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Diffstat (limited to 'doc/development/migration_style_guide.md')
| -rw-r--r-- | doc/development/migration_style_guide.md | 48 |
1 files changed, 45 insertions, 3 deletions
diff --git a/doc/development/migration_style_guide.md b/doc/development/migration_style_guide.md index 28dedf3978c..02e024ca15a 100644 --- a/doc/development/migration_style_guide.md +++ b/doc/development/migration_style_guide.md @@ -8,7 +8,10 @@ In addition, having to take a server offline for a an upgrade small or big is a big burden for most organizations. For this reason it is important that your migrations are written carefully, can be applied online and adhere to the style guide below. -It's advised to have offline migrations only in major GitLab releases. +Migrations should not require GitLab installations to be taken offline unless +_absolutely_ necessary. If a migration requires downtime this should be +clearly mentioned during the review process as well as being documented in the +monthly release post. When writing your migrations, also consider that databases might have stale data or inconsistencies and guard for that. Try to make as little assumptions as possible @@ -58,6 +61,45 @@ remove_index :namespaces, column: :name if index_exists?(:namespaces, :name) If you need to add an unique index please keep in mind there is possibility of existing duplicates. If it is possible write a separate migration for handling this situation. It can be just removing or removing with overwriting all references to these duplicates depend on situation. +When adding an index make sure to use the method `add_concurrent_index` instead +of the regular `add_index` method. The `add_concurrent_index` method +automatically creates concurrent indexes when using PostgreSQL, removing the +need for downtime. To use this method you must disable transactions by calling +the method `disable_ddl_transaction!` in the body of your migration class like +so: + +``` +class MyMigration < ActiveRecord::Migration + disable_ddl_transaction! + + def change + + end +end +``` + +## Adding Columns With Default Values + +When adding columns with default values you should use the method +`add_column_with_default`. This method ensures the table is updated without +requiring downtime. This method is not reversible so you must manually define +the `up` and `down` methods in your migration class. + +For example, to add the column `foo` to the `projects` table with a default +value of `10` you'd write the following: + +``` +class MyMigration < ActiveRecord::Migration + def up + add_column_with_default(:projects, :foo, :integer, 10) + end + + def down + remove_column(:projects, :foo) + end +end +``` + ## Testing Make sure that your migration works with MySQL and PostgreSQL with data. An empty database does not guarantee that your migration is correct. @@ -74,7 +116,7 @@ Example with Arel: users = Arel::Table.new(:users) users.group(users[:user_id]).having(users[:id].count.gt(5)) -#updtae other tables with this results +#update other tables with these results ``` Example with plain SQL and `quote_string` helper: @@ -89,4 +131,4 @@ select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(i execute("UPDATE taggings SET tag_id = #{origin_tag_id} WHERE tag_id IN(#{duplicate_ids.join(",")})") execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})") end -```
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