Batched background migrations

Batched background migrations should be used to perform data migrations whenever a migration exceeds the time limits in our guidelines. For example, you can use batched background migrations to migrate data that's stored in a single JSON column to a separate table instead.

NOTE: Batched background migrations replaced the legacy background migrations framework. Check that documentation in reference to any changes involving that framework.

NOTE: The batched background migrations framework has ChatOps support. Using ChatOps, GitLab engineers can interact with the batched background migrations present in the system.

When to use batched background migrations

Use a batched background migration when you migrate data in tables containing so many rows that the process would exceed the time limits in our guidelines if performed using a regular Rails migration.

• Batched background migrations should be used when migrating data in high-traffic tables.
• Batched background migrations may also be used when executing numerous single-row queries for every item on a large dataset. Typically, for single-record patterns, runtime is largely dependent on the size of the dataset. Split the dataset accordingly, and put it into background migrations.
• Don't use batched background migrations to perform schema migrations.

Background migrations can help when:

• Migrating events from one table to multiple separate tables.
• Populating one column based on JSON stored in another column.
• Migrating data that depends on the output of external services. (For example, an API.)

Notes

• If the batched background migration is part of an important upgrade, it must be announced in the release post. Discuss with your Project Manager if you're unsure if the migration falls into this category.
• You should use the generator to create batched background migrations, so that required files are created by default.

How batched background migrations work

Batched background migrations (BBM) are subclasses of Gitlab::BackgroundMigration::BatchedMigrationJob that define a perform method. As the first step, a regular migration creates a batched_background_migrations record with the BBM class and the required arguments. By default, batched_background_migrations is in an active state, and those are picked up by the Sidekiq worker to execute the actual batched migration.

All migration classes must be defined in the namespace Gitlab::BackgroundMigration. Place the files in the directory lib/gitlab/background_migration/.

Execution mechanism

Batched background migrations are picked from the queue in the order they are enqueued. Multiple migrations are fetched and executed in parallel, as long they are in active state and do not target the same database table. The default number of migrations processed in parallel is 2, for GitLab.com this limit is configured to 4. Once migration is picked for execution, a job is created for the specific batch. After each job execution, migration's batch size may be increased or decreased, based on the performance of the last 20 jobs.

@startuml
hide empty description
skinparam ConditionEndStyle hline
left to right direction
rectangle "Batched background migration queue" as migrations {
  rectangle "Migration N (active)" as migrationn
  rectangle "Migration 1 (completed)" as migration1
  rectangle "Migration 2 (active)" as migration2
  rectangle "Migration 3 (on hold)" as migration3
  rectangle "Migration 4 (active)" as migration4
  migration1 -[hidden]> migration2
  migration2 -[hidden]> migration3
  migration3 -[hidden]> migration4
  migration4 -[hidden]> migrationn
}
rectangle "Execution Workers" as workers {
 rectangle "Execution Worker 1 (busy)" as worker1
 rectangle "Execution Worker 2 (available)" as worker2
 worker1 -[hidden]> worker2
}
migration2 --> [Scheduling Worker]
migration4 --> [Scheduling Worker]
[Scheduling Worker] --> worker2
@enduml

Soon as a worker is available, the BBM is processed by the runner.

@startuml
hide empty description
start
rectangle Runner {
  :Migration;
  if (Have reached batching bounds?) then (Yes)
    if (Have jobs to retry?) then (Yes)
      :Fetch the batched job;
    else (No)
      :Finish active migration;
      stop
    endif
  else (No)
    :Create a batched job;
  endif
  :Execute batched job;
  :Evaluate DB health;
  note right: Checks for table autovacuum, Patroni Apdex, Write-ahead logging
  if (Evaluation signs to stop?) then (Yes)
    :Put migration on hold;
  else (No)
    :Optimize migration;
  endif
}
@enduml

Idempotence

Batched background migrations are executed in a context of a Sidekiq process. The usual Sidekiq rules apply, especially the rule that jobs should be small and idempotent. Ensure that in the case where your migration job is retried, data integrity is guaranteed.

See Sidekiq best practices guidelines for more details.

Migration optimization

After each job execution, a verification takes place to check if the migration can be optimized. The optimization underlying mechanic is based on the concept of time efficiency. It calculates the exponential moving average of time efficiencies for the last N jobs and updates the batch size of the batched background migration to its optimal value.

For GitLab SAAS

When updating a large dataset specify different batch sizes for GitLab SAAS.

