Zero downtime migrations

August 15, 2015

For any sufficiently large application, you want to minimize interruptions to service while deploying new code. This is especially challenging for migrations on Heroku and other Platforms-as-a-Service, where you have a Catch-22 problem; you want to run your migrations, first, but you can’t run them until you’ve deployed the migrations to production. Usually, at this point, your new migration-dependent features are in the deploy, too. So when your application restarts, and the migration hasn’t run, yet, your poor application will trip some exceptions, and perhaps, create problems for your users. Of course, you could put the app into maintenance mode, but that creates more downtime for your users.

The technique described hereWe try very hard to make migrations non-destructive so there’s no rollback required. The alternative is much more complicated. This also assumes that all QA tests are complete and the next task on your checklist is to deploy to production. will allow you to gather your migrations into a separate, deployable, commit, that you can run before you deploy your features, thus minimizing downtime.

The Recipe

0. On a clean, working copy of your code

git checkout master

1. Create a patch file of all migrations, added since your last production deploy

git fetch production
 git diff production/master... -- 'db/migrate/' > patch0

The production [git] remote is on Heroku. It can be named anything, but we call it that.

2. Check out a branch, starting at production/master’s head

 git checkout production/master -b pre-deploy-migrations

3. Reset your development database to current state of the production database

WARNING This will delete all of your data in your development environment. But that should not be a problem, right? If it is, you have a problem with your development environment.

 rake db:reset

This also assumes that you’ve been diligent, OCD-diligent, about keeping your schema.rb file synchronized with production.

4. Apply the patch file

patch -p1 < patch0

5. Run your migrations

rake db:migrate

6. Add and commit your changes

git add db/migrate/*
 git add db/schema.rb
 git commit -m "pre-deploy migrations only"
 git status # => should be "reasonably" empty

7. Re-run your test suite to make sure nothing ‘asplodes

What happens if your tests do break? You might have a destructive migration! For example, I recently changed a table from single to polymorphic ownership. Instead of adding the columns, owner_id and owner_type, I renamed organization_id to owner_id. A whole bunch of tests broke. If you continue with these steps, your app will crash when you run the migrations. See Appendix A for some work-arounds.

8. Compare your commit against master

Make sure that nothing snuck in by accident. Conceptually, schema.rb will represent the state of the database after the migrations have been run, and should exactly match the schema that we want to have for our new feature.

git diff head..origin/master -- 'db/migrate/' # => This should return zero changes!
 git diff head..origin/master -- 'db/schema.rb' # => This should return zero changes!

There might be differences that are okay. You’ll have to do a manual code review to be sure.

9. Push just this commit into production

You can do it by pushing the branch, or merging the commit into your deploy / mainline, rebase it ahead of your other commits, then push just that sha. We use the heroku_san gem, so, for us, it is a simple shell command (while still on this branch).

rake production deploy

10. Run your migrations on your production instance

heroku run:detached rake db:migrate -r production

Is this a long running migration? Don’t want Heroku to hang up before your done? What’s this run:detached?

11. Merge the branch into master

It should be a no-op since the code is already there. This is optional, but it keeps git happy for the next deploy.

git checkout master
 git merge -m "Merge branch 'pre-deploy-migrations'" pre-deploy-migrations 
 git push origin master

12. You are ready to deploy to production

rake production deploy

13. Clean up

git branch -D pre-deploy-migrations
 rm patch0

14. Bask in the glow of your mastery!


Appendix A: How to Handle Deprecated Columns

Don’t remove them! That would break your existing production code and any rollbacks become infinitely more difficult. Many practitioners never remove old columns, but I think that’s bad too. Code costs. Maintaining these extra columns, over time, will cost you time and money. Here’s a way to “soft” remove columns until you are sure that you won’t need them again. I schedule a chore a few iterations in the future with the task of doing the actual removal.

class YourModel < ActiveRecord::Base
  ...
  # TODO: https://www.pivotaltracker.com/story/show/...
  def self.columns
    super.reject { |c| c.name == "_column_name_" }
  end
  ...
 end

Appendix B: Copy Data to New Columns

Making copies to a new column is safe (and “disks” are “cheap”), although it does open a window where your data will be inconsistent. In your migrations, make SQL calls to copy a column from the old to the new. There is a down side to this, especially on very large tables. You want tall, narrow tables for efficiency and performance. This does neither. So plan accordingly.

class YourMigration < ActiveRecord::Migration
  def change
    add_column :_table_name, :_new_column_name_, :string

    ActiveRecord::Base.connection.execute <<-SQL
      UPDATE _table_name_ SET _new_column_name_ = _old_column_name_;
    SQL

If you are using MySQL, you might need the help of the Large Hadron Migrator to keep your site up and running while you copy over the data.

Zero Downtime Migrations - August 15, 2015 - Ken Mayer