Tutorial: Relations with Juju

This post teaches you how to build juju charms with relations by studying two example charms made for this purpose. I work professionally with juju since many years and I’ve always thought juju relations should have a more tutorials to help beginner level programmers get start with this extremely powerful tool. You can find other tutorials I’ve written in the open source community on https://discourse.juju.is

Difficulty: Intermediate

What you will learn

This tutorial will teach you implementing a simple relation in juju. We will use two existing charms that implements a master-worker pattern and study that code for reference.

Get the code here: git clone https://github.com/erik78se/masterworker

You will:

  • Learn what a relation is and how to use it in your charms.
  • Learn more about hooks and how hook-tools drives relation data exchanges.
  • Learn about relation-hooks and when they run.
  • How to debug/introspect a relation with hook-tools


  • You need a basic understanding of what juju, charms and models are.
  • You should have got through the getting started chapter of official juju documentation.
  • You need to be able to deploy charms to a cloud.
  • You have read: the lifecycle of charm relations
  • You have read: charm writing relations.
  • You need beginner level python programming skills.

Refreshing our memory a little

Some key elements of juju we are worth mentioning before we dig into the code.

Juju hook-tools

When working with relations and juju in general, what goes on under the hood are calls to juju hook-tools.

Looking at what those hook-tools can be done via:

juju help hook-tools

Two specific hook-tools are of high importance when working with juju relations:

relation-get & relation-set

Those are the primary tools when working with juju relations because they get/set data in relations.

Important: You set data on the local unit, and get data from remote units.

Hooks, their environment & context

Hooks-tools are normally executed from within a ‘hook’ where environment variables are set by the Juju agent, dependent on the context/hook. These variables are used when writing code for relations.

An example below show logging of some of those environment variables.

juju-log "This is the relation id: $JUJU_RELATION_ID"
juju-log "This is the remote unit: $JUJU_REMOTE_UNIT"
juju-log "This is the local unit: $JUJU_UNIT_NAME"


When building charms with python, the python package charmhelpers provides a set of functions that wrapps the hook-tools. Charmhelpers can be installed with

pip install charmhelpers

Here is the documentation: charmhelpers-docs

Installing charmhelpers for use within you charm, could be part of your install-hook, or even better, cloned into the “./lib/” of your charm, making it part of your charm software.

Cloning charmhelpers into your charm is a good practice since it isolates your charms software requirements from other charms that may live on the same host.

Feeling all refreshed on juju basics, lets now introduce the “master” and “worker” charms.

Master worker

Clone the “masterworker” repo to your client.

git clone https://github.com/erik78se/masterworker

The repo contains:

├── bundle.yaml         # <--- A bundle with a related master + 2 workers
├── master              # <--- The master charm
├── worker              # <--- The worker charm
├── ./lib/hookenv.py    # <--- Part of charmhelpers

The idea here is that:

  • The master is a single unit, whereas the workers can be many.
  • The master send some unique information to the individual worker units.
  • The master send some common information to all workers units.
  • The workers don’t send (relation-set) any information at all.

This pattern is useful in a lot of situations in computer science, such as when implementing client-server solutions.

Lets deploy the master and two workers so we can see how it looks and how the charms are related.

juju deploy master
juju deploy worker -n 2
juju relate master worker

Note: You could of course deploy the bundle instead:

juju deploy ./bundle.yaml


So, lets go through the steps required to produce the relation between these charms.

The first step in implementing the relation between two charms starts with defining the relation endpoint for the charms and its interface name. This is done in metadata.yaml

Step 1. Define an endpoint and select an interface name

A starting point to create a relation charm, is to modify the the metadata.yaml file. We do this for both master and worker since they have different roles in the relation.

The endpoints for the master and worker are defined as below.


provides:                # <--- Role
  master-application:    # <--- Relation Name
    interface: exchange  # <--- Interface name 
    limit: 1


requires:                # <--- Role
  master:                # <--- Relation name
    interface: exchange  # <--- Interface name

The interface name must be same for the master/worker endpoints or juju will refuse to relate the charms.

Step 2. Decide what data to pass

As we described above, the master is the only part of the relation that exchanges information in our invented exchange interface with the worker.

  1. worker-key for each unique worker. The worker-key is created by the master.
  2. message from the master to all the workers.

So, this data is all what we will “get/set” in the relation.

This is all done as part of the “relation hooks” that we will look into now.

Step 3. Use the relation hooks to set/get data.

Lets follow the events following the call to juju relate:

juju relate master worker

What happens now, is that Juju triggers a specific set of hooks on all units involved in the relation called “relation hooks”. The picture below shows how these hooks are called and in what order when a relation is formed.


