858 lines
27 KiB
Markdown
858 lines
27 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 run 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 an upgrade small or big is a
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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
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guide below.
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Migrations are **not** allowed to require GitLab installations to be taken
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offline unless _absolutely necessary_.
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When downtime is necessary the migration has to be approved by:
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1. The VP of Engineering
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1. A Backend Maintainer
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1. A Database Maintainer
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An up-to-date list of people holding these titles can be found at
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<https://about.gitlab.com/company/team/>.
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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 few assumptions as
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possible about the state of the database.
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Please don't depend on GitLab-specific code since it can change in future
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versions. If needed copy-paste GitLab code into the migration to make it forward
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compatible.
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For GitLab.com, please take into consideration that regular migrations (under `db/migrate`)
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are run before [Canary is deployed](https://about.gitlab.com/handbook/engineering/infrastructure/library/canary/#configuration-and-deployment),
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and post-deployment migrations (`db/post_migrate`) are run after the deployment to production has finished.
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## Schema Changes
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Changes to the schema should be commited to `db/structure.sql`. This
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file is automatically generated by Rails, so you normally should not
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edit this file by hand. If your migration is adding a column to a
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table, that column will be added at the bottom. Please do not reorder
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columns manually for existing tables as this will cause confusing to
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other people using `db/structure.sql` generated by Rails.
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When your local database in your GDK is diverging from the schema from
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`master` it might be hard to cleanly commit the schema changes to
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Git. In that case you can use the `scripts/regenerate-schema` script to
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regenerate a clean `db/structure.sql` for the migrations you're
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adding. This script will apply all migrations found in `db/migrate`
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or `db/post_migrate`, so if there are any migrations you don't want to
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commit to the schema, rename or remove them. If your branch is not
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targetting `master` you can set the `TARGET` environment variable.
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```sh
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# Regenerate schema against `master`
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scripts/regenerate-schema
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# Regenerate schema against `12-9-stable-ee`
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TARGET=12-9-stable-ee scripts/regenerate-schema
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```
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## What Requires Downtime?
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The document ["What Requires Downtime?"](what_requires_downtime.md) specifies
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various database operations, such as
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- [adding, dropping, and renaming columns](what_requires_downtime.md#adding-columns)
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- [changing column constraints and types](what_requires_downtime.md#changing-column-constraints)
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- [adding and dropping indexes, tables, and foreign keys](what_requires_downtime.md#adding-indexes)
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and whether they require downtime and how to work around that whenever possible.
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## Downtime Tagging
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Every migration must specify if it requires downtime or not, and if it should
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require downtime it must also specify a reason for this. This is required even
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if 99% of the migrations won't require downtime as this makes it easier to find
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the migrations that _do_ require downtime.
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To tag a migration, add the following two constants to the migration class'
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body:
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- `DOWNTIME`: a boolean that when set to `true` indicates the migration requires
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downtime.
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- `DOWNTIME_REASON`: a String containing the reason for the migration requiring
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downtime. This constant **must** be set when `DOWNTIME` is set to `true`.
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For example:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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DOWNTIME = true
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DOWNTIME_REASON = 'This migration requires downtime because ...'
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def change
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...
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end
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end
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```
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It is an error (that is, CI will fail) if the `DOWNTIME` constant is missing
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from a migration class.
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## Reversibility
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Your migration **must 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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Some migrations cannot be reversed. For example, some data migrations can't be
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reversed because we lose information about the state of the database before the migration.
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You should still create a `down` method with a comment, explaining why
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the changes performed by the `up` method can't be reversed, so that the
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migration itself can be reversed, even if the changes performed during the migration
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can't be reversed:
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```ruby
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def down
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# no-op
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# comment explaining why changes performed by `up` cannot be reversed.
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end
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```
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## Atomicity
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By default, migrations are single transaction. That is, a transaction is opened
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at the beginning of the migration, and committed after all steps are processed.
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Running migrations in a single transaction makes sure that if one of the steps fails,
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none of the steps will be executed, leaving the database in valid state.