# frozen_string_literal: true

class BatchedMigration < Gitlab::Database::Migration[2.2]
  BATCH_SIZE = 1000
  SUB_BATCH_SIZE = 100
  GITLAB_OPTIMIZED_BATCH_SIZE = 75_000
  GITLAB_OPTIMIZED_SUB_BATCH_SIZE = 250

  def up
    queue_batched_background_migration(
      MIGRATION,
      TABLE_NAME,
      COLUMN_NAME,
      job_interval: DELAY_INTERVAL,
      **batch_sizes
    )
  end

  private

  def batch_sizes
    if Gitlab.com_except_jh?
      {
        batch_size: GITLAB_OPTIMIZED_BATCH_SIZE,
        sub_batch_size: GITLAB_OPTIMIZED_SUB_BATCH_SIZE
      }
    else
      {
        batch_size: BATCH_SIZE,
        sub_batch_size: SUB_BATCH_SIZE
      }
    end
  end
end

Job retry mechanism

The batched background migrations retry mechanism ensures that a job is executed again in case of failure. The following diagram shows the different stages of our retry mechanism:

@startuml
hide empty description
note as N1
  can_split?:
  the failure is due to a query timeout
end note
    [*] --> Running
Running --> Failed
note on link
  if number of retries <= MAX_ATTEMPTS
end note
Running --> Succeeded
Failed --> Running
note on link
  if number of retries > MAX_ATTEMPTS
  and can_split? == true
  then two jobs with smaller
  batch size will be created
end note
Failed --> [*]
Succeeded --> [*]
@enduml

• MAX_ATTEMPTS is defined in the Gitlab::Database::BackgroundMigration class.
• can_split? is defined in the Gitlab::Database::BatchedJob class.

Failed batched background migrations

The whole batched background migration is marked as failed (/chatops run batched_background_migrations status MIGRATION_ID shows the migration as failed) if any of the following is true:

• There are no more jobs to consume, and there are failed jobs.
• More than half of the jobs failed since the background migration was started.

Throttling batched migrations

Because batched migrations are update heavy and there have been incidents due to the heavy load from these migrations while the database was underperforming, a throttling mechanism exists to mitigate future incidents.

These database indicators are checked to throttle a migration. Upon receiving a stop signal, the migration is paused for a set time (10 minutes):

• WAL queue pending archival crossing the threshold.
• Active autovacuum on the tables on which the migration works on.
• Patroni apdex SLI dropping below the SLO.
• WAL rate crossing the threshold.

There is an ongoing effort to add more indicators to further enhance the database health check framework. For more details, see epic 7594.

Isolation

Batched background migrations must be isolated and cannot use application code (for example, models defined in app/models except the ApplicationRecord classes). Because these migrations can take a long time to run, it's possible for new versions to deploy while the migrations are still running.

Depending on migrated data

Unlike a regular or a post migration, waiting for the next release is not enough to guarantee that the data was fully migrated. That means that you shouldn't depend on the data until the BBM is finished. If having 100% of the data migrated is a requirement, then, the ensure_batched_background_migration_is_finished helper can be used to guarantee that the migration was finished and the data fully migrated. (See an example).

How to

Generate a batched background migration

The custom generator batched_background_migration scaffolds necessary files and accepts table_name, column_name, and feature_category as arguments. When choosing the column_name, ensure that you are using a column type that can be iterated over distinctly, preferably the table's primary key. The table will be iterated over based on the column defined here. For more information, see Batch over non-distinct columns.

Usage:

bundle exec rails g batched_background_migration my_batched_migration --table_name=<table-name> --column_name=<column-name> --feature_category=<feature-category>

This command creates the following files:

• db/post_migrate/20230214231008_queue_my_batched_migration.rb
• spec/migrations/20230214231008_queue_my_batched_migration_spec.rb
• lib/gitlab/background_migration/my_batched_migration.rb
• spec/lib/gitlab/background_migration/my_batched_migration_spec.rb

Enqueue a batched background migration

Queueing a batched background migration should be done in a post-deployment migration. Use this queue_batched_background_migration example, queueing the migration to be executed in batches. Replace the class name and arguments with the values from your migration:

queue_batched_background_migration(
  JOB_CLASS_NAME,
  TABLE_NAME,
  JOB_ARGUMENTS,
  JOB_INTERVAL
  )

NOTE: This helper raises an error if the number of provided job arguments does not match the number of job arguments defined in JOB_CLASS_NAME.

Make sure the newly-created data is either migrated, or saved in both the old and new version upon creation. Removals in turn can be handled by defining foreign keys with cascading deletes.

Finalize a batched background migration

Finalizing a batched background migration is done by calling ensure_batched_background_migration_is_finished.

It is important to finalize all batched background migrations when it is safe to do so. Leaving around old batched background migration is a form of technical debt that needs to be maintained in tests and in application behavior. It is important to note that you cannot depend on any batched background migration being completed until after it is finalized.

We recommend that batched background migrations are finalized after all of the following conditions are met:

• The batched background migration is completed on GitLab.com
• The batched background migration was added in or before the last required stop

The ensure_batched_background_migration_is_finished call must exactly match the migration that was used to enqueue it. Pay careful attention to:

• The job arguments: Needs to exactly match or it will not find the queued migration
• The gitlab_schema: Needs to exactly match or it will not find the queued migration. Even if the gitlab_schema of the table has changed from gitlab_main to gitlab_main_cell in the meantime you must finalize it with gitlab_main if that's what was used when queueing the batched background migration.

