OnlineMigrations

Catch unsafe PostgreSQL migrations in development and run them easier in production (code helpers for table/column renaming, changing column type, adding columns with default, background migrations, etc).

Description

Catch unsafe migrations in development and run them easier in production.

✅ Detects potentially dangerous operations ✅ Prevents them from running by default ✅ Provides instructions and migrations helpers on safer ways to do what you want

It has migrations helpers for:

* renaming tables/columns * changing columns types (including changing primary/foreign keys from `integer` to `bigint`) * adding columns with default values * adding different types of constraints * and others

Additionally, it has an internal framework for running data migrations on very large tables using background migrations. For example, you can use background migrations to migrate data that’s stored in a single JSON column to a separate table instead; backfill values from one column to another (as one of the steps when changing column type); or backfill some column’s value from an API.

Programming language: Ruby

License: MIT License

Tags: Database Tools     Ruby     Active Record     Ruby On Rails    

Latest version: v0.5.2

README

OnlineMigrations

Catch unsafe PostgreSQL migrations in development and run them easier in production.

:white_check_mark: Detects potentially dangerous operations\ :white_check_mark: Prevents them from running by default\ :white_check_mark: Provides instructions and helpers on safer ways to do what you want

Note: You probably don't need this gem for smaller projects, as operations that are unsafe at scale can be perfectly safe on smaller, low-traffic tables.

Cool, but there is a strong_migrations already

See comparison to strong_migrations

Requirements

• Ruby 2.1+
• Rails 4.2+
• PostgreSQL 9.6+

Installation

Add this line to your application's Gemfile:

gem 'online_migrations'

And then run:

$ bundle install
$ bin/rails generate online_migrations:install

Note: If you do not have plans on using [background migrations](BACKGROUND_MIGRATIONS.md) feature, then you can delete the generated migration and regenerate it later, if needed.

Motivation

Writing a safe migration can be daunting. Numerous articles have been written on the topic and a few gems are trying to address the problem. Even for someone who has a pretty good command of PostgreSQL, remembering all the subtleties of explicit locking can be problematic.

Online Migrations was created to catch dangerous operations and provide a guidance and code helpers to run them safely.

An operation is classified as dangerous if it either:

• Blocks reads or writes for more than a few seconds (after a lock is acquired)
• Has a good chance of causing application errors

Example

Consider the following migration:

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :admin, :boolean, default: false, null: false
  end
end

If the users table is large, running this migration on a live PostgreSQL < 11 database will likely cause downtime.

A safer approach would be to run something like the following:

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  # Do not wrap the migration in a transaction so that locks are held for a shorter time.
  disable_ddl_transaction!

  def change
    # Lower PostgreSQL's lock timeout to avoid statement queueing.
    execute "SET lock_timeout TO '5s'" # The lock_timeout duration is customizable.

    # Add the column without the default value and the not-null constraint.
    add_column :users, :admin, :boolean

    # Set the column's default value.
    change_column_default :users, :admin, false

    # Backfill the column in batches.
    User.in_batches.update_all(admin: false)

    # Add the not-null constraint. Beforehand, set a short statement timeout so that
    # Postgres does not spend too much time performing the full table scan to verify
    # the column contains no nulls.
    execute "SET statement_timeout TO '5s'"
    change_column_null :users, :admin, false
  end
end

When you actually run the original migration, you will get an error message:

⚠️  [online_migrations] Dangerous operation detected ⚠️

Adding a column with a non-null default blocks reads and writes while the entire table is rewritten.

A safer approach is to:
1. add the column without a default value
2. change the column default
3. backfill existing rows with the new value
4. add the NOT NULL constraint

add_column_with_default takes care of all this steps:

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_column_with_default :users, :admin, :boolean, default: false, null: false
  end
end

It suggests how to safely implement a migration, which essentially runs the steps similar to described in the previous example.

