Lucky Relation

Communicate over Juju relations.

Usage

relation [FLAGS] <SUBCOMMAND>

Subcommands

• get: Get data from a relation
• set: Set local values on a relation
• list-units: List the units collected to a relation
• list-ids: List relation ids connected to this unit

Understanding Relations

The lucky relation command has all of the tools that you need to interact with Juju's concept of relations. Relations are a somwhat complicated concept to grasp but they are probably the most important concept in Juju and they are what powers Juju's ability to deploy apps in a conceptually simple manner.

Juju relations allow the units on each side of a relation to set values on their side of the relation that can be read by the unit on the other side. A unit can set and get values on its own side of the relation, but it can only get values from the other side of the relation.

While we hope to provide more documentation on using relations later, currently the best reference is the Juju documentation. The lucky relation commands serve the same functions as the built-in Juju hook tools. The mappings are as follows:

Examples

Iterating over charm relations:

This will work inside or outside of a relation-* hook.

servers=""
# For each related application in the `http` relation
for relation_id in $(lucky relation list-ids --relation-name http); do
    
    # For every unit of that application
    for related_unit in $(lucky relation list-units -r $relation_id); do
        
        # Get the unit's hostname
        addr=$(lucky relation get -r $relation_id -u $related_unit hostname)
        
        # Get the unit's port
        port=$(lucky relation get -r $relation_id -u $related_unit port)
        
        # Add it to the server list
        servers="$servers $addr:$port"

        # You can also set values on your side of the relation
        lucky relation set -r $relation_id -u $related_unit key=value
    done

    # You can also get the values that you have set on this relation by specifying
    # your unit name ( from the $JUJU_UNIT_NAME env var ) as the -u argument
    lucky relation get -r $relation_id -u $JUJU_UNIT_NAME hostname
done

Get the hostname and port of a related unit in a http-relation-changed hook:

In this example, because we are in a http-relation-changed hook, we don't have to specify the relation id or the related unit. Those are set in environment variables by Juju and the lucky relation get command will automatically default to getting from the relation and unit that triggered the relation-changed hook.

hostname=$(lucky relation get hostname)
port=$(lucky relation get port)

You can also set data on relations in hooks in the same way.

lucky relation set user=my-username