When finalizing a batched background migration you also need to update the finalized_by in the corresponding db/docs/batched_background_migrations file. The value should be the timestamp/version of the migration you added to finalize it.

See the below Examples for specific details on what the actual migration code should be.

Use job arguments

BatchedMigrationJob provides the job_arguments helper method for job classes to define the job arguments they need.

Batched migrations scheduled with queue_batched_background_migration must use the helper to define the job arguments:

queue_batched_background_migration(
  'CopyColumnUsingBackgroundMigrationJob',
  TABLE_NAME,
  'name', 'name_convert_to_text',
  job_interval: DELAY_INTERVAL
)

NOTE: If the number of defined job arguments does not match the number of job arguments provided when scheduling the migration, queue_batched_background_migration raises an error.

In this example, copy_from returns name, and copy_to returns name_convert_to_text:

class CopyColumnUsingBackgroundMigrationJob < BatchedMigrationJob
  job_arguments :copy_from, :copy_to
  operation_name :update_all

  def perform
    from_column = connection.quote_column_name(copy_from)
    to_column = connection.quote_column_name(copy_to)

    assignment_clause = "#{to_column} = #{from_column}"

    each_sub_batch do |relation|
      relation.update_all(assignment_clause)
    end
  end
end

Use filters

By default, when creating background jobs to perform the migration, batched background migrations iterate over the full specified table. This iteration is done using the PrimaryKeyBatchingStrategy. If the table has 1000 records and the batch size is 100, the work is batched into 10 jobs. For illustrative purposes, EachBatch is used like this:

# PrimaryKeyBatchingStrategy
Namespace.each_batch(of: 100) do |relation|
  relation.where(type: nil).update_all(type: 'User') # this happens in each background job
end

In some cases, only a subset of records must be examined. If only 10% of the 1000 records need examination, apply a filter to the initial relation when the jobs are created:

Namespace.where(type: nil).each_batch(of: 100) do |relation|
  relation.update_all(type: 'User')
end

In the first example, we don't know how many records will be updated in each batch. In the second (filtered) example, we know exactly 100 will be updated with each batch.

BatchedMigrationJob provides a scope_to helper method to apply additional filters and achieve this:

1. Create a new migration job class that inherits from BatchedMigrationJob and defines the additional filter:

   class BackfillNamespaceType < BatchedMigrationJob
     scope_to ->(relation) { relation.where(type: nil) }
     operation_name :update_all
     feature_category :source_code_management
   
     def perform
       each_sub_batch do |sub_batch|
         sub_batch.update_all(type: 'User')
       end
     end
   end

   NOTE: For EE migrations that define scope_to, ensure the module extends ActiveSupport::Concern. Otherwise, records are processed without taking the scope into consideration.

2. In the post-deployment migration, enqueue the batched background migration:

   class BackfillNamespaceType < Gitlab::Database::Migration[2.1]
     MIGRATION = 'BackfillNamespaceType'
     DELAY_INTERVAL = 2.minutes
   
     restrict_gitlab_migration gitlab_schema: :gitlab_main
   
     def up
       queue_batched_background_migration(
         MIGRATION,
         :namespaces,
         :id,
         job_interval: DELAY_INTERVAL
       )
     end
   
     def down
       delete_batched_background_migration(MIGRATION, :namespaces, :id, [])
     end
   end

NOTE: When applying additional filters, it is important to ensure they are properly covered by an index to optimize EachBatch performance. In the example above we need an index on (type, id) to support the filters. See the EachBatch documentation for more information.

Access data for multiple databases

Background migration contrary to regular migrations does have access to multiple databases and can be used to efficiently access and update data across them. To properly indicate a database to be used it is desired to create ActiveRecord model inline the migration code. Such model should use a correct ApplicationRecord depending on which database the table is located. As such usage of ActiveRecord::Base is disallowed as it does not describe a explicitly database to be used to access given table.

# good
class Gitlab::BackgroundMigration::ExtractIntegrationsUrl
  class Project < ::ApplicationRecord
    self.table_name = 'projects'
  end

  class Build < ::Ci::ApplicationRecord
    self.table_name = 'ci_builds'
  end
end

# bad
class Gitlab::BackgroundMigration::ExtractIntegrationsUrl
  class Project < ActiveRecord::Base
    self.table_name = 'projects'
  end

  class Build < ActiveRecord::Base
    self.table_name = 'ci_builds'
  end
end

Similarly the usage of ActiveRecord::Base.connection is disallowed and needs to be replaced preferably with the usage of model connection.