Checks

Potentially dangerous operations:

• removing a column
• adding a column with a default value
• backfilling data
• changing the type of a column
• renaming a column
• renaming a table
• creating a table with the force option
• adding a check constraint
• setting NOT NULL on an existing column
• executing SQL directly
• adding an index non-concurrently
• removing an index non-concurrently
• replacing an index
• adding a reference
• adding a foreign key
• adding a json column
• using primary key with short integer type
• hash indexes
• adding multiple foreign keys
• removing a table with multiple foreign keys
• mismatched reference column types
• adding a single table inheritance column

You can also add custom checks or disable specific checks.

Removing a column

:x: Bad

ActiveRecord caches database columns at runtime, so if you drop a column, it can cause exceptions until your app reboots.

class RemoveNameFromUsers < ActiveRecord::Migration[7.0]
  def change
    remove_column :users, :name
  end
end

:white_check_mark: Good

1. Ignore the column:

  # For ActiveRecord 5+
  class User < ApplicationRecord
    self.ignored_columns = ["name"]
  end

  # For ActiveRecord < 5
  class User < ActiveRecord::Base
    def self.columns
      super.reject { |c| c.name == "name" }
    end
  end

1. Deploy
2. Wrap column removing in a safety_assured block:

  class RemoveNameFromUsers < ActiveRecord::Migration[7.0]
    def change
      safety_assured { remove_column :users, :name }
    end
  end

1. Remove column ignoring from User model
2. Deploy

Adding a column with a default value

:x: Bad

In earlier versions of PostgreSQL adding a column with a non-null default value to an existing table blocks reads and writes while the entire table is rewritten.

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :admin, :boolean, default: false
  end
end

In PostgreSQL 11+ this no longer requires a table rewrite and is safe. Volatile expressions, however, such as random(), will still result in table rewrites.

:white_check_mark: Good

A safer approach is to:

1. add the column without a default value
2. change the column default
3. backfill existing rows with the new value

add_column_with_default helper takes care of all this steps:

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_column_with_default :users, :admin, :boolean, default: false
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

Backfilling data

:x: Bad

ActiveRecord wraps each migration in a transaction, and backfilling in the same transaction that alters a table keeps the table locked for the duration of the backfill.

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :admin, :boolean
    User.update_all(admin: false)
  end
end

Also, running a single query to update data can cause issues for large tables.

:white_check_mark: Good

There are three keys to backfilling safely: batching, throttling, and running it outside a transaction. Use a update_column_in_batches helper in a separate migration with disable_ddl_transaction!.

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :admin, :boolean
  end
end

class BackfillUsersAdminColumn < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def up
    update_column_in_batches(:users, :admin, false, pause_ms: 10)
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail. Note: You may consider background migrations to run data changes on large tables.

Changing the type of a column

:x: Bad

Changing the type of an existing column blocks reads and writes while the entire table is rewritten.

class ChangeFilesSizeType < ActiveRecord::Migration[7.0]
  def change
    change_column :files, :size, :bigint
  end
end

A few changes don't require a table rewrite (and are safe) in PostgreSQL:

Type	Safe Changes
bit	Changing to bit_varying
bit_varying	Increasing or removing :limit
cidr	Changing to inet
citext	Changing to text if not indexed, changing to string with no :limit if not indexed
datetime	Increasing or removing :precision, changing to timestamptz when session time zone is UTC in PostgreSQL 12+
decimal	Increasing :precision at same :scale, removing :precision and :scale
interval	Increasing or removing :precision
numeric	Increasing :precision at same :scale, removing :precision and :scale
string	Increasing or removing :limit, changing to text, changing to citext if not indexed
text	Changing to string with no :limit, changing to citext if not indexed
timestamptz	Increasing or removing :limit, changing to datetime when session time zone is UTC in PostgreSQL 12+
xml	Changing to text, changing to string with no :limit

:white_check_mark: Good

Note: The following steps can also be used to change the primary key's type (e.g., from integer to bigint).

