88 lines
3.3 KiB
Markdown
88 lines
3.3 KiB
Markdown
# Migration Style Guide
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When writing migrations for GitLab, you have to take into account that
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these will be ran by hundreds of thousands of organizations of all sizes, some with
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many years of data in their database.
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In addition, having to take a server offline for a an upgrade small or big is
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a big burden for most organizations. For this reason it is important that your
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migrations are written carefully, can be applied online and adhere to the style guide below.
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When writing your migrations, also consider that databases might have stale data
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or inconsistencies and guard for that. Try to make as little assumptions as possible
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about the state of the database.
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Please don't depend on GitLab specific code since it can change in future versions.
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If needed copy-paste GitLab code into the migration to make make it forward compatible.
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## Comments in the migration
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Each migration you write needs to have the two following pieces of information
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as comments.
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### Online, Offline, errors?
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First, you need to provide information on whether the migration can be applied:
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1. online without errors (works on previous version and new one)
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2. online with errors on old instances after migrating
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3. online with errors on new instances while migrating
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4. offline (needs to happen without app servers to prevent db corruption)
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It is always preferable to have a migration run online. If you expect the migration
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to take particularly long (for instance, if it loops through all notes),
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this is valuable information to add.
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### Reversibility
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Your migration should be reversible. This is very important, as it should
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be possible to downgrade in case of a vulnerability or bugs.
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In your migration, add a comment describing how the reversibility of the
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migration was tested.
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## Removing indices
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If you need to remove index, please add a condition like in following example:
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```
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remove_index :namespaces, column: :name if index_exists?(:namespaces, :name)
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```
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## Adding indices
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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.
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## Testing
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Make sure that your migration works with MySQL and PostgreSQL with data. An empty database does not guarantee that your migration is correct.
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Make sure your migration can be reversed.
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## Data migration
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Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of using plain SQL you need to quote all input manually with `quote_string` helper.
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Example with Arel:
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```
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users = Arel::Table.new(:users)
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users.group(users[:user_id]).having(users[:id].count.gt(5))
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#updtae other tables with this results
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```
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Example with plain SQL and `quote_string` helper:
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```
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select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(id) > 1").each do |tag|
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tag_name = quote_string(tag["name"])
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duplicate_ids = select_all("SELECT id FROM tags WHERE name = '#{tag_name}'").map{|tag| tag["id"]}
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origin_tag_id = duplicate_ids.first
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duplicate_ids.delete origin_tag_id
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execute("UPDATE taggings SET tag_id = #{origin_tag_id} WHERE tag_id IN(#{duplicate_ids.join(",")})")
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execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})")
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end
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```
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