# good
Project.connection.execute("SELECT * FROM projects")

# acceptable
ApplicationRecord.connection.execute("SELECT * FROM projects")

# bad
ActiveRecord::Base.connection.execute("SELECT * FROM projects")

Re-queue batched background migrations

A batched background migration might need to be re-run for one of several reasons:

• The migration contains a bug (example).
• The migration cleaned up data but the data became de-normalized again due to a bypass in application logic (example).
• The batch size of the original migration causes the migration to fail (example).

To requeue a batched background migration, you must:

• No-op the contents of the #up and #down methods of the original migration file. Otherwise, the batched background migration is created, deleted, then created again on systems that are upgrading multiple patch releases at once.
• Add a new post-deployment migration that re-runs the batched background migration.
• In the new post-deployment migration, delete the existing batched background migration using the delete_batched_background_migration method at the start of the #up method to ensure that any existing runs are cleaned up.
• Update the db/docs/batched_background_migration/*.yml file from the original migration to include information about the requeue.

Example

Original Migration:

# frozen_string_literal: true

class QueueResolveVulnerabilitiesForRemovedAnalyzers < Gitlab::Database::Migration[2.2]
  milestone '17.3'

  MIGRATION = "ResolveVulnerabilitiesForRemovedAnalyzers"

  def up
    # no-op because there was a bug in the original migration, which has been
    # fixed by
  end

  def down
    # no-op because there was a bug in the original migration, which has been
    # fixed in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/162527
  end
end

Requeued migration:

# frozen_string_literal: true

class RequeueResolveVulnerabilitiesForRemovedAnalyzers < Gitlab::Database::Migration[2.2]
  milestone '17.4'

  restrict_gitlab_migration gitlab_schema: :gitlab_main

  MIGRATION = "ResolveVulnerabilitiesForRemovedAnalyzers"
  DELAY_INTERVAL = 2.minutes
  BATCH_SIZE = 10_000
  SUB_BATCH_SIZE = 100

  def up
    # Clear previous background migration execution from QueueResolveVulnerabilitiesForRemovedAnalyzers
    delete_batched_background_migration(MIGRATION, :vulnerability_reads, :id, [])

    queue_batched_background_migration(
      MIGRATION,
      :vulnerability_reads,
      :id,
      job_interval: DELAY_INTERVAL,
      batch_size: BATCH_SIZE,
      sub_batch_size: SUB_BATCH_SIZE
    )
  end

  def down
    delete_batched_background_migration(MIGRATION, :vulnerability_reads, :id, [])
  end
end

Batched migration dictionary:

The milestone and queued_migration_version should be the ones of requeued migration (in this eg: RequeueResolveVulnerabilitiesForRemovedAnalyzers).

---
migration_job_name: ResolveVulnerabilitiesForRemovedAnalyzers
description: Resolves all detected vulnerabilities for removed analyzers.
feature_category: static_application_security_testing
introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/162691
milestone: '17.4'
queued_migration_version: 20240814085540
finalized_by: # version of the migration that finalized this BBM

Batch over non-distinct columns

The default batching strategy provides an efficient way to iterate over primary key columns. However, if you need to iterate over columns where values are not unique, you must use a different batching strategy.

The LooseIndexScanBatchingStrategy batching strategy uses a special version of EachBatch to provide efficient and stable iteration over the distinct column values.

This example shows a batched background migration where the issues.project_id column is used as the batching column.

Database post-migration:

class ProjectsWithIssuesMigration < Gitlab::Database::Migration[2.1]
  MIGRATION = 'BatchProjectsWithIssues'
  INTERVAL = 2.minutes
  BATCH_SIZE = 5000
  SUB_BATCH_SIZE = 500
  restrict_gitlab_migration gitlab_schema: :gitlab_main

  disable_ddl_transaction!
  def up
    queue_batched_background_migration(
      MIGRATION,
      :issues,
      :project_id,
      job_interval: INTERVAL,
      batch_size: BATCH_SIZE,
      batch_class_name: 'LooseIndexScanBatchingStrategy', # Override the default batching strategy
      sub_batch_size: SUB_BATCH_SIZE
    )
  end

  def down
    delete_batched_background_migration(MIGRATION, :issues, :project_id, [])
  end
end

Implementing the background migration class:

module Gitlab
  module BackgroundMigration
    class BatchProjectsWithIssues < Gitlab::BackgroundMigration::BatchedMigrationJob
      include Gitlab::Database::DynamicModelHelpers

      operation_name :backfill_issues

      def perform
        distinct_each_batch do |batch|
          project_ids = batch.pluck(batch_column)
          # do something with the distinct project_ids
        end
      end
    end
  end
end

NOTE: Additional filters defined with scope_to are ignored by LooseIndexScanBatchingStrategy and distinct_each_batch.

