81 lines
2.8 KiB
Markdown
81 lines
2.8 KiB
Markdown
---
|
|
stage: Data Stores
|
|
group: Database
|
|
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
|
|
---
|
|
|
|
# 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 must 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:
|
|
|
|
```shell
|
|
bundle exec rake db:migrate
|
|
```
|
|
|
|
This however skips post deployment migrations:
|
|
|
|
```shell
|
|
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:
|
|
|
|
```shell
|
|
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 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:
|
|
|
|
```shell
|
|
bundle exec rails g post_deployment_migration migration_name_here
|
|
```
|
|
|
|
This generates 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
|
|
1. Run the migration removing the column
|
|
1. 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
|
|
1. 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
|