By default, an application is named after the deployed charm. However, you can
specify a custom name by adding an extra argument to the
juju deploy mysql wikidb
This will create a unit of the
This can lend to greater organisation as your infrastructure gets more complex:
juju deploy mysql website-db juju deploy mysql app-master-db juju deploy mysql app-slave-db -n2
But there are other reasons to do this: application groups, which are nothing other than named applications. They are called out as a separate feature because they're quite useful in a few different ways.
Some applications acquire a role at runtime based on the relations that are joined.
A great example of this is the hadoop charm. It can instantiate applications
juju deploy hadoop namenode juju deploy hadoop datacluster -n40
which are identical at this point except for the application name that Juju's using for the various units.
These applications acquire roles at relation-time via
juju add-relation namenode:namenode datacluster:datanode juju add-relation namenode:jobtracker datacluster:tasktracker
The relations determine the application role.
Another example of this is mysql replication.
juju deploy mysql masterdb juju deploy mysql slavedb -n2
where the different applications are related to each other
juju add-relation masterdb:master slavedb:slave
and to other applications via
juju deploy mediawiki mywiki juju add-relation mywiki:db masterdb:db juju add-relation mywiki:slave slavedb:db
There are also interesting use-cases for breaking large applications down into separate groups of units. Instead of a single 5000-node hadoop application named hadoop-slave, you might build that cluster from multiple smaller application groups.
juju deploy hadoop hadoop-master juju deploy hadoop hadoop-slave-A -n2500 juju deploy hadoop hadoop-slave-B -n2500 juju add-relation hadoop-master:namenode hadoop-slave-A:datanode juju add-relation hadoop-master:namenode hadoop-slave-B:datanode ...
These application groups can be managed independently by Juju for upgrades and configuration
juju config hadoop-slave-B some_param=new_value
This technique can potentially be a way for Juju to manage rolling upgrades for an application. Of course, this depends heavily on the applications in question and how well they support version management, schema changes, etc.