gocardless/activerecord-safer_migrations

ActiveRecord safer migration helpers

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Note: this library only supports PostgreSQL 9.3+. If you're interested in adding support for other databases, we're open to pull requests!

Postgres holds ACCESS EXCLUSIVE locks for almost all DDL operations. ACCESS EXCLUSIVE locks conflict with all other table-level locks, which can cause issues in several situations. For instance:

1. If the lock is held for a long time, all other access to the table will be blocked, which can result in downtime.
2. Even if the lock is only held briefly, it will block all other access to the table while it is in the lock queue, as it conflicts with all other locks. The lock can't be acquired until all other queries ahead of it have finished, so having to wait on long-running queries can also result in downtime. See here for more details.

Both these issues can be avoided by setting timeouts on the migration connection - statement_timeout and lock_timeout respectively.

Once this gem is loaded, all migrations will automatically have a lock_timeout and a statement_timeout set. The initial lock_timeout default is 750ms, and the initial statement_timeout default is 1500ms. Both defaults can be easily changed (e.g. in a Rails initializer).

ActiveRecord::SaferMigrations.default_lock_timeout = 1000
ActiveRecord::SaferMigrations.default_statement_timeout = 2000

To explicitly set timeouts for a given migration, use the set_lock_timeout and set_statement_timeout class methods in the migration.

class LockTest < ActiveRecord::Migration
  set_lock_timeout(250)
  set_statement_timeout(750)

  def change
    create_table :lock_test
  end
end

To disable timeouts for a migration, use the disable_lock_timeout! and disable_statement_timeout! class methods. Note that this is extremely dangerous if you're doing any schema alterations in your migration.

class LockTest < ActiveRecord::Migration
  # Only do this if you really know what you're doing!
  disable_lock_timeout!
  disable_statement_timeout!

  def change
    create_table :lock_test
  end
end

Use with PgBouncer

This gem sets session-level settings on Postgres connections. If you're using PgBouncer in transaction pooling mode, using session-level settings is dangerous, as you can't guarantee which connection will receive the setting. For this reason, this gem is incompatible with transaction-pooling and should only be used if migrations are run on connections that support session-level features.