A safer approach can be accomplished in several steps:

1. Create a new column and keep column's data in sync:

  class InitializeChangeFilesSizeType < ActiveRecord::Migration[7.0]
    def change
      initialize_column_type_change :files, :size, :bigint
    end
  end

1. Backfill data from the old column to the new column:

  class BackfillChangeFilesSizeType < ActiveRecord::Migration[7.0]
    disable_ddl_transaction!

    def up
      backfill_column_for_type_change :files, :size
    end

    def down
      # no op
    end
  end

1. Copy indexes, foreign keys, check constraints, NOT NULL constraint, swap new column in place:

  class FinalizeChangeFilesSizeType < ActiveRecord::Migration[7.0]
    disable_ddl_transaction!

    def change
      finalize_column_type_change :files, :size
    end
  end

1. Deploy
2. Finally, if everything is working as expected, remove copy trigger and old column:

  class CleanupChangeFilesSizeType < ActiveRecord::Migration[7.0]
    def up
      cleanup_column_type_change :files, :size
    end

    def down
      initialize_column_type_change :files, :size, :integer
    end
  end

1. Deploy

Renaming a column

:x: Bad

Renaming a column that's in use will cause errors in your application.

class RenameUsersNameToFirstName < ActiveRecord::Migration[7.0]
  def change
    rename_column :users, :name, :first_name
  end
end

:white_check_mark: Good

The "classic" approach suggests creating a new column and copy data/indexes/etc to it from the old column. This can be costly for very large tables. There is a trick that helps to avoid such heavy operations.

The technique is built on top of database views, using the following steps:

1. Rename the table to some temporary name
2. Create a VIEW using the old table name with addition of a new column as an alias of the old one
3. Add a workaround for ActiveRecord's schema cache

For the previous example, to rename name column to first_name of the users table, we can run:

BEGIN;
ALTER TABLE users RENAME TO users_column_rename;
CREATE VIEW users AS SELECT *, first_name AS name FROM users;
COMMIT;

As database views do not expose the underlying table schema (default values, not null constraints, indexes, etc), further steps are needed to update the application to use the new table name. ActiveRecord heavily relies on this data, for example, to initialize new models.

To work around this limitation, we need to tell ActiveRecord to acquire this information from original table using the new table name.

Online Migrations provides several helpers to implement column renaming:

1. Instruct Rails that you are going to rename a column:

OnlineMigrations.config.column_renames = {
  "users" => {
    "name" => "first_name"
  }
}

1. Deploy
2. Create a VIEW with aliased column:

class InitializeRenameUsersNameToFirstName < ActiveRecord::Migration[7.0]
  def change
    initialize_column_rename :users, :name, :first_name
  end
end

1. Replace usages of the old column with a new column in the codebase
2. Deploy
3. Remove the column rename config from step 1
4. Remove the VIEW created in step 3:

class FinalizeRenameUsersNameToFirstName < ActiveRecord::Migration[7.0]
  def change
    finalize_column_rename :users, :name, :first_name
  end
end

1. Deploy

Renaming a table

:x: Bad

Renaming a table that's in use will cause errors in your application.

class RenameClientsToUsers < ActiveRecord::Migration[7.0]
  def change
    rename_table :clients, :users
  end
end

:white_check_mark: Good

The "classic" approach suggests creating a new table and copy data/indexes/etc to it from the old table. This can be costly for very large tables. There is a trick that helps to avoid such heavy operations.

The technique is built on top of database views, using the following steps:

1. Rename the database table
2. Create a VIEW using the old table name by pointing to the new table name
3. Add a workaround for ActiveRecord's schema cache

For the previous example, to rename name column to first_name of the users table, we can run:

BEGIN;
ALTER TABLE clients RENAME TO users;
CREATE VIEW clients AS SELECT * FROM users;
COMMIT;

As database views do not expose the underlying table schema (default values, not null constraints, indexes, etc), further steps are needed to update the application to use the new table name. ActiveRecord heavily relies on this data, for example, to initialize new models.

To work around this limitation, we need to tell ActiveRecord to acquire this information from original table using the new table name.

Online Migrations provides several helpers to implement table renaming:

1. Instruct Rails that you are going to rename a table:

OnlineMigrations.config.table_renames = {
  "clients" => "users"
}

1. Deploy
2. Create a VIEW:

class InitializeRenameClientsToUsers < ActiveRecord::Migration[7.0]
  def change
    initialize_table_rename :clients, :users
  end
end

1. Replace usages of the old table with a new table in the codebase
2. Remove the table rename config from step 1
3. Deploy
4. Remove the VIEW created in step 3:

class FinalizeRenameClientsToUsers < ActiveRecord::Migration[7.0]
  def change
    finalize_table_rename :clients, :users
  end
end

1. Deploy

Creating a table with the force option

:x: Bad

The force option can drop an existing table.

