Richard Huang's Website

after_commit

We are using RabbitMQ as our message queue system, ruby client is workling. This week we encountered a strange issue, we create a notification, and define an after_create callback to ask workling to find that notification and then push the notification to twitter or facebook, it works fine except that sometimes it will raise an error said "can't find the notification with the specified ID"

class Notification < ActiveRecord::Base
  after_create :asyns_send_notification
  ......
  def async_send_notification
    NotificationWorker.async_send_notification({:notification_id => id})
  end
end

class NotificationWorker < Workling::Base
  def send_notification(params)
    notification = Notification.find(params[:notification_id])
    ......
  end
end

It's wierd the notification_id is passed to NotificationWorker, that means the notification is already created, the notification is supposed to be existed.

After talking with MySQL DBA, we find the problem is the find sql is executed before insert transaction is committed.

Let me describe it step by step.

Notification sends "Transaction Begin" command
Notification sends "INSERT" command
Notification gets "next sequence value" as new object id
Notification sends "new object id" to NotificationWorker
NotificationWorker sends "SELECT" command to find notification object
Notification sends "Transaction Commit" command

As you seen, at step 5, the new notification is not existed in the mysql database yet, so the error "Not found" will be raised.

To solve this issue, we can use after_commit callback.

In rails 2.x, we should install after_commit gem, in rails 3.x, after_commit callback is supported by default.

class Notification < ActiveRecord::Base
  after_commit_on_create :asyns_send_notification
  ......
  def async_send_notification
    NotificationWorker.async_send_notification({:notification_id => id})
  end
end