What is a relation?

Relations define how applications should interact with one another. The definition of a relation is handled through an interface, and does not restrict the user to a traditional RFC approach.

Relations are "loosely typed", meaning there is no de-facto specification for:

• What information a relation must send/receive
• What actions are to be taken with data sent/received over the wire

With that being said, picking an interface is a strong statement; you need to set the same settings as do all the other charms with the same role for the interface; and you should only expect to be able to read those settings set by the other charms with the counterpart role.

Relation composition

Provides and Requires

The provides and requires keys defined in metadata.yaml are used to define pairings of charms that are likely to be fruitful. Consider mongodb's metadata:

name: mongodb
...
provides:
  database:
    interface: mongodb

...and that of the node.js charm:

name: my-node-app
...
requires:
  database:
    interface: mongodb
provides:
  website:
    interface: http

Put together, these files indicate that a relation can be made between applications. The mongodb charm provides a relation named database with the mongodb interface, and the my-node-app charm requires a relation named database with the mongodb interface.

The my-node-app charm also provides a relation named website with the http interface, but that's irrelevant to the mongodb charm. (But an haproxy charm might well define, say, reverseproxy, that requires the http interface provided by my-node-app.)

Relation namespace

The relation namespace is unrestricted with the exception that you may not provide a relation named juju nor have its name begin with juju-. Charms attempting to provide relations in this namespace will trigger an error.

Relation interfaces

An interface name is a string that must only contain characters a-z and -, and neither start nor end with -. It's the single determiner of compatibility between charms; and it carries with it nothing more than a mutual promise that the provider and requirer somehow know the communication protocol implied by the name.

This allows for rapid development in some situations; but, in the example above, there is a potential problem: we've picked two interface names that already have meanings in the charm ecosystem, and that means we have to be compatible. That's a concern for later, when we're actually writing the relation hooks.

Implicit relations

Implicit relations allow for an application to gather data about another application without requiring any modifications on the part of the author of the other application's charm.

Implicit relations are named in the reserved namespace and there is currently only one such relation provided to all deployed applications: juju-info.

If specified, it would look like this:

provides:
  juju-info:
    interface: juju-info

To be clear, the charm author does not declare the juju-info relation; the above is provided for demonstrative purposes only.

Furthermore, this relation presently captures very select bits of data from the remote unit:

private-address
public-address

If you want to write a subordinate charm that can be related to by any other charm, the juju-info relation can be used. For example:

Charm rsyslog-forwarder is a subordinate charm and requires a valid scope: container relationship in order to deploy. In the event that the principal charm doesn't provide this the logging charm author can leverage juju-info:

requires:
  logging:
      interface: logging-directory
      scope: container
  juju-info:
      interface: juju-info
      scope: container

The admin then issues the following:

juju add-relation wordpress rsyslog-forwarder

If the 'wordpress' charm author doesn't define the logging-directory interface, Juju will use the less-specific (in the sense that it likely provides less information) juju-info interface.

Peers

Charms can declare relations under peers which causes each unit of a single application to respond to the other units in the same application. A peer relation is defined in exactly the same way as any other relation.

Looking at the MongoDB peering relation, we see the charm defines replica-set as the relation, with the interface mongodb-replica-set

peers:
  replica-set:
    interface: mongodb-replica-set

As outlined in the relation - peering relations are particularly useful when your application supports clustering. Think about the implications of applications such as MongoDB, PostgreSQL, and ElasticSearch where clusters must exchange information amongst one another to perform proper clustering.

Configuring relations

There's an alternative syntax for specifying relations, which allows you to set additional fields by replacing the interface name with a dictionary. In this case, the interface key must be specified explicitly, and a number of other possibilities become available:

• scope defaults to global, but may be set to container. The scope controls the set of remote units that are reported to the unit as members of the relation: container-scoped relations are restricted to reporting details of a single principal unit to a single subordinate, and vice versa, while global relations consider all possible remote units. Subordinate charms are only valid if they have at least one requires relation with container scope.

• limit is ignored by Juju, but if present should be a positive integer N indicating that the charm is not designed to use this interface in more than N relations at once.