Calculate overall time estimation of a batched background migration

It's possible to estimate how long a BBM takes to complete. GitLab already provides an estimation through the db:gitlabcom-database-testing pipeline. This estimation is built based on sampling production data in a test environment and represents the max time that the migration could take and, not necessarily, the actual time that the migration takes. In certain scenarios, estimations provided by the db:gitlabcom-database-testing pipeline may not be enough to calculate all the singularities around the records being migrated, making further calculations necessary. As it made necessary, the formula interval * number of records / max batch size can be used to determine an approximate estimation of how long the migration takes. Where interval and max batch size refer to options defined for the job, and the total tuple count is the number of records to be migrated.

NOTE: Estimations may be affected by the migration optimization mechanism.

Cleaning up a batched background migration

NOTE: Cleaning up any remaining background migrations must be done in either a major or minor release. You must not do this in a patch release.

Because background migrations can take a long time, you can't immediately clean things up after queueing them. For example, you can't drop a column used in the migration process, as jobs would fail. You must add a separate post-deployment migration in a future release that finishes any remaining jobs before cleaning things up. (For example, removing a column.)

To migrate the data from column foo (containing a big JSON blob) to column bar (containing a string), you would:

1. Release A:
   1. Create a migration class that performs the migration for a row with a given ID.
   2. Update new rows using one of these techniques:
      • Create a new trigger for copy operations that don't need application logic.
      • Handle this operation in the model/service as the records are created or updated.
      • Create a new custom background job that updates the records.
   3. Queue the batched background migration for all existing rows in a post-deployment migration.
2. Release B:
   1. Add a post-deployment migration that checks if the batched background migration is completed.
   2. Deploy code so that the application starts using the new column and stops to update new records.
   3. Remove the old column.

Bumping the import/export version may be required, if importing a project from a prior version of GitLab requires the data to be in the new format.

Add indexes to support batched background migrations

Sometimes it is necessary to add a new or temporary index to support a batched background migration. To do this, create the index in a post-deployment migration that precedes the post-deployment migration that queues the background migration.

See the documentation for adding database indexes for additional information about some cases that require special attention to allow the index to be used directly after creation.

Execute a particular batch on the database testing pipeline

NOTE: Only database maintainers can view the database testing pipeline artifacts. Ask one for help if you need to use this method.

Let's assume that a batched background migration failed on a particular batch on GitLab.com and you want to figure out which query failed and why. At the moment, we don't have a good way to retrieve query information (especially the query parameters) and rerunning the entire migration with more logging would be a long process.

Fortunately you can leverage our database migration pipeline to rerun a particular batch with additional logging and/or fix to see if it solves the problem.

For an example see Draft: Test PG::CardinalityViolation fix but make sure to read the entire section.

To do that, you need to:

1. Find the batch start_id and end_id
2. Create a regular migration
3. Apply a workaround for our migration helpers (optional)
4. Start the database migration pipeline

Find the batch start_id and end_id

You should be able to find those in Kibana.

Create a regular migration

Schedule the batch in the up block of a regular migration:

def up
  instance = Gitlab::BackgroundMigration::YourBackgroundMigrationClass.new(
      start_id: <batch start_id>,
      end_id: <batch end_id>,
      batch_table: <table name>,
      batch_column: <batching column>,
      sub_batch_size: <sub batch size>,
      pause_ms: <miliseconds between batches>,
      job_arguments: <job arguments if any>,
      connection: connection
    )

    instance.perform
end

def down
  # no-op
end

Apply a workaround for our migration helpers (optional)

If your batched background migration touches tables from a schema other than the one you specified by using restrict_gitlab_migration helper (example: the scheduling migration has restrict_gitlab_migration gitlab_schema: :gitlab_main but the background job uses tables from the :gitlab_ci schema) then the migration will fail. To prevent that from happening you must to monkey patch database helpers so they don't fail the testing pipeline job:

1. Add the schema names to RestrictGitlabSchema

diff --git a/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb b/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb
index b8d1d21a0d2d2a23d9e8c8a0a17db98ed1ed40b7..912e20659a6919f771045178c66828563cb5a4a1 100644
--- a/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb
+++ b/lib/gitlab/database/migration_helpers/restrict_gitlab_schema.rb
@@ -55,7 +55,7 @@ def unmatched_schemas
         end

         def allowed_schemas_for_connection
-          Gitlab::Database.gitlab_schemas_for_connection(connection)
+          Gitlab::Database.gitlab_schemas_for_connection(connection) << :gitlab_ci
         end
       end
     end

1. Add the schema names to RestrictAllowedSchemas

diff --git a/lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb b/lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb
index 4ae3622479f0800c0553959e132143ec9051898e..d556ec7f55adae9d46a56665ce02de782cb09f2d 100644
--- a/lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb
+++ b/lib/gitlab/database/query_analyzers/restrict_allowed_schemas.rb
@@ -79,7 +79,7 @@ def restrict_to_dml_only(parsed)
             tables = self.dml_tables(parsed)
             schemas = self.dml_schemas(tables)

-            if (schemas - self.allowed_gitlab_schemas).any?
+            if (schemas - (self.allowed_gitlab_schemas << :gitlab_ci)).any?
               raise DMLAccessDeniedError, \
                 "Select/DML queries (SELECT/UPDATE/DELETE) do access '#{tables}' (#{schemas.to_a}) " \
                 "which is outside of list of allowed schemas: '#{self.allowed_gitlab_schemas}'. " \

Start the database migration pipeline

Create a Draft merge request with your changes and trigger the manual db:gitlabcom-database-testing job.

