Scaling Charms
One of the killer features of computing in the cloud is that it (should) seamlessly allow you to scale up or down your services to meet your needs and whims. Juju not only makes it simple to deploy services, but crucially makes it easy to manage them too. It won't anticipate you getting slashdotted or on the front page of hacker news (yet), but it does mean that when you do you can reliably scale your services to meet the demand.
Adding Units
Scaling up services is really as simple as asking for more instances. Consider the following setup for a mediawiki:
juju bootstrap
juju deploy mysql
juju deploy mediawiki
juju add-relation mysql mediawiki:db
juju expose mediawiki
When you notice the mediawiki instance is struggling under the load of people making edits, you can simply scale up the service using the command:
juju add-unit mediawiki
This will cause a new instance to be run and configured to work alongside the currently running one.
If that isn't the sort of scale you had in mind, you can use a --count or -c switch to specify a number of instances to add:
Scaling Back
Sometimes you also want to scale back some of your services, and this too is easy with Juju.
juju remove-unit mediawiki/1
The remove-unit command can be run to remove running units safely. The running services should automatically adjust to the change. Should you wish to remove a service, please see the section on destroying services.