For example, if you're writing a really simple exploratory charm for your particular data store, you could just create a single shared store and write the same access credentials for every relation. A limit of 1 is thus useful in that it does document the restriction, even though it's not automatically enforced today.

• optional is ignored by Juju, but if present should only be set to true, on requires relations, to indicate that the charm can still function effectively without having those relations added. For example, the my-node-app charm might also define:

requires:
  database:
    interface: mongodb
  memcache:
    interface: memcached
    optional: true

...to indicate that it can integrate with memcached if it's available, but that it can't be expected to do anything useful without a MongoDB application available.

Relation execution in charms

When applications are related, Juju decides which hooks to call within each charm based on this local relation name. When WordPress is related to MySQL, the "database-relation-joined, database-relation-changed, etc" hooks are called on the WordPress end. Corresponding hooks will be called on the 'mysql' charm "db- relation-joined, db-relation-changed" (based on the 'mysql' relation names).

Authoring charm interfaces

Relations are basically a bidirectional channel of communication between applications. They're not actually talking directly, the agents communicate via the state server, but it helps to think of it as direct communication between the applications. Relation hooks can call tools such as relation-get and relation- set to pass information back and forth between the application endpoints.

Pseudo relation talk

For example, wordpress and mysql might have a conversation like the following:

wordpress:
  I'm here and my application name is "wordpress"
mysql:
  I'm here, let me create a db for you
  your database/schema name is "wordpress"
  your credentials are "admin/pass1234"
  you can access the db on "my.host.addr:port"
wordpress:
  let me write the wordpress config files needed to access that
  database (and bounce the server to pick up those changes)
  bye
mysql:
  see-ya

We'll go over some more detailed versions of this, but this is the high-level conversation that occurs between two applications when they are related in a relation implementing the mysql interface.

At first glance, it would appear that the interface called mysql might be defined simply by the set of variables that get passed along the channel. Something like:

interface:
  name: mysql
  variables_set:
    - database_host
    - database_port
    - database_name
    - database_user
    - database_password
    - encoding

but really, that's not complete. In fact, it's not even enough information to implement hooks for a new application that needs to talk to MySQL. The timing and sequencing are critical components of this conversation! They can't be left out.

So let's dig a little deeper into this interface. Consider only the relation- joined and relation-changed hooks for now. The remaining broken and departed hooks are covered elsewhere.

Actually, if we start from provisioning, the hooks that are called for 'wordpress' are:

# juju deploy wordpress
install
config-changed
start
# juju add-relation wordpress mysql
database-relation-joined
database-relation-changed

similarly, for 'mysql':

# juju deploy mysql
install
config-changed
start
# juju add-relation wordpress mysql
db-relation-joined
db-relation-changed

and we can fill in a little of what the relation hooks are doing:

# wordpress
database-relation-joined
  <no-op>
database-relation-changed
  relation-get database_name, creds, host/port
  write config for wordpress
  bounce wordpress
# mysql
db-relation-joined
  relation-get application-name
  create db, creds
  relation-set db, creds, host/port
database-relation-changed
  <no-op>

This conversation is the actual interface.

Sample metadata.yaml files

The 'mysql' charm metadata:

name: mysql
summary: MySQL is a fast, stable and true multi-user, multi-threaded SQL database
maintainer: Marco Ceppi
description: |
MySQL is a fast, stable and true multi-user, multi-threaded SQL database
server. SQL (Structured Query Language) is the most popular database query
language in the world. The main goals of MySQL are speed, robustness and
ease of use.
categories:
- databases
provides:
  db:
    interface: mysql
  db-admin:
    interface: mysql-root
  shared-db:
    interface: mysql-shared
  master:
    interface: mysql-oneway-replication
  munin:
    interface: munin-node
  monitors:
    interface: monitors
  local-monitors:
    interface: local-monitors
    scope: container
peers:
    cluster:
      interface: mysql-ha
requires:
    slave:
      interface: mysql-oneway-replication
    ceph:
      interface: ceph-client
    ha:
      interface: hacluster
      scope: container

Interface documentation

Although we have described above that interfaces arrive by convention, there are several well-used interfaces which have enough implementations to define a defacto standard.

Below is a list of the interfaces for which we have compiled documentation and reference implementations:

• mysql - the database interface used by MySQL and client services.