Robby on Rails

Not terribly long ago, I announced Active Delegate, which was a really lightweight plugin that I developed to allow models to talk to multiple databases for specific methods. The plugin worked great for really simple situations, like individual models.. but when it came time to test with associations it fell apart. I haven’t had a chance to work on any updates and knew that it was going to take more work to get it going.

Earlier this week, we helped one of our bigger clients launch their new web site¹. For the deployment, we needed to send all writes to a master database and a reads to slaves (initial deployment is talking to almost 10 slaves spread around the globe!). We needed something to get integrated quickly and decided to ditch Active Delegate for the time being and began looking at the following options.

I spoke with Rick Olson² and he pointed me to a new plugin that he hasn’t really released yet. So, I’m going to do him a favor and announce it for him. Of course… I got his permission first… ;-)

Announcing Masochism!

Masochism³ is a new plugin for Ruby on Rails that allows you to delegate all writes to a master database and reads to a slave database. The configuration process is just a few lines in your environment file and the plugin takes care of the rest.

Installing Masochism

With piston, you can import Masochism with:

  $ cd vendor/plugins
  $ piston import http://ar-code.svn.engineyard.com/plugins/masochism/

• To learn more about piston, read Every Second Counts with a Piston in your trunk

You can also install it with the old-fashioned way:

  $ ./script/plugin install -x http://ar-code.svn.engineyard.com/plugins/masochism/

Configuring Masochism

The first thing that you’ll need to do is add another database connection in config/database.yml for master_database. By default, Masochism expects you to have a production database, which will be the read-only/slave database. The master_database will be the connection details for your (you guessed it…) master database.

# config/database.yml  
production:
  database: masochism_slave_database
  adapter: postgresql
  host: slavedb1.hostname.tld
  ...

master_database:
  database: masochism_master_database
  adapter: postgresql
  host: masterdb.hostname.tld
  ...

The idea here is that replication will be handled elsewhere and your application can reap the benefits of talking to the slave database for all of it’s read-only operations and let the master database(s) spend their time writing data.

The next step is to set this up in your environment file. In our scenario, this was config/environments/production.rb.

# Add this to config/environments/production.rb
config.after_initialize do 
  ActiveReload::ConnectionProxy.setup!    
end

Voila, you should be good to go now. As I mentioned, we’ve only been using this for this past week and we’ve had to address a few problems that the initial version of the plugin didn’t address. One of our developers, Andy Delcambre, just posted an article to show how we had a problem with using ActiveRecord observers and masochism, which we’re sending over a patch for now.

As we continue to monitor how this solution works, we’ll report any findings on our blog. In the meantime, I’d be interested in knowing what you’re using to solve this problem. :-)

¹ Contiki, a cool travel company we’re working with

² Rick just moved to Portland… welcome to stump town!

³ The Masochism plugin README

We have a client that already has some database replication going on in their deployment and needed to have most of their Ruby on Rails application pull from slave servers, but the few writes would go to the master, which would then end up in their slaves.

So, I was able to quickly extend ActiveRecord with just two methods to achieve this. Anyhow, earlier today, someone in #caboose asked if there was any solutions to this and it prompted me to finally package this up into a quick and dirty Rails plugin.

Introducing… Active Delegate!

To install, do the following:

cd vendor/plugins;
piston import http://svn.planetargon.org/rails/plugins/active_delegate

Next, you’ll need to create another database entry in your database.yml.

login: &login
  adapter: postgresql
  host: localhost
  port: 5432

development:
  database: rubyurl_development
  <<: *login

test:
  database: rubyurl_test
  <<: *login

production:
  database: rubyurl_servant
  <<: *login

# NOTICE THE NEXT ENTRY/KEY
master_database:
  database: rubyurl_master
  <<: *login

At this point, your Rails application won’t talk to the master_database, because nothing is being told to connect to it. So, the current solution with Active Delegate is to create an ActiveRecord model that will act as a connection handler.

  # app/models/master_database.rb
  class MasterDatabase < ActiveRecord::Base
    handles_connection_for :master_database # <-- this matches the key from our database.yml
  end  

Now, in the model(s) that we’ll want to have talk to this database, we’ll do add the following.

  # app/models/animal.rb
  class Animal < ActiveRecord::Base
     delegates_connection_to :master_database, :on => [:create, :save, :destroy]
  end

Now, when your application performs a create, save, or destroy, it’ll talk to the master database and your find calls will retrieve data from your servant database.

It’s late on a Friday afternoon and I felt compelled to toss this up for everyone. I think that this could be improved quite a bit, but it’s working great for the original problem that needed to be solved.

If you have feedback and/or bugs, please send us tickets.

Yesterday, I wrote a short post titled, Observers Big and Small, about using Observers in your Rails applications.

The following questions were raised in the comments.

When should I use an Observer?

Eric Allam asks…

“Why not just use ActiveRecord callback hooks instead of Observers? Are Observers more powerful or is it just a matter of preference?”

Eric, this is an excellent question. I’d say that a majority of the time, using the ActiveRecord callbacks in your models is going to work for your situation. However, there are times that you want the same methods to be called through callbacks. For example, let’s take a recent problem that we used an observer to solve.

Graeme is working on implementing Ferret into a project that we’re developing for a client. With the use of Ferret, we can index and later search through content over several objects into a format that makes sense for our implementation goals. Each time an object is created and updated, we have to update our Ferret indexes to reflect these changes. The most obvious location that we can call our indexing methods is in each models’ callbacks, but this violates the DRY[1] principle. So, we created an Observer, which observes each of the models that need these methods to be called. In fact, as far as we’re concerned, the fact that we’re indexing some of its data, is none of its business. We only want our models to be concerned with that they’re designed to be concerned about. We may opt to change our indexing solution in the future and we’d just need to rethink that at the Observer level and not change anything about the business logic in our models.

