Migrating to Single Table Inheritance

How Shopify migrated from a regular ActiveRecord model to a Single Table Inheritance (STI) with zero downtime

Steps include:
- Adding a type column
- Renaming the table
- Creating the STI

Transcript

1. Migrating to STI with no downtime! The purpose being ‘no downtime’
2. gmalette @  
4. class CustomerGroup < AR::Base Based on a real-life scenario at shopify
5. CustomerGroups are really just filters applied to a merchant's customers.
6. class CustomerSearch class ProductSearch class OrderSearch Merchants wanted to have it on more models
7. You could go with different tables All tables have the same structure Technical debt
8. class CustomerSearch < Search class ProductSearch < Search class Search < AR::Base class OrderSearch < Search Single Table Inheritance Allows having `Search < AR::Base` and models inheriting from Search
9. Single Table Inheritance (STI) Using a single table to manage model inheritance tree
10. Single Table Inheritance
11. Class Single Table Inheritance When Rails loads the models, it instantiates the class in the `type` column
12. Multiple step operation This migration is a multi-step operation
13. Step 1: Add the `type` column Step 1: Add the `type` column Adding columns in MySQL takes time and blocks the table.
14. Large Hadron Migrator (LHM) from SoundCloud
16. New table Large Hadron Migrator
17. Setup triggers Large Hadron Migrator → →
18. Copy data Large Hadron Migrator →
19. Rename tables atomically Large Hadron Migrator → →
20.       Code, Push, Migrate, Restart
21.        Add a step to deployment, LHM Think of LHM as asynchronous, non-blocking migrations
22. def self.up Lhm.change_table :customer_groups do |m| m.add_column :type, "VARCHAR(32) DEFAULT 'CustomerSearch'" end end Large Hadron Migrator After this we’ll have a `type` column What will be the problem if we push this as-is?
23. ActiveRecord::SubclassNotFound Rails cannot find the `CustomerSearch` class
24. def self.columns super.reject { |col| col.name == "type"} end Ignore the type for now
25. Step 2: Creating a view At some point in time, both the old and new code will be running Both `customer_groups` and `searches` tables will be expected Table rename is out of the scope of responsibilities for LHM
27. def up execute "CREATE VIEW searches AS SELECT * FROM customer_groups;" end
28. Step 3: Pushing the STI
29. class CustomerSearch < Search class ProductSearch < Search class Search < AR::Base class OrderSearch < Search Write these classes
30. Easy enough? This should be enough?
31. Easy enough? Not so fast!
32. Notice the default value is 0, and extra doesn’t show autoincrement The id is also not described as being the primary key
33. class Search < ActiveRecord::Base self.primary_key = :id after_initialize :reset_id def reset_id self.id = nil if self.id == 0 end
34. self.primary_key = :id Tell AR to use id as the PK
35. after_initialize :reset_id def reset_id self.id = nil if self.id == 0 end Whenever a model is initialized with ID 0, reset it to nil Rails will insert as-is, MySQL will auto increment and everything will be fine
36. # customer_group.rb CustomerGroup = CustomerSearch Alias CustomerGroup to CustomerSearch Useful if models have polymorphic associations to this model
37. Step 4: Rename tables + Cleanup
38. N def up execute("RENAME TABLE searches TO searches_view_to_drop, customer_groups TO searches") execute("DROP VIEW searches_view_to_drop") end
39. N class Search < ActiveRecord::Base self.primary_key = :id after_initialize :reset_id def reset_id self.id = nil if self.id == 0 end
40. Thanks! Questions?