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users, force: true do |t|
      # ...
    end
  end
end

:white_check_mark: Good

Create tables without the force option.

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users do |t|
      # ...
    end
  end
end

If you intend to drop an existing table, run drop_table first.

Adding a check constraint

:x: Bad

Adding a check constraint blocks reads and writes while every row is checked.

class AddCheckConstraint < ActiveRecord::Migration[7.0]
  def change
    add_check_constraint :users, "char_length(name) >= 1", name: "name_check"
  end
end

:white_check_mark: Good

Add the check constraint without validating existing rows, and then validate them in a separate transaction:

class AddCheckConstraint < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_check_constraint :users, "char_length(name) >= 1", name: "name_check", validate: false
    validate_check_constraint :users, name: "name_check"
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

Setting NOT NULL on an existing column

:x: Bad

Setting NOT NULL on an existing column blocks reads and writes while every row is checked.

class ChangeUsersNameNull < ActiveRecord::Migration[7.0]
  def change
    change_column_null :users, :name, false
  end
end

:white_check_mark: Good

Instead, add a check constraint and validate it in a separate transaction:

class ChangeUsersNameNull < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_not_null_constraint :users, :name, name: "users_name_null", validate: false
    validate_not_null_constraint :users, :name, name: "users_name_null"
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

A NOT NULL check constraint is functionally equivalent to setting NOT NULL on the column (but it won't show up in schema.rb in Rails < 6.1). In PostgreSQL 12+, once the check constraint is validated, you can safely set NOT NULL on the column and drop the check constraint.

class ChangeUsersNameNullDropCheck < ActiveRecord::Migration[7.0]
  def change
    # in PostgreSQL 12+, you can then safely set NOT NULL on the column
    change_column_null :users, :name, false
    remove_check_constraint :users, name: "users_name_null"
  end
end

Executing SQL directly

Online Migrations does not support inspecting what happens inside an execute call, so cannot help you here. Make really sure that what you're doing is safe before proceeding, then wrap it in a safety_assured { ... } block:

class ExecuteSQL < ActiveRecord::Migration[7.0]
  def change
    safety_assured { execute "..." }
  end
end

Adding an index non-concurrently

:x: Bad

Adding an index non-concurrently blocks writes.

class AddIndexOnUsersEmail < ActiveRecord::Migration[7.0]
  def change
    add_index :users, :email, unique: true
  end
end

:white_check_mark: Good

Add indexes concurrently.

class AddIndexOnUsersEmail < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_index :users, :email, unique: true, algorithm: :concurrently
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail. Also, note that indexes on new tables (those created in the same migration) don't require this.

Removing an index non-concurrently

:x: Bad

While actual removing of an index is usually fast, removing it non-concurrently tries to obtain an ACCESS EXCLUSIVE lock on the table, waiting for all existing queries to complete and blocking all the subsequent queries (even SELECTs) on that table until the lock is obtained and index is removed.

class RemoveIndexOnUsersEmail < ActiveRecord::Migration[7.0]
  def change
    remove_index :users, :email
  end
end

:white_check_mark: Good

Remove indexes concurrently.

class RemoveIndexOnUsersEmail < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    remove_index :users, :email, algorithm: :concurrently
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

Replacing an index

:x: Bad

Removing an old index before replacing it with the new one might result in slow queries while building the new index.

class AddIndexOnCreationToProjects < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    remove_index :projects, :creator_id, algorithm: :concurrently
    add_index :projects, [:creator_id, :created_at], algorithm: :concurrently
  end
end

Note: If removed index is covered by any existing index, then it is safe to remove the index before replacing it with the new one.