Establish dependencies

In some instances, migrations depended on the completion of previously enqueued BBMs. If the BBMs are still running, the dependent migration fails. For example: introducing an unique index on a large table can depend on the previously enqueued BBM to handle any duplicate records.

The following process has been configured to make dependencies more evident while writing a migration.

• Version of the migration that queued the BBM is stored in batched_background_migrations table and in BBM dictionary file.
• DEPENDENT_BATCHED_BACKGROUND_MIGRATIONS constant is added (commented by default) in each migration file. To establish the dependency, add queued_migration_version of the dependent BBMs. If not, remove the commented line.
• Migration::UnfinishedDependencies cop complains if the dependent BBMs are not yet finished. It determines whether they got finished by looking up the finalized_by key in the BBM dictionary.

Example:

# db/post_migrate/20231113120650_queue_backfill_routes_namespace_id.rb
class QueueBackfillRoutesNamespaceId < Gitlab::Database::Migration[2.1]
  MIGRATION = 'BackfillRouteNamespaceId'

  restrict_gitlab_migration gitlab_schema: :gitlab_main
  ...
  ...

  def up
    queue_batched_background_migration(
      MIGRATION,
      ...
    )
  end
end

# This depends on the finalization of QueueBackfillRoutesNamespaceId BBM
class AddNotNullToRoutesNamespaceId < Gitlab::Database::Migration[2.1]
  DEPENDENT_BATCHED_BACKGROUND_MIGRATIONS = ["20231113120650"]

  def up
    add_not_null_constraint :routes, :namespace_id
  end

  def down
    remove_not_null_constraint :routes, :namespace_id
  end
end

Managing

NOTE: BBM management takes place through chatops integration, which is limited to GitLab team members only.

List batched background migrations

To list the batched background migrations in the system, run this command:

/chatops run batched_background_migrations list

This command supports the following options:

• Database selection:
  • --database DATABASE_NAME: Connects to the given database:
    • main: Uses the main database (default).
    • ci: Uses the CI database.
• Environment selection:
  • --dev: Uses the dev environment.
  • --staging: Uses the staging environment.
  • --staging_ref: Uses the staging_ref environment.
  • --production : Uses the production environment (default).

Output example:

NOTE: ChatOps returns 20 batched background migrations order by created_at (DESC).

Monitor the progress and status of a batched background migration

To see the status and progress of a specific batched background migration, run this command:

/chatops run batched_background_migrations status MIGRATION_ID

This command supports the following options:

• Database selection:
  • --database DATABASE_NAME: Connects to the given database:
    • main: Uses the main database (default)
    • ci: Uses the CI database
• Environment selection:
  • --dev: Uses the dev environment.
  • --staging: Uses the staging environment.
  • --staging_ref: Uses the staging_ref environment.
  • --production : Uses the production environment (default).

Output example:

Progress represents the percentage of the background migration that has been completed.

Definitions of the batched background migration states:

• Active: Either:
  • Ready to be picked by the runner.
  • Running batched jobs.
• Finalizing: Running batched jobs.
• Failed: Failed batched background migration.
• Finished: Completed batched background migration.
• Paused: Not visible to the runner.

Pause a batched background migration

If you want to pause a batched background migration, you need to run the following command:

/chatops run batched_background_migrations pause MIGRATION_ID

This command supports the following options:

• Database selection:
  • --database DATABASE_NAME: Connects to the given database:
    • main: Uses the main database (default).
    • ci: Uses the CI database.
• Environment selection:
  • --dev: Uses the dev environment.
  • --staging: Uses the staging environment.
  • --staging_ref: Uses the staging_ref environment.
  • --production : Uses the production environment (default).

Output example:

NOTE: You can pause only active batched background migrations.

Resume a batched background migration

If you want to resume a batched background migration, you need to run the following command:

/chatops run batched_background_migrations resume MIGRATION_ID

This command supports the following options:

• Database selection:
  • --database DATABASE_NAME: Connects to the given database:
    • main: Uses the main database (default).
    • ci: Uses the CI database.
• Environment selection:
  • --dev: Uses the dev environment.
  • --staging: Uses the staging environment.
  • --staging_ref: Uses the staging_ref environment.
  • --production : Uses the production environment (default).