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Therefore, either:
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- Put all migrations in one single-transaction migration.
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- If necessary, put most actions in one migration and create a separate migration
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for the steps that cannot be done in a single transaction.
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For example, if you create an empty table and need to build an index for it,
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it is recommended to use a regular single-transaction migration and the default
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rails schema statement: [`add_index`](https://api.rubyonrails.org/v5.2/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index).
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This is a blocking operation, but it won't cause problems because the table is not yet used,
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and therefore it does not have any records yet.
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## Heavy operations in a single transaction
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When using a single-transaction migration, a transaction will hold on a database connection
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for the duration of the migration, so you must make sure the actions in the migration
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do not take too much time: In general, queries executed in a migration need to fit comfortably
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within `15s` on GitLab.com.
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In case you need to insert, update, or delete a significant amount of data, you:
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- Must disable the single transaction with `disable_ddl_transaction!`.
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- Should consider doing it in a [Background Migration](background_migrations.md).
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## Retry mechanism when acquiring database locks
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When changing the database schema, we use helper methods to invoke DDL (Data Definition
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Language) statements. In some cases, these DDL statements require a specific database lock.
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Example:
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```ruby
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def change
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remove_column :users, :full_name, :string
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end
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```
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Executing this migration requires an exclusive lock on the `users` table. When the table
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is concurrently accessed and modified by other processes, acquiring the lock may take
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a while. The lock request is waiting in a queue and it may also block other queries
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on the `users` table once it has been enqueued.
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More information about PostgresSQL locks: [Explicit Locking](https://www.postgresql.org/docs/current/explicit-locking.html)
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For stability reasons, GitLab.com has a specific [`statement_timeout`](../user/gitlab_com/index.md#postgresql)
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set. When the migration is invoked, any database query will have
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a fixed time to execute. In a worst-case scenario, the request will sit in the
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lock queue, blocking other queries for the duration of the configured statement timeout,
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then failing with `canceling statement due to statement timeout` error.
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This problem could cause failed application upgrade processes and even application
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stability issues, since the table may be inaccessible for a short period of time.
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To increase the reliability and stability of database migrations, the GitLab codebase
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offers a helper method to retry the operations with different `lock_timeout` settings
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and wait time between the attempts. Multiple smaller attempts to acquire the necessary
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lock allow the database to process other statements.
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### Examples
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**Removing a column:**
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```ruby
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include Gitlab::Database::MigrationHelpers
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def up
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with_lock_retries do
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remove_column :users, :full_name
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end
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end
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def down
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with_lock_retries do
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add_column :users, :full_name, :string
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end
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end
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```
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**Removing a foreign key:**
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```ruby
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include Gitlab::Database::MigrationHelpers
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def up
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with_lock_retries do
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remove_foreign_key :issues, :projects
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end
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end
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def down
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with_lock_retries do
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add_foreign_key :issues, :projects
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end
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end
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```
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**Changing default value for a column:**
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```ruby
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include Gitlab::Database::MigrationHelpers
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def up
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with_lock_retries do
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change_column_default :merge_requests, :lock_version, from: nil, to: 0
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end
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end
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def down
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with_lock_retries do
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change_column_default :merge_requests, :lock_version, from: 0, to: nil
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end
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end
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```
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**Creating a new table with a foreign key:**
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We can simply wrap the `create_table` method with `with_lock_retries`:
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```ruby
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def up
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with_lock_retries do
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create_table :issues do |t|
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t.references :project, index: true, null: false, foreign_key: { on_delete: :cascade }
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t.string :title, limit: 255
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end
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end
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end
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def down
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drop_table :issues
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end
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```
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**Creating a new table when we have two foreign keys:**
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For this, we'll need three migrations:
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1. Creating the table without foreign keys (with the indices).
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1. Add foreign key to the first table.
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1. Add foreign key to the second table.