:white_check_mark: Good

A safer approach is to create the new index and then delete the old one.

class AddIndexOnCreationToProjects < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_index :projects, [:creator_id, :created_at], algorithm: :concurrently
    remove_index :projects, :creator_id, algorithm: :concurrently
  end
end

Adding a reference

:x: Bad

Rails adds an index non-concurrently to references by default, which blocks writes. Additionally, if foreign_key option (without validate: false) is provided, both tables are blocked while it is validated.

class AddUserToProjects < ActiveRecord::Migration[7.0]
  def change
    add_reference :projects, :user, foreign_key: true
  end
end

:white_check_mark: Good

Make sure the index is added concurrently and the foreign key is added in a separate migration. Or you can use add_reference_concurrently helper. It will create a reference and take care of safely adding index and/or foreign key.

class AddUserToProjects < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_reference_concurrently :projects, :user
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

Adding a foreign key

:x: Bad

Adding a foreign key blocks writes on both tables.

class AddForeignKeyToProjectsUser < ActiveRecord::Migration[7.0]
  def change
    add_foreign_key :projects, :users
  end
end

or

class AddReferenceToProjectsUser < ActiveRecord::Migration[7.0]
  def change
    add_reference :projects, :user, foreign_key: true
  end
end

:white_check_mark: Good

Add the foreign key without validating existing rows, and then validate them in a separate transaction.

class AddForeignKeyToProjectsUser < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_foreign_key :projects, :users, validate: false
    validate_foreign_key :projects, :users
  end
end

Note: If you forget disable_ddl_transaction!, the migration will fail.

Adding a json column

:x: Bad

There's no equality operator for the json column type, which can cause errors for existing SELECT DISTINCT queries in your application.

class AddSettingsToProjects < ActiveRecord::Migration[7.0]
  def change
    add_column :projects, :settings, :json
  end
end

:white_check_mark: Good

Use jsonb instead.

class AddSettingsToProjects < ActiveRecord::Migration[7.0]
  def change
    add_column :projects, :settings, :jsonb
  end
end

Using primary key with short integer type

:x: Bad

When using short integer types as primary key types, there is a risk of running out of IDs on inserts. The default type in ActiveRecord < 5.1 for primary and foreign keys is INTEGER, which allows a little over of 2 billion records. Active Record 5.1 changed the default type to BIGINT.

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users, id: :integer do |t|
      # ...
    end
  end
end

:white_check_mark: Good

Use one of bigint, bigserial, uuid instead.

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users, id: :bigint do |t| # bigint is the default for Active Record >= 5.1
      # ...
    end
  end
end

Hash indexes

:x: Bad - PostgreSQL < 10

Hash index operations are not WAL-logged, so hash indexes might need to be rebuilt with REINDEX after a database crash if there were unwritten changes. Also, changes to hash indexes are not replicated over streaming or file-based replication after the initial base backup, so they give wrong answers to queries that subsequently use them. For these reasons, hash index use is discouraged.

class AddIndexToUsersOnEmail < ActiveRecord::Migration[7.0]
  def change
    add_index :users, :email, unique: true, using: :hash
  end
end

:white_check_mark: Good - PostgreSQL < 10

Use B-tree indexes instead.

class AddIndexToUsersOnEmail < ActiveRecord::Migration[7.0]
  def change
    add_index :users, :email, unique: true # B-tree by default
  end
end

Adding multiple foreign keys

:x: Bad

Adding multiple foreign keys in a single migration blocks reads and writes on all involved tables until migration is completed. Avoid adding foreign key more than once per migration file, unless the source and target tables are identical.

class CreateUserProjects < ActiveRecord::Migration[7.0]
  def change
    create_table :user_projects do |t|
      t.belongs_to :user, foreign_key: true
      t.belongs_to :project, foreign_key: true
    end
  end
end

:white_check_mark: Good

Add additional foreign keys in separate migration files. See adding a foreign key for how to properly add foreign keys.

class CreateUserProjects < ActiveRecord::Migration[7.0]
  def change
    create_table :user_projects do |t|
      t.belongs_to :user, foreign_key: true
      t.belongs_to :project, foreign_key: false
    end
  end
end

class AddForeignKeyFromUserProjectsToProject < ActiveRecord::Migration[7.0]
  def change
    add_foreign_key :user_projects, :projects
  end
end

Removing a table with multiple foreign keys

:x: Bad

Removing a table with multiple foreign keys blocks reads and writes on all involved tables until migration is completed. Remove all the foreign keys first.

