How to manage relations

Add a relation

The procedure differs slightly depending on whether the applications that you want to integrate are on the same model or rather on different models.

Add a same-model relation

To set up a relation between two applications on the same model, run the integrate command followed by the names of the applications. For example:

juju integrate mysql wordpress

This will satisfy WordPress’s database requirement where MySQL provides the appropriate schema and access credentials required for WordPress to run properly.

The code above however works only if there is no ambiguity in what relation the charm requires and what the related charm provides.

If the charms in question are able to establish multiple relation types, Juju may need to be supplied with more information as to how the charms should be joined. For example, if we try instead to relate the ‘mysql’ charm to the ‘mediawiki’ charm:

juju integrate mysql mediawiki

the result is an error:

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 integrate mysql mediawiki:db

Note

The integration endpoints provided or required by a charm are listed in the result of the juju info command. They are also listed on the page for the charmed operator at Charmhub.

See more: juju integrate

Add a cross-model relation

See also: Cross-model

In a cross-model relation there is also an ‘offering’ model and a ‘consuming’ model. The admin of the ‘offering’ model ‘offers’ an application for consumption outside of the model and grants an external user access to it. The user on the ‘consuming’ model can then find an offer to use, consume the offer, and integrate an application on their model with the ‘offer’ via the same integrate command as in the same-model case (just that the offer must be specified in terms of its offer URL or its consume alias). This creates a local proxy for the offer in the consuming model, and the application is subsequently treated as any other application in the model.

View all the current relations

To view the current relations in the model, run juju status --relations. The example below shows a peer relation and a regular relation:

[...]
Relation provider  Requirer       Interface  Type     Message
mysql:cluster      mysql:cluster  mysql-ha   peer     
mysql:db           mediawiki:db   mysql      regular

To view just a specific relation and the applications it integrates, run juju status --relations followed by the provider and the requirer application (and endpoint). For example, based on the output above, juju status --relations mysql mediawiki would output:

[...]
Relation provider  Requirer       Interface  Type     Message  
mysql:db           mediawiki:db   mysql      regular

See more: juju status

Get the relation ID

To get the ID of a relation, for any unit participating in the relation, run the show-unit command – the output will also include the relation ID. For example:

$ juju show-unit synapse/0
  
...
  - relation-id: 7
    endpoint: synapse-peers
    related-endpoint: synapse-peers
   application-data:
      secret-id: secret://1234
    local-unit:
      in-scope: true

Remove a relation

Regardless of whether the relation is same-model or cross-model, to remove an relation, run the remove-relation command followed by the names of the two applications involved in the integration:

juju remove-relation <application-name> <application-name>

For example:

juju remove-relation mediawiki mysql

In cases where there is more than one relation between the two applications, specify the interface at least for one of the applications:

juju remove-relation mediawiki mysql:db