This is the sort of scenario when using an Observer makes great sense in your application.

Logging from an Observer

Adam R. asks…

“I’d also like the ability to use the logger from within an observer, but that’s another issue.”

I assume that you are referring to the logger method? I always forget to even use that method. I do know that the following works just fine in an Observer.

class IndexObserver < ActiveRecord::Observer
  observer Article, Editorial, BlogPost, ClassifiedAd

  def after_save(model)
    RAILS_DEFAULT_LOGGER.warn("Every single day. Every word you say. Every game you play. Every night you stay. I'll be watching you.")
    # execute something fun
  end
end  

This will output to your log file without any problem.

This reminded me of when I used to want to log from Unit Tests.

(few minutes later)

Okay, I just attempted to use logger from an Observer and you’re right… it doesn’t currently work. There is a simple fix though, just extend ActiveRecord::Observer to add a logger method like so and require it in config/environment.rb (much like I did in with unit tests).

# lib/observer_extensions.rb
class ActiveRecord::Observer
  def logger
    RAILS_DEFAULT_LOGGER
  end
end

This will give you a solution to that problem.

class FooObserver < ActiveRecord::Observer
  observer Foo

  def after_save(model)
    logger.warn("I wonder if the #{address.class} knows that I've been watching it all along?")
  end
end  

Observers Spy for Us

Most often, I look at Observers as being the guys that I hire to spy on my models. I don’t want my models to know that they’re being spied on and I’d like to keep it that way. They don’t solve all of our problems and it’s easy to overuse them. However, I have found several cases that they made a lot of sense and most of those cases have been where we’ve had the same things occurring in our model’s callbacks.

If you have other questions related to Observers, feel free to let me know. If you’re already using Observers, perhaps you could post a comment and/or blog post response with an example of when and how you use Observers in your Rails applications.

Related Posts

• Observers Big and Small
• Where did my Observer go?
• Observers in Ruby on Rails, Tom Eric

¹ Don’t Repeat Yourself

My colleague, Gary, keeps a stack of Ruby and Rails books on his desk and was implementing an Observer into a client project. It appears that the Agile Web Development with Rails book is still encouraging people to do the following in order to load an Observer.

# app/models/flower_observer.rb
class FlowerObserver < ActiveRecord::Observer
  observe Flower

  def after_create(model)
    # model.do_something!
  end
end

# controller(s)
class FlowerController < ApplicationController
  observer :flower_observer
end

What is wrong with this approach?

Well, in order for your Observer to be used, the model(s) callbacks that it is observing need to be triggered through a controller. If you end up writing any scheduled rake tasks, your observer will not be called. In my opinion, the controller shouldn’t know this much about the model. In fact, the model doesn’t even really know about it’s observer… so why should a controller?

This was actually changed a long time ago (I previously blogged about a different solution here) and the Rails docs for ActiveRecord::Observer are currently correct.

Observers in the Environment

If you open up a recent version of config/environment.rb, you notice in the comments the following.

  # Activate observers that should always be running
  # config.active_record.observers = :cacher, :garbage_collector

Take a moment to go ahead and specify which observer(s) you’d like to load into your Rails environment.

config.active_record.observers = :flower_observer

Then you can remove your observer calls in all your controllers, because that’s not where you should be defining them.

Also, if you’re not using Observers yet, I’d really encourage you to consider reading up on them and giving them a try.

I was playing around with an application from script/console and all of a sudden… my Observers just stopped working. It took me a while to figure out that when I would call Dispatcher.reset_application!... it wouldn’t reload config/environment.rb where I have defined the observers.

I really don’t like that this is the current implementation for invoking Observers and adding them to the controllers is not kosher with my (do-it-in-script-console) approach.

The current solution?

# app/models/foo_bar.rb
class FooBar < ActiveRecord::Base
end

FooBarObserver.instance

... there needs to be a better way!

A question was posed on the Rails mailing list concerning how one would go about adding CONSTRAINTs to the database tables with ActiveRecord::Migration.

One argument was raised stating that it is easier to handle these in plain SQL schema files. I disagree. :-)

Migrations to the Rescue

Databases evolve and I have recently found the Migration structure to be perfect for handling iterations and schema changes. Using the #execute method has helped move more of my code into the Ruby/Rails framework… and that just makes things easier to manage in the long-run. This is the approach that we are using at PLANET ARGON with some of our current client projects.

# db/migrate/6_add_foreign_key.rb
class AddForeignKey < ActiveRecord::Migration
  def self.up
    execute "ALTER TABLE bees ADD CONSTRAINT beehive_id_fkey FOREIGN KEY
(beehive_id) REFERENCES beehives (id);"
  end

  def self.down
    execute "ALTER TABLE bees DROP CONSTRAINT beehive_id_fkey;"
  end
end

This gives us an easy way to use the standard, #create_table syntax for building our tables with Ruby… and then we can slap these constraints on later.

This would add the constraints…

rake migrate VERSION=6

rake

rake migrate VERSION=5

I have found that this approach is really useful with testing in Rails. When I think that I have everything working great (without CONSTRAINTS in PostgreSQL), I run another migration to add a bunch of foreign key and data constraints to the tables and… run my tests again.

Let’s give Active Record a Hug

This has helped me gain some trust in Active Record while still giving me that comforting feeling that PostgreSQL is acting as the body guard for my data.

Even if you don’t end up using Migrations to handle these types of database schema changes, I would highly suggest that you model your implementation after this. I’ve worked with many database schemas and this just makes it easy to add your new change and run one command to commit it to the database.

...and now I go play with beehives…