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Creating the table:
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```ruby
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def up
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create_table :imports do |t|
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t.bigint :project_id, null: false
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t.bigint :user_id, null: false
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t.string :jid, limit: 255
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end
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add_index :imports, :project_id
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add_index :imports, :user_id
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end
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def down
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drop_table :imports
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end
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```
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Adding foreign key to `projects`:
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```ruby
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include Gitlab::Database::MigrationHelpers
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def up
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with_lock_retries do
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add_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
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end
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end
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def down
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with_lock_retries do
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remove_foreign_key :imports, column: :project_id
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end
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end
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```
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Adding foreign key to `users`:
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```ruby
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include Gitlab::Database::MigrationHelpers
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def up
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with_lock_retries do
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add_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
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end
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end
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def down
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with_lock_retries do
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remove_foreign_key :imports, column: :user_id
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end
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end
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```
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### When to use the helper method
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The `with_lock_retries` helper method can be used when you normally use
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standard Rails migration helper methods. Calling more than one migration
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helper is not a problem if they're executed on the same table.
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Using the `with_lock_retries` helper method is advised when a database
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migration involves one of the high-traffic tables:
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- `users`
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- `projects`
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- `namespaces`
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- `ci_pipelines`
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- `ci_builds`
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- `notes`
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Example changes:
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- `add_foreign_key` / `remove_foreign_key`
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- `add_column` / `remove_column`
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- `change_column_default`
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- `create_table` / `drop_table`
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**Note:** `with_lock_retries` method **cannot** be used with `disable_ddl_transaction!`.
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**Note:** `with_lock_retries` method **cannot** be used within the `change` method, you must manually define the `up` and `down` methods to make the migration reversible.
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### How the helper method works
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1. Iterate 50 times.
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1. For each iteration, set a pre-configured `lock_timeout`.
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1. Try to execute the given block. (`remove_column`).
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1. If `LockWaitTimeout` error is raised, sleep for the pre-configured `sleep_time`
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and retry the block.
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1. If no error is raised, the current iteration has successfully executed the block.
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For more information check the [`Gitlab::Database::WithLockRetries`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database/with_lock_retries.rb) class. The `with_lock_retries` helper method is implemented in the [`Gitlab::Database::MigrationHelpers`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/database/migration_helpers.rb) module.
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In a worst-case scenario, the method:
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- Executes the block for a maximum of 50 times over 40 minutes.
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- Most of the time is spent in a pre-configured sleep period after each iteration.
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- After the 50th retry, the block will be executed without `lock_timeout`, just
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like a standard migration invocation.
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- If a lock cannot be acquired, the migration will fail with `statement timeout` error.
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The migration might fail if there is a very long running transaction (40+ minutes)
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accessing the `users` table.
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## Multi-Threading
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Sometimes a migration might need to use multiple Ruby threads to speed up a
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migration. For this to work your migration needs to include the module
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`Gitlab::Database::MultiThreadedMigration`:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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include Gitlab::Database::MigrationHelpers
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include Gitlab::Database::MultiThreadedMigration
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end
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```
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You can then use the method `with_multiple_threads` to perform work in separate
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threads. For example:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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include Gitlab::Database::MigrationHelpers
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include Gitlab::Database::MultiThreadedMigration
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def up
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with_multiple_threads(4) do
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disable_statement_timeout
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# ...
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end
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end
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end
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```
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Here the call to `disable_statement_timeout` will use the connection local to
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the `with_multiple_threads` block, instead of re-using the global connection
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pool. This ensures each thread has its own connection object, and won't time
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out when trying to obtain one.
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**NOTE:** PostgreSQL has a maximum amount of connections that it allows. This
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limit can vary from installation to installation. As a result, it's recommended
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you do not use more than 32 threads in a single migration. Usually, 4-8 threads
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should be more than enough.
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## Removing indexes
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If the table is not empty when removing an index, make sure to use the method
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`remove_concurrent_index` instead of the regular `remove_index` method.
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The `remove_concurrent_index` method drops indexes concurrently, so no locking is required,
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and there is no need for downtime. To use this method, you must disable single-transaction mode
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by calling the method `disable_ddl_transaction!` in the body of your migration
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class like so:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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include Gitlab::Database::MigrationHelpers
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disable_ddl_transaction!
