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CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
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Rails ships with a few different session storage mechanisms. You can choose to store session data directly on the application server via the file system (the current default), directly in the browser cookies (there is some controversy regarding the security of this approach), or in the database. We like the database approach the best, because it fits well with Rails architectural principles.
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The Shared-Nothing Architecture
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Rails is built on the principle of a shared-nothing architecture. When we say sharednothing, we mean that no piece of data is shared between the servers that host the application. Storing session data on the server would violate this principal. Imagine that you were storing your session data on the file system of the server that runs your application. This would work fine as long as you were using only one server for your application. But what happens when your site becomes busy and you decide that you need multiple servers to keep up with the traffic You would need to use a load balancer a hardware or software layer that routes requests to one of several application servers to spread out the load. Say a request comes in to server 1, and you store some session data in the file system. Then server 1 gets busy, and the load balancer decides to send the next request to server 2. But the session data isn t on server 2; it s on server 1. This presents a dilemma. The shared-nothing architecture avoids this problem by keeping the state somewhere other than on the application server, like the database, as shown in Figure 6-4.
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Note David Heinemeier Hansson (the creator of Rails) has written about his thoughts on the sharednothing architecture on his blog at http://loudthinking.com/arc/000479.html.
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Figure 6-4. Shared-nothing architecture
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CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
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Note Purists will argue that this isn t really shared-nothing. After all, we re sharing the database server.
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But since a single database server can service dozens of application servers without a hitch, using it as a centralized storage location for session data is a worthy compromise.
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Storing Sessions in the Database
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The default file system session storage mechanism will just work with no configuration required. However, we feel that storing session data in the database is important enough that it s worth taking a brief detour to set up. Fortunately, Rails makes this an easy affair. There s a built-in Rake task called db:sessions:create that will make a migration to create the sessions table. To run it, enter the following from your application s directory on the command line:
$ rake db:sessions:create
exists create
db/migrate db/migrate/007_add_sessions.rb
This will create a migration to create the necessary sessions table in the database. All we need to do is run it.
$ rake db:migrate
Now that the sessions table has been created, we need to tell Rails that we want to use the database for session storage. This is matter of configuration, and is therefore specified in the config/environment.rb file. Open this file in your editor and you ll see the session configuration options are already there, though they re commented out.
# Use the database for sessions instead of the file system # (create the session table with 'rake db:sessions:create') # config.action_controller.session_store = :active_record_store
Remove the comment from the last line to activate the active_record_store option for sessions, as shown in Listing 6-13.
Listing 6-13. Activating active_record_store in config/environment.rb
# Use the database for sessions instead of the file system # (create the session table with 'rake db:sessions:create') config.action_controller.session_store = :active_record_store
CHAPTER 6 ACTION PACK: WORKING WITH THE VIEW AND THE CONTROLLER
You ll need to start and stop your web server for this change to take effect. Remember to use Ctrl+C to stop the server (and ./script/server to start it up again).
Using the Session
Secure in the knowledge that Rails will take care of all the low-level details of sessions for us, using the session object couldn t be easier. The session is implemented as a hash, just like the flash. We should come clean here. The flash is, in fact, a session in disguise (you can think of it as a specialized session due to its auto-expiring properties). Not surprisingly, then, the flash and session interfaces are identical. We store values in the session according to a key.
session[:account_id] = @account.id session[:account_id] # => 1 session['message'] = "Hello world!" session['message'] # => "Hello world!"
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