Output example:

NOTE: You can resume only active batched background migrations

Enable or disable background migrations

In extremely limited circumstances, a GitLab administrator can disable either or both of these feature flags:

• execute_background_migrations
• execute_batched_migrations_on_schedule

These flags are enabled by default. Disable them only as a last resort to limit database operations in special circumstances, like database host maintenance.

WARNING: Do not disable either of these flags unless you fully understand the ramifications. If you disable the execute_background_migrations or execute_batched_migrations_on_schedule feature flag, GitLab upgrades might fail and data loss might occur.

Batched background migrations for EE-only features

All the background migration classes for EE-only features should be present in GitLab FOSS. For this purpose, create an empty class for GitLab FOSS, and extend it for GitLab EE as explained in the guidelines for implementing Enterprise Edition features.

NOTE: Background migration classes for EE-only features that use job arguments should define them in the GitLab FOSS class. Definitions are required to prevent job arguments validation from failing when migration is scheduled in the GitLab FOSS context.

You can use the generator to generate an EE-only migration scaffold by passing --ee-only flag when generating a new batched background migration.

Debug

Viewing failure error logs

You can view failures in two ways:

• Via GitLab logs:

  1. After running a batched background migration, if any jobs fail, view the logs in Kibana. View the production Sidekiq log and filter for:

     • json.new_state: failed
     • json.job_class_name: <Batched Background Migration job class name>
     • json.job_arguments: <Batched Background Migration job class arguments>

  2. Review the json.exception_class and json.exception_message values to help understand why the jobs failed.

  3. Remember the retry mechanism. Having a failure does not mean the job failed. Always check the last status of the job.

• Via database:

  1. Get the batched background migration CLASS_NAME.

  2. Execute the following query in the PostgreSQL console:

      SELECT migration.id, migration.job_class_name, transition_logs.exception_class, transition_logs.exception_message
      FROM batched_background_migrations as migration
      INNER JOIN batched_background_migration_jobs as jobs
      ON jobs.batched_background_migration_id = migration.id
      INNER JOIN batched_background_migration_job_transition_logs as transition_logs
      ON transition_logs.batched_background_migration_job_id = jobs.id
      WHERE transition_logs.next_status = '2' AND migration.job_class_name = "CLASS_NAME";

Testing

Writing tests is required for:

• The batched background migrations' queueing migration.
• The batched background migration itself.
• A cleanup migration.

The :migration and schema: :latest RSpec tags are automatically set for background migration specs. Refer to the Testing Rails migrations style guide.

Remember that before and after RSpec hooks migrate your database down and up. These hooks can result in other batched background migrations being called. Using spy test doubles with have_received is encouraged, instead of using regular test doubles, because your expectations defined in a it block can conflict with what is called in RSpec hooks. Refer to issue #35351 for more details.

Best practices

1. Know how much data you're dealing with.

2. Make sure the batched background migration jobs are idempotent.

3. Confirm the tests you write are not false positives.

4. If the data being migrated is critical and cannot be lost, the clean-up migration must also check the final state of the data before completing.

5. Discuss the numbers with a database specialist. The migration may add more pressure on DB than you expect. Measure on staging, or ask someone to measure on production.

6. Know how much time is required to run the batched background migration.

7. Be careful when silently rescuing exceptions inside job classes. This may lead to jobs being marked as successful, even in a failure scenario.

   # good
   def perform
     each_sub_batch do |sub_batch|
       sub_batch.update_all(name: 'My Name')
     end
   end
   
   # acceptable
   def perform
     each_sub_batch do |sub_batch|
       sub_batch.update_all(name: 'My Name')
     rescue Exception => error
       logger.error(message: error.message, class: error.class)
   
       raise
     end
   end
   
   # bad
   def perform
     each_sub_batch do |sub_batch|
       sub_batch.update_all(name: 'My Name')
     rescue Exception => error
       logger.error(message: error.message, class: self.class.name)
     end
   end

Examples

Routes use-case

The routes table has a source_type field that's used for a polymorphic relationship. As part of a database redesign, we're removing the polymorphic relationship. One step of the work is migrating data from the source_id column into a new singular foreign key. Because we intend to delete old rows later, there's no need to update them as part of the background migration.

1. Start by using the generator to create batched background migration files:

   bundle exec rails g batched_background_migration BackfillRouteNamespaceId --table_name=routes --column_name=id --feature_category=source_code_management

2. Update the migration job (subclass of BatchedMigrationJob) to copy source_id values to namespace_id:

   class Gitlab::BackgroundMigration::BackfillRouteNamespaceId < BatchedMigrationJob
     # For illustration purposes, if we were to use a local model we could
     # define it like below, using an `ApplicationRecord` as the base class
     # class Route < ::ApplicationRecord
     #   self.table_name = 'routes'
     # end
   
     operation_name :update_all
     feature_category :source_code_management
   
     def perform
       each_sub_batch(
         batching_scope: -> (relation) { relation.where("source_type <> 'UnusedType'") }
       ) do |sub_batch|
         sub_batch.update_all('namespace_id = source_id')
       end
     end
   end

   NOTE: Job classes inherit from BatchedMigrationJob to ensure they are correctly handled by the batched migration framework. Any subclass of BatchedMigrationJob is initialized with the necessary arguments to execute the batch, and a connection to the tracking database.

