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.
Important: Applications form relations based on interface names only. They have to match.
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 toglobal
, but may be set tocontainer
. 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 onerequires
relation withcontainer
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, onrequires
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.