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def up
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remove_concurrent_index :table_name, :column_name
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end
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end
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```
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Note that it is not necessary to check if the index exists prior to
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removing it.
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For a small table (such as an empty one or one with less than `1,000` records),
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it is recommended to use `remove_index` in a single-transaction migration,
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combining it with other operations that don't require `disable_ddl_transaction!`.
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## Adding indexes
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Before adding an index, consider if this one is necessary. There are situations in which an index
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might not be required, like:
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- The table is small (less than `1,000` records) and it's not expected to exponentially grow in size.
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- Any existing indexes filter out enough rows.
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- The reduction in query timings after the index is added is not significant.
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Additionally, wide indexes are not required to match all filter criteria of queries, we just need
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to cover enough columns so that the index lookup has a small enough selectivity. Please review our
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[Adding Database indexes](adding_database_indexes.md) guide for more details.
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When adding an index to a non-empty table make sure to use the method
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`add_concurrent_index` instead of the regular `add_index` method.
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The `add_concurrent_index` method automatically creates concurrent indexes
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when using PostgreSQL, removing the need for downtime.
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To use this method, you must disable single-transactions mode
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by calling the method `disable_ddl_transaction!` in the body of your migration
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class like so:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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include Gitlab::Database::MigrationHelpers
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disable_ddl_transaction!
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def up
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add_concurrent_index :table, :column
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end
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def down
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remove_concurrent_index :table, :column
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end
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end
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```
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If you need to add a unique index, please keep in mind there is the possibility
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of existing duplicates being present in the database. This means that should
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always _first_ add a migration that removes any duplicates, before adding the
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unique index.
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For a small table (such as an empty one or one with less than `1,000` records),
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it is recommended to use `add_index` in a single-transaction migration, combining it with other
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operations that don't require `disable_ddl_transaction!`.
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## Adding foreign-key constraints
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When adding a foreign-key constraint to either an existing or a new column also
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remember to add an index on the column.
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This is **required** for all foreign-keys, e.g., to support efficient cascading
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deleting: when a lot of rows in a table get deleted, the referenced records need
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to be deleted too. The database has to look for corresponding records in the
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referenced table. Without an index, this will result in a sequential scan on the
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table, which can take a long time.
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Here's an example where we add a new column with a foreign key
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constraint. Note it includes `index: true` to create an index for it.
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```ruby
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class Migration < ActiveRecord::Migration[4.2]
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def change
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add_reference :model, :other_model, index: true, foreign_key: { on_delete: :cascade }
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end
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end
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```
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When adding a foreign-key constraint to an existing column in a non-empty table,
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we have to employ `add_concurrent_foreign_key` and `add_concurrent_index`
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instead of `add_reference`.
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For an empty table (such as a fresh one), it is recommended to use
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`add_reference` in a single-transaction migration, combining it with other
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operations that don't require `disable_ddl_transaction!`.
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You can read more about adding [foreign key constraints to an existing column](database/add_foreign_key_to_existing_column.md).
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## Adding Columns With Default Values
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When adding columns with default values to non-empty tables, you must use
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`add_column_with_default`. This method ensures the table is updated without
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requiring downtime. This method is not reversible so you must manually define
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the `up` and `down` methods in your migration class.
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For example, to add the column `foo` to the `projects` table with a default
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value of `10` you'd write the following:
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
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include Gitlab::Database::MigrationHelpers
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disable_ddl_transaction!
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def up
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add_column_with_default(:projects, :foo, :integer, default: 10)
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end
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def down
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remove_column(:projects, :foo)
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end
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end
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```
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Keep in mind that this operation can easily take 10-15 minutes to complete on
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larger installations (e.g. GitLab.com). As a result, you should only add
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default values if absolutely necessary. There is a RuboCop cop that will fail if
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this method is used on some tables that are very large on GitLab.com, which
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would cause other issues.