Assuming, projects has foreign keys on users.id and repositories.id:

class DropProjects < ActiveRecord::Migration[7.0]
  def change
    drop_table :projects
  end
end

:white_check_mark: Good

Remove all the foreign keys first:

class RemoveProjectsUserFk < ActiveRecord::Migration[7.0]
  def change
    remove_foreign_key :projects, :users
  end
end

class RemoveProjectsRepositoryFk < ActiveRecord::Migration[7.0]
  def change
    remove_foreign_key :projects, :repositories
  end
end

Then remove the table:

class DropProjects < ActiveRecord::Migration[7.0]
  def change
    drop_table :projects
  end
end

Mismatched reference column types

:x: Bad

Reference columns should be of the same type as the referenced primary key. Otherwise, there's a risk of bugs caused by IDs representable by one type but not the other.

Assuming, there is a users table with bigint primary key type:

class AddUserIdToProjects < ActiveRecord::Migration[7.0]
  def change
    add_column :projects, :user_id, :integer
  end
end

:white_check_mark: Good

Add a reference column of the same type as a referenced primary key.

Assuming, there is a users table with bigint primary key type:

class AddUserIdToProjects < ActiveRecord::Migration[7.0]
  def change
    add_column :projects, :user_id, :bigint
  end
end

Adding a single table inheritance column

:x: Bad

Adding a single table inheritance column might cause errors in old instances of your application.

class AddTypeToUsers < ActiveRecord::Migration[7.0]
  def change
    add_column :users, :string, :type, default: "Member"
  end
end

After the migration was ran and the column was added, but before the code is fully deployed to all instances, an old instance may be restarted (due to an error etc). And when it will fetch 'User' records from the database, 'User' will look for a 'Member' subclass (from the 'type' column) and fail to locate it unless it is already defined.

:white_check_mark: Good

A safer approach is to:

1. ignore the column:

  # For ActiveRecord 5+
  class User < ApplicationRecord
    self.ignored_columns = ["type"]
  end

  # For ActiveRecord < 5
  class User < ActiveRecord::Base
    def self.columns
      super.reject { |c| c.name == "type" }
    end
  end

1. deploy
2. remove the column ignoring from step 1 and apply initial code changes
3. deploy

Assuring Safety

To mark a step in the migration as safe, despite using a method that might otherwise be dangerous, wrap it in a safety_assured block.

class MySafeMigration < ActiveRecord::Migration[7.0]
  def change
    safety_assured { remove_column :users, :some_column }
  end
end

Certain methods like execute and change_table cannot be inspected and are prevented from running by default. Make sure what you're doing is really safe and use this pattern.

Configuring the gem

There are a few configurable options for the gem. Custom configurations should be placed in a online_migrations.rb initializer.

OnlineMigrations.configure do |config|
  # ...
end

Note: Check the source code for the list of all available configuration options.

Custom checks

Add your own custom checks with:

# config/initializers/online_migrations.rb

config.add_check do |method, args|
  if method == :add_column && args[0].to_s == "users"
    stop!("No more columns on the users table")
  end
end

Use the stop! method to stop migrations.

Note: Since remove_column, execute and change_table always require a safety_assured block, it's not possible to add a custom check for these operations.

Disable Checks

Disable specific checks with:

# config/initializers/online_migrations.rb

config.disable_check(:remove_index)

Check the source code for the list of keys.

Down Migrations / Rollbacks

By default, checks are disabled when migrating down. Enable them with:

# config/initializers/online_migrations.rb

config.check_down = true

Custom Messages

You can customize specific error messages:

# config/initializers/online_migrations.rb

config.error_messages[:add_column_default] = "Your custom instructions"

Check the source code for the list of keys.

Migration Timeouts

It’s extremely important to set a short lock timeout for migrations. This way, if a migration can't acquire a lock in a timely manner, other statements won't be stuck behind it.

