Few applications are so simple that they can run independently - most rely on other applications. A certain charm knows that it requires, say, a database and, correspondingly, a database charm knows that is capable of accommodating the other charm's requirements. The act of joining such mutually-dependent charms causes code (hooks) to run in each charm in such a way that both charms can effectively talk to one another. When charms have joined logically in this manner they are said to have formed a relation.
Note: A criteria for forming a relation is that both applications are currently deployed. See the Deploying applications page for guidance.
Creating a relation is straightforward enough. The
add-relation command is
used to set up a relation between two applications:
juju add-relation mysql wordpress
This will satisfy WordPress's database requirement where MySQL provides the appropriate structures (e.g. tables) needed for WordPress to run properly.
If the charms in question are versatile enough, Juju may need to be supplied with more information as to how the charms should be joined.
To demonstrate, if we try instead to relate the 'mysql' charm to the 'mediawiki' charm:
juju add-relation mysql mediawiki
This is what will happen:
error: ambiguous relation: "mediawiki mysql" could refer to "mediawiki:db mysql:db"; "mediawiki:slave mysql:db"
The solution is to be explicit when referring to an endpoint, where the
latter has a format of
<application>:<application endpoint>. In this case, it
is 'db' for both applications. However, it is not necessary to specify the
mysql endpoint because only the mediawiki endpoint is ambiguous (according to
the error message). Therefore, the command becomes:
juju add-relation mysql mediawiki:db
Note: An application endpoint can be discovered by looking at the metadata of the corresponding charm. This can be done by examining the charm on the Charm Store or by querying the Store with the Charm Tools (using a command like
charm show <application> charm-metadata).
The output to
juju status --relations will display the relations:
Model Controller Cloud/Region Version SLA Timestamp default lxd localhost/localhost 2.4-beta4 unsupported 20:22:45Z App Version Status Scale Charm Store Rev OS Notes mediawiki 1.19.14 active 1 mediawiki jujucharms 19 ubuntu mysql 5.7.22 active 1 mysql jujucharms 58 ubuntu Unit Workload Agent Machine Public address Ports Message mediawiki/0* active idle 2 10.115.37.227 80/tcp Ready mysql/0* active idle 1 10.115.37.45 3306/tcp Ready Machine State DNS Inst id Series AZ Message 1 started 10.115.37.45 juju-db874f-1 xenial Running 2 started 10.115.37.227 juju-db874f-2 trusty Running Relation provider Requirer Interface Type Message mysql:cluster mysql:cluster mysql-ha peer mysql:db mediawiki:db mysql regular
The final section of the status output shows all current established relations.
There are times when a relation just isn't working and it is time to move on. See Removing Juju objects for how to do this.
Relations can also work across models, even across multiple controllers. See Cross model relations for more information.