Commit e6fdeda

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+# [Routing](http://www.rabbitmq.com/tutorials/tutorial-four-ruby.html)
+
+In this example we us a direct exchange to route specific types of
+messages to specific queues. A direct exchange named "direct_logs" is
+created. When subscribers come oneline they register with the direct
+exchange for specific types of messages.
+
+In this example, each subscriber to can register for one of the
+following severity levels: info, warning or error.
+
+This allows subscribers to select which types of message they wish to
+process based on a routing key (info, warning, error).
+
+This time when you start the subscriber, you will need to specify the
+routing key to bind to.
+
+```bash
+λ ruby lib/4-routing/subscribe.rb warning
+ [*] Waiting for logs. To exit press CTRL+C
+ [x] warning:blah
+```
+
+When you publish a message you will need to specify the routing key to
+use or else it will default to 'info'.
+
+```bash
+λ ruby lib/4-routing/publish.rb error blah
+ [x] Sent 'blah'
+```
+
+In the above example the routing key 'error' was used. So that message
+will not be delivered to our subscriber that was bound the to 'warning'
+routing key.
+
+To deliver a message to the subscriber of the 'warning' routing key we
+need to send a message using the 'warning' routing key.
+
+```bash
+λ ruby lib/4-routing/publish.rb warning 'no need for alarm'
+ [x] Sent 'blah'
+```