Add timeouts to config/database.yml:

production:
  connect_timeout: 5
  variables:
    lock_timeout: 10s
    statement_timeout: 15s

Or set the timeouts directly on the database user that runs migrations:

ALTER ROLE myuser SET lock_timeout = '10s';
ALTER ROLE myuser SET statement_timeout = '15s';

Lock Timeout Retries

You can configure this gem to automatically retry statements that exceed the lock timeout:

# config/initializers/online_migrations.rb

config.lock_retrier = OnlineMigrations::ExponentialLockRetrier.new(
  attempts: 30,                # attempt 30 retries
  base_delay: 0.01.seconds,    # starting with delay of 10ms between each unsuccessful try, increasing exponentially
  max_delay: 1.minute,         # maximum delay is 1 minute
  lock_timeout: 0.05.seconds   # and 50ms set as lock timeout for each try
)

When statement within transaction fails - the whole transaction is retried.

To permanently disable lock retries, you can set lock_retrier to nil. To temporarily disable lock retries while running migrations, set DISABLE_LOCK_RETRIES env variable.

Note: Statements are retried by default, unless lock retries are disabled. It is possible to implement more sophisticated lock retriers. See source code for the examples.

Existing Migrations

To mark migrations as safe that were created before installing this gem, configure the migration version starting after which checks are performed:

# config/initializers/online_migrations.rb

config.start_after = 20220101000000

# or if you use multiple databases (ActiveRecord 6+)
config.start_after = { primary: 20211112000000, animals: 20220101000000 }

Use the version from your latest migration.

Target Version

If your development database version is different from production, you can specify the production version so the right checks run in development.

# config/initializers/online_migrations.rb

config.target_version = 10 # or "12.9" etc

# or if you use multiple databases (ActiveRecord 6+)
config.target_version = { primary: 10, animals: 14.1 }

For safety, this option only affects development and test environments. In other environments, the actual server version is always used.

Small Tables

Most projects have tables that are known to be small in size. These are usually "settings", "prices", "plans" etc. It is considered safe to perform most of the dangerous operations on them, like adding indexes, columns etc.

To mark tables as small:

config.small_tables = [:settings, :prices]

Verbose SQL logs

For any operation, Online Migrations can output the performed SQL queries.

This is useful to demystify online_migrations inner workings, and to better investigate migration failure in production. This is also useful in development to get a better grasp of what is going on for high-level statements like add_column_with_default.

Consider migration, running on PostgreSQL < 11:

class AddAdminToUsers < ActiveRecord::Migration[7.0]
  disable_ddl_transaction!

  def change
    add_column_with_default :users, :admin, :boolean, default: false
  end
end

Instead of the traditional output:

== 20220106214827 AddAdminToUsers: migrating ==================================
-- add_column_with_default(:users, :admin, :boolean, {:default=>false})
   -> 0.1423s
== 20220106214827 AddAdminToUsers: migrated (0.1462s) =========================

Online Migrations will output the following logs:

== 20220106214827 AddAdminToUsers: migrating ==================================
   (0.3ms)  SHOW lock_timeout
   (0.2ms)  SET lock_timeout TO '50ms'
-- add_column_with_default(:users, :admin, :boolean, {:default=>false})
  TRANSACTION (0.1ms)  BEGIN
   (37.7ms)  ALTER TABLE "users" ADD "admin" boolean DEFAULT NULL
   (0.5ms)  ALTER TABLE "users" ALTER COLUMN "admin" SET DEFAULT FALSE
  TRANSACTION (0.3ms)  COMMIT
   Load (0.3ms)  SELECT "users"."id" FROM "users" WHERE ("users"."admin" != FALSE OR "users"."admin" IS NULL) ORDER BY "users"."id" ASC LIMIT $1  [["LIMIT", 1]]
   Load (0.5ms)  SELECT "users"."id" FROM "users" WHERE ("users"."admin" != FALSE OR "users"."admin" IS NULL) AND "users"."id" >= 1 ORDER BY "users"."id" ASC LIMIT $1 OFFSET $2  [["LIMIT", 1], ["OFFSET", 1000]]
  #<Class:0x00007f8ae3703f08> Update All (9.6ms)  UPDATE "users" SET "admin" = $1 WHERE ("users"."admin" != FALSE OR "users"."admin" IS NULL) AND "users"."id" >= 1 AND "users"."id" < 1001  [["admin", false]]
   Load (0.8ms)  SELECT "users"."id" FROM "users" WHERE ("users"."admin" != FALSE OR "users"."admin" IS NULL) AND "users"."id" >= 1001 ORDER BY "users"."id" ASC LIMIT $1 OFFSET $2  [["LIMIT", 1], ["OFFSET", 1000]]
  #<Class:0x00007f8ae3703f08> Update All (1.5ms)  UPDATE "users" SET "admin" = $1 WHERE ("users"."admin" != FALSE OR "users"."admin" IS NULL) AND "users"."id" >= 1001  [["admin", false]]
   -> 0.1814s
   (0.4ms)  SET lock_timeout TO '5s'
== 20220106214827 AddAdminToUsers: migrated (0.1840s) =========================

