bug in GitLab
+* Removing tables
+* Migrating jobs from one Sidekiq queue to another