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