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diff --git a/doc/development/post_deployment_migrations.md b/doc/development/post_deployment_migrations.md new file mode 100644 index 00000000000..cfc91539bee --- /dev/null +++ b/doc/development/post_deployment_migrations.md @@ -0,0 +1,75 @@ +# 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 |