3. Create a database migration that adds a new trigger to the database. Example:

   class AddTriggerToRoutesToCopySourceIdToNamespaceId < Gitlab::Database::Migration[2.1]
     FUNCTION_NAME = 'example_function'
     TRIGGER_NAME = 'example_trigger'
   
     def up
       execute(<<~SQL)
         CREATE OR REPLACE FUNCTION #{FUNCTION_NAME}() RETURNS trigger
         LANGUAGE plpgsql
         AS $$
         BEGIN
           NEW."namespace_id" = NEW."source_id"
           RETURN NEW;
         END;
         $$;
   
         CREATE TRIGGER #{TRIGGER_NAME}() AFTER INSERT OR UPDATE
         ON routes
         FOR EACH ROW EXECUTE FUNCTION #{FUNCTION_NAME}();
       SQL
     end
   
     def down
       drop_trigger(TRIGGER_NAME, :routes)
       drop_function(FUNCTION_NAME)
     end
   end

4. Update the created post-deployment migration with required delay and batch sizes:

   class QueueBackfillRoutesNamespaceId < Gitlab::Database::Migration[2.1]
     MIGRATION = 'BackfillRouteNamespaceId'
     DELAY_INTERVAL = 2.minutes
     BATCH_SIZE = 1000
     SUB_BATCH_SIZE = 100
   
     restrict_gitlab_migration gitlab_schema: :gitlab_main
   
     def up
       queue_batched_background_migration(
         MIGRATION,
         :routes,
         :id,
         job_interval: DELAY_INTERVAL,
         batch_size: BATCH_SIZE,
         sub_batch_size: SUB_BATCH_SIZE
       )
     end
   
     def down
       delete_batched_background_migration(MIGRATION, :routes, :id, [])
     end
   end

    # db/docs/batched_background_migrations/backfill_route_namespace_id.yml
    ---
    migration_job_name: BackfillRouteNamespaceId
    description: Copies source_id values from routes to namespace_id
    feature_category: source_code_management
    introduced_by_url: "https://mr_url"
    milestone: 16.6
    queued_migration_version: 20231113120650
    finalized_by: # version of the migration that ensured this bbm

   NOTE: When queuing a batched background migration, you need to restrict the schema to the database where you make the actual changes. In this case, we are updating routes records, so we set restrict_gitlab_migration gitlab_schema: :gitlab_main. If, however, you need to perform a CI data migration, you would set restrict_gitlab_migration gitlab_schema: :gitlab_ci.

   After deployment, our application:

   • Continues using the data as before.
   • Ensures that both existing and new data are migrated.

5. Add a new post-deployment migration that checks that the batched background migration is complete. Also update finalized_by attribute in BBM dictionary with the version of this migration.

   class FinalizeBackfillRouteNamespaceId < Gitlab::Database::Migration[2.1]
     MIGRATION = 'BackfillRouteNamespaceId'
     disable_ddl_transaction!
   
     restrict_gitlab_migration gitlab_schema: :gitlab_main
   
     def up
       ensure_batched_background_migration_is_finished(
         job_class_name: MIGRATION,
         table_name: :routes,
         column_name: :id,
         job_arguments: [],
         finalize: true
       )
     end
   
     def down
       # no-op
     end
   end

    # db/docs/batched_background_migrations/backfill_route_namespace_id.yml
    ---
    migration_job_name: BackfillRouteNamespaceId
    description: Copies source_id values from routes to namespace_id
    feature_category: source_code_management
    introduced_by_url: "https://mr_url"
    milestone: 16.6
    queued_migration_version: 20231113120650
    finalized_by: 20231115120912

   NOTE: If the batched background migration is not finished, the system will execute the batched background migration inline. If you don't want to see this behavior, you need to pass finalize: false.

   If the application does not depend on the data being 100% migrated (for instance, the data is advisory, and not mission-critical), then you can skip this final step. This step confirms that the migration is completed, and all of the rows were migrated.

6. Add a database migration to remove the trigger.

   class RemoveNamepaceIdTriggerFromRoutes < Gitlab::Database::Migration[2.1]
     FUNCTION_NAME = 'example_function'
     TRIGGER_NAME = 'example_trigger'
   
     def up
       drop_trigger(TRIGGER_NAME, :routes)
       drop_function(FUNCTION_NAME)
     end
   
     def down
       # Should reverse the trigger and the function in the up method of the migration that added it
     end
   end

After the batched migration is completed, you can safely depend on the data in routes.namespace_id being populated.