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Set transaction isolation level in the ActiveRecord in a database agnostic way. Works with MySQL, PostgreSQL and SQLite as long as you are using new adapters mysql2, pg or sqlite3. Supports all ANSI SQL isolation levels: :serializable, :repeatable_read, :read_committed, :read_uncommitted.

See also transaction_retry gem for auto-retrying transactions on deadlocks and serialization errors.

ActiveRecord::Base.isolation_level( :serializable ) do
  # your code
end

Add this to your Gemfile:

gem 'transaction_isolation'

Then run:

bundle

It works out of the box with Ruby on Rails.

If you have a standalone ActiveRecord-based project you'll need to call:

TransactionIsolation.apply_activerecord_patch     # after connecting to the database

after connecting to the database. This is because ActiveRecord loads adapters lazilly and only then they can be patched.

• Setting transaction isolation level: :serializable, :repeatable_read, :read_committed, :read_uncommitted
• Auto-reverting to the original isolation level after the block
• Database agnostic
• MySQL, PostgreSQL and SQLite supported
• Exception translation. All deadlocks and serialization errors are wrapped in a ActiveRecord::TransactionIsolationConflict exception
• Use it in your Rails application or a standalone ActiveRecord-based project

This gem was initially developed for and successfully works in production at Kontomierz.pl - the finest Polish personal finance app.

When implementing a table-based job queue you should ensure that only one worker process can pop a particular job from the queue. Wrapping your code in a transaction is not enough because by default databases do not isolate transactions to the full extent, which leads to occasional phantom reads. It is therefore necessary to manually raise the transaction isolation level. The highest level of transaction isolation is called "serializable" and that's what we need here:

class QueuedJob < ActiveRecord::Base

  # Job status
  TODO = 1
  PROCESSING = 2
  DONE = 3

  # Returns first job from the queue or nil if the queue is empty
  def pop
    QueuedJob.isolation_level( :serializable ) do
      QueuedJob.transaction do
        queued_job = find_by_status( TODO )
        if queud_job
          queued_job.update_attribute( :status, PROCESSING )
          return queued_job
        else
          return nil
        end
      end
    end
  rescue ActiveRecord::TransactionIsolationConflict => e
    logger.warn( e.message )
    retry
  end

end

Read more about isolation levels in Wikipedia

• Ruby 1.9.2
• ActiveRecord 3.0.11+

Run tests on the selected database (mysql2 by default):

db=mysql2 bundle exec rake test
db=postgresql bundle exec rake test
db=sqlite3 bundle exec rake test

Run tests on all supported databases:

./tests

Database configuration is hardcoded in test/db/db.rb; feel free to improve this and submit a pull request.

You should be very suspicious about any gem that monkey patches your stock Ruby on Rails framework.

This gem is carefully written to not be more intrusive than it needs to be:

• introduces several new methods to Mysql2Adapter, PostgreSQLAdapter, SQLite3Adapter; names are carefully taken to not collide with future changes
• wraps #translate_exception method using alias_method_chain to add new translation
• introduces new class ActiveRecord::TransactionIsolationConflict in the ActiveRecord module
• introduces new convenience method ActiveRecord::Base.isolation_level akin to ActiveRecord::Base.transaction

Released under the MIT license. Copyright (C) 2012 Piotr 'Qertoip' Włodarek.