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## Changing the column default
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One might think that changing a default column with `change_column_default` is an
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expensive and disruptive operation for larger tables, but in reality it's not.
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Take the following migration as an example:
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```ruby
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class DefaultRequestAccessGroups < ActiveRecord::Migration[5.2]
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DOWNTIME = false
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def change
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change_column_default(:namespaces, :request_access_enabled, from: false, to: true)
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end
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end
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```
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Migration above changes the default column value of one of our largest
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tables: `namespaces`. This can be translated to:
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```sql
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ALTER TABLE namespaces
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ALTER COLUMN request_access_enabled
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DEFAULT false
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```
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In this particular case, the default value exists and we're just changing the metadata for
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`request_access_enabled` column, which does not imply a rewrite of all the existing records
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in the `namespaces` table. Only when creating a new column with a default, all the records are going be rewritten.
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NOTE: **Note:** A faster [ALTER TABLE ADD COLUMN with a non-null default](https://www.depesz.com/2018/04/04/waiting-for-postgresql-11-fast-alter-table-add-column-with-a-non-null-default/)
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was introduced on PostgresSQL 11.0, removing the need of rewriting the table when a new column with a default value is added.
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For the reasons mentioned above, it's safe to use `change_column_default` in a single-transaction migration
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without requiring `disable_ddl_transaction!`.
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## Updating an existing column
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To update an existing column to a particular value, you can use
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`update_column_in_batches` (`add_column_with_default` uses this internally to
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fill in the default value). This will split the updates into batches, so we
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don't update too many rows at in a single statement.
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This updates the column `foo` in the `projects` table to 10, where `some_column`
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is `'hello'`:
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```ruby
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update_column_in_batches(:projects, :foo, 10) do |table, query|
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query.where(table[:some_column].eq('hello'))
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end
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```
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If a computed update is needed, the value can be wrapped in `Arel.sql`, so Arel
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treats it as an SQL literal. It's also a required deprecation for [Rails 6](https://gitlab.com/gitlab-org/gitlab/issues/28497).
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The below example is the same as the one above, but
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the value is set to the product of the `bar` and `baz` columns:
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```ruby
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update_value = Arel.sql('bar * baz')
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update_column_in_batches(:projects, :foo, update_value) do |table, query|
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query.where(table[:some_column].eq('hello'))
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end
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```
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Like `add_column_with_default`, there is a RuboCop cop to detect usage of this
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on large tables. In the case of `update_column_in_batches`, it may be acceptable
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to run on a large table, as long as it is only updating a small subset of the
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rows in the table, but do not ignore that without validating on the GitLab.com
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staging environment - or asking someone else to do so for you - beforehand.
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## Dropping a database table
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Dropping a database table is uncommon, and the `drop_table` method
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provided by Rails is generally considered safe. Before dropping the table,
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please consider the following:
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If your table has foreign keys on a high-traffic table (like `projects`), then
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the `DROP TABLE` statement might fail with **statement timeout** error. Determining
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what tables are high traffic can be difficult. Self-managed instances might
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use different features of GitLab with different usage patterns, thus making
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assumptions based on GitLab.com is not enough.
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Table **has no records** (feature was never in use) and **no foreign
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keys**:
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- Simply use the `drop_table` method in your migration.
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```ruby
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def change
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drop_table :my_table
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end
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```
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Table **has records** but **no foreign keys**:
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- First release: Remove the application code related to the table, such as models,
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controllers and services.
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- Second release: Use the `drop_table` method in your migration.
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```ruby
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def up
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drop_table :my_table
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end
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def down
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# create_table ...
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end
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```
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Table **has foreign keys**:
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- First release: Remove the application code related to the table, such as models,
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controllers, and services.
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- Second release: Remove the foreign keys using the `with_lock_retries`
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helper method. Use `drop_table` in another migration file.
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**Migrations for the second release:**
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Removing the foreign key on the `projects` table:
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```ruby
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# first migration file
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def up
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with_lock_retries do
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remove_foreign_key :my_table, :projects
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end
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end
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def down
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with_lock_retries do
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add_foreign_key :my_table, :projects
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end
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end
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```
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Dropping the table:
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```ruby
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# second migration file
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def up
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drop_table :my_table
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end
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def down
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# create_table ...
