Monitor database table for external changes from within Rails application

Question

I'm integrating some non-rails-model tables in my Rails application. Everything works out very nicely, the way I set up the model is:

class Change < ActiveRecord::Base
  establish_connection(ActiveRecord::Base.configurations["otherdb_#{RAILS_ENV}"])
  set_table_name  "change"
end

This way I can use the Change model for all existing records with find etc.

Now I'd like to run some sort of notification, when a record is added to the table. Since the model never gets created via Change.new and Change.save using ActiveRecord::Observer is not an option.

Is there any way I can get some of my Rails code to be executed, whenever a new record is added? I looked at delayed_job but can't quite get my head around, how to set that up. I imagine it evolves around a cron-job, that selects all rows that where created since the job last ran and then calls the respective Rails code for each row.

Update Currently looking at Javan's Whenever, looks like it can solve the 'run rails code from cron part'.

Answer 1

Yeah, you'll either want some sort of background task processor (Delayed::Job is one of the popular ones, or you can fake your own with the Daemon library or similar) or to setup a cronjob that runs on some sort of schedule. If you want to check frequently (every minute, say) I'd recommend the Delayed::Job route, if it's longer (every hour or so) a cron job will do it just fine.

Going the DJ route, you'd need to create a job that would check for new records, process them if there are any, then requeue the job, as each job is marked "completed" when it's finished.

-jon

Answer 2

This is what I finally did: Use Whenever, because it integrates nicely with Capistrano and showed me how to run Rails code from within cron. My missing peace was basically

script/runner -e production 'ChangeObserver.recentchanges'

which is now run every 5 minutes. The recentchanges reads the last looked-at ID from a tmp-file, pulls all new Change records which have a higher ID than that and runs the normal observer code for each record (and saves the highest looked-at ID to the tmp-file, of course).

Answer 3

As usual with monitoring state changes, there are two approaches : polling and notification. You seem to have chose to go the polling way for now (having a cron job look at the state of the database on a regular basis and execute some code if that changed) You can do the same thing using one of the rails schedulers, there are a few out there (google will find them readily, they have various feature sets, I'll let you choose the one which suits your need if you got that way)

You could also try to go the notification way depending on your database. Some database support both triggers and external process execution or specific notification protocols. In this case you are notified by the database itself that the table changed. there are many such options for various DBMS in http://stackoverflow.com/questions/812233/getting-events-from-a-database