So you can actually check which steps are performed.

Note: The SHOW statements are used by Online Migrations to query settings for their original values in order to restore them after the work is done.

To enable verbose sql logs:

# config/initializers/online_migrations.rb

config.verbose_sql_logs = true

This feature is enabled by default in a production Rails environment. You can override this setting via ONLINE_MIGRATIONS_VERBOSE_SQL_LOGS environment variable.

Background Migrations

Read [BACKGROUND_MIGRATIONS.md](BACKGROUND_MIGRATIONS.md) on how to perform data migrations on large tables.

Credits

Thanks to strong_migrations gem, GitLab and maintenance_tasks gem for the original ideas.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/fatkodima/online_migrations.

Development

After checking out the repo, run bundle install to install dependencies. Run createdb online_migrations_test to create a test database. Then, run bundle exec rake test to run the tests. This project uses multiple Gemfiles to test against multiple versions of ActiveRecord; you can run the tests against the specific version with BUNDLE_GEMFILE=gemfiles/activerecord_61.gemfile bundle exec rake test.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Additional resources

Alternatives:

• https://github.com/ankane/strong_migrations
• https://github.com/LendingHome/zero_downtime_migrations
• https://github.com/braintree/pg_ha_migrations
• https://github.com/doctolib/safe-pg-migrations

Interesting reads:

• Explicit Locking
• When Postgres blocks: 7 tips for dealing with locks
• PostgreSQL rocks, except when it blocks: Understanding locks
• PostgreSQL at Scale: Database Schema Changes Without Downtime
• Adding a NOT NULL CONSTRAINT on PG Faster with Minimal Locking
• Adding columns with default values to really large tables in Postgres + Rails
• Safe Operations For High Volume PostgreSQL
• Stop worrying about PostgreSQL locks in your Rails migrations
• Avoiding integer overflows with zero downtime

Maybe TODO

• support MySQL
• support other ORMs

Background migrations:

• extract as a separate gem
• add UI
• support batching over non-integer and multiple columns

Comparison to strong_migrations

This gem was heavily inspired by the strong_migrations and GitLab's approaches to database migrations. This gem is a superset of strong_migrations, feature-wise, and has the same APIs.

The main differences are:

1. strong_migrations provides you text guidance on how to run migrations safer and you should implement them yourself. This new gem has actual code helpers (and suggests them when fails on unsafe migrations) you can use to do what you want. See example for an example.

It has migrations helpers for:

• renaming tables/columns
• changing columns types (including changing primary/foreign keys from integer to bigint)
• adding columns with default values
• backfilling data
• adding different types of constraints
• and others

1. This gem has a powerful internal framework for running data migrations on very large tables using background migrations.

For example, you can use background migrations to migrate data that’s stored in a single JSON column to a separate table instead; backfill values from one column to another (as one of the steps when changing column type); or backfill some column’s value from an API.

1. Yet, it has more checks for unsafe changes (see checks).

2. Currently, this gem supports only PostgreSQL, while strong_migrations also checks MySQL and MariaDB migrations.

3. This gem is more flexible in terms of configuration - see config file for additional configuration options.

License

The gem is available as open source under the terms of the MIT License.