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end
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```
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## Integer column type
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By default, an integer column can hold up to a 4-byte (32-bit) number. That is
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a max value of 2,147,483,647. Be aware of this when creating a column that will
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hold file sizes in byte units. If you are tracking file size in bytes, this
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restricts the maximum file size to just over 2GB.
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To allow an integer column to hold up to an 8-byte (64-bit) number, explicitly
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set the limit to 8-bytes. This will allow the column to hold a value up to
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`9,223,372,036,854,775,807`.
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Rails migration example:
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```ruby
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add_column_with_default(:projects, :foo, :integer, default: 10, limit: 8)
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```
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## Timestamp column type
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By default, Rails uses the `timestamp` data type that stores timestamp data
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without timezone information. The `timestamp` data type is used by calling
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either the `add_timestamps` or the `timestamps` method.
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Also, Rails converts the `:datetime` data type to the `timestamp` one.
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Example:
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```ruby
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# timestamps
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create_table :users do |t|
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t.timestamps
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end
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# add_timestamps
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def up
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add_timestamps :users
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end
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# :datetime
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def up
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add_column :users, :last_sign_in, :datetime
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end
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```
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Instead of using these methods, one should use the following methods to store
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timestamps with timezones:
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- `add_timestamps_with_timezone`
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- `timestamps_with_timezone`
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- `datetime_with_timezone`
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This ensures all timestamps have a time zone specified. This, in turn, means
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existing timestamps won't suddenly use a different timezone when the system's
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timezone changes. It also makes it very clear which timezone was used in the
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first place.
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## Storing JSON in database
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The Rails 5 natively supports `JSONB` (binary JSON) column type.
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Example migration adding this column:
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```ruby
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class AddOptionsToBuildMetadata < ActiveRecord::Migration[5.0]
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DOWNTIME = false
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def change
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add_column :ci_builds_metadata, :config_options, :jsonb
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end
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end
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```
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You have to use a serializer to provide a translation layer:
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```ruby
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class BuildMetadata
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serialize :config_options, Serializers::JSON # rubocop:disable Cop/ActiveRecordSerialize
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end
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```
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## Testing
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See the [Testing Rails migrations](testing_guide/testing_migrations_guide.md) style guide.
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## Data migration
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Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of
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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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```ruby
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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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#update other tables with these results
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```
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Example with plain SQL and `quote_string` helper:
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```ruby
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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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If you need more complex logic, you can define and use models local to a
|
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migration. For example:
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|
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```ruby
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class MyMigration < ActiveRecord::Migration[4.2]
|
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class Project < ActiveRecord::Base
|
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self.table_name = 'projects'
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end
|
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end
|
|
```
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When doing so be sure to explicitly set the model's table name, so it's not
|
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derived from the class name or namespace.
|
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|
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### Renaming reserved paths
|
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|
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When a new route for projects is introduced, it could conflict with any
|
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existing records. The path for these records should be renamed, and the
|
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related data should be moved on disk.
|
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|
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Since we had to do this a few times already, there are now some helpers to help
|
|
with this.
|
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|
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To use this you can include `Gitlab::Database::RenameReservedPathsMigration::V1`
|
|
in your migration. This will provide 3 methods which you can pass one or more
|
|
paths that need to be rejected.
|
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|
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**`rename_root_paths`**: This will rename the path of all _namespaces_ with the
|
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given name that don't have a `parent_id`.
|
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|
|
**`rename_child_paths`**: This will rename the path of all _namespaces_ with the
|
|
given name that have a `parent_id`.
|
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|
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**`rename_wildcard_paths`**: This will rename the path of all _projects_, and all
|
|
_namespaces_ that have a `project_id`.
|
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|
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The `path` column for these rows will be renamed to their previous value followed
|
|
by an integer. For example: `users` would turn into `users0`
|