The master set data in master-application-relation-joined

The worker get data in master-relation-changed

A best practice here, is to use relation-joined and/or relation-created to set initial data and relation-changed to retrieve them just as we have done in the master and worker charms.

The reason for this is that we can’t know in relation-created or relation-joined that the other end of the relation has set any relation data yet.

Only a few relation keys (such as, the remote unit ‘private-address’) are available at these early stages (Available in relation-joined) and its not until in relation-change that your own relation data should be expected to be available.

Apart from these considerations, all we do to manage data is via: “relation-set” and “relation-get”.

Now, lets look a bit closer on how the master sends out data that is unique to our worker units.

Communicating unit unique data

Data exchanged on juju relations is a dictionary.

So to pass individual data to workers, the master creates a composite dictionary key, made up by the joining remote unit-name + key-name and relation-set data for that composite key.


    log(" ========= hook: master-application-relation-joined  ========")

    # Generate a worker-key
    workerKey = generateWorkerKey()

    # Get the remote unit name so that we can use that for a composite key.
    remoteUnitName = os.environ.get('JUJU_REMOTE_UNIT', None) # remote_unit()

    # Get the worker remote unit private-address for logging
    workerAddr = relation_get('private-address', unit=remoteUnitName)

    log(f"Joined with WORKER at private-address: {workerAddr}")

    # Assemble the relation data.
    relation_data = { f"{remoteUnitName}-worker-key": workerKey }

    # Set the relation data on the relation.
    relation_set(relation_id(), relation_settings=relation_data )

The worker access its individual ‘worker-key’ in the master-relation-changed hook:


    log(" ========= hook: master-relation-changed  ========")
    localunitname = os.environ['JUJU_UNIT_NAME']

    ## If we have data that belong to this unit
    if relation_get(f"{localunitname}-worker-key"):

        # Get the worker-key with our unit name on it, e.g.: 'worker/0-worker-key'
        workerKey = relation_get(f"{localunitname}-worker-key")

Pretty straight forward, right?

Lets explore further how we use an alternative way to send out a message to the workers outside of the relation hooks.

Triggering a relation-change via a juju action.

So, juju takes care of making sure that any change on a relation triggers the hook relation_name-relation-change on the remote units, we can trigger this from other non relation hooks since we can access the relations by their id:s.

Look at the juju-action broadcast-message to show how this is achieved:


# Assume that the first relation_id is the only and use that.
relation_id = relation_ids('master-application')[0]

# Get the message from the juju function/action
message = function_get('message')

relation_data = { 'message': message }

# ... set the relation data.
relation_set(relation_id, relation_settings=relation_data)

If you run the action ‘broadcast-message’ and watch the “juju debug-log” you will see all units logging the message sent.

juju run-action master/0 broadcast-message message="Hello there"

Look into the relations (debugging)

We will often need to see what goes on on a relation, what data is set etc. Lets see how that is done using the hook-tools.

Here we retrieve the relation-ids for the master/0 unit.

juju run --unit master/0 "relation-ids master-application"

Removing and adding back a relation shows how the relation-id changes from master-application:0 to master-application:1

juju remove-relation master worker
juju relate master worker
juju run --unit master/0 'relation-ids master-application'

We can see from the command below, how the worker can access all (-) keys/data on the master/0 unit.

juju run --unit worker/0 'relation-get -r master:1 - master/0'
worker/0-worker-key: "5914"
worker/1-worker-key: ADA1

We can from the command below, see that on the master/0 there is no information from the worker, which is expected. Remember that the workers don’t set any data.

juju run --unit master/0 'relation-get -r master-application:1 - worker/0'

Individual keys can be retrieved as well with their key names:

juju run --unit master/0 "relation-get -r master-application:1 worker/1-worker-key master/0"

Step 4. Departing the relation

The last step to implement in juju relation is taking case of when a unit departs from a relation, the programmer should:

  1. Remove any relation data associated with the departing unit from the relation dictionary with the relation-set hook tool.
  2. Do whatever is needed to remove a departing unit from the service e.g. perform reconfiguration, removing databases etc.

Lets walk this through by removing a worker. Follow the events with juju debug-log.

juju remove-unit worker/1

The master (and worker/1) gets notified of the event and executes their respective relation-departed hook.

Departing – as it happens on the master

The master cleans up the relation data associated with the departing (remote) unit. ./master/hooks/master-application-relation-departed

    # Set a None value on the key (removes it from the relation data dictionary)
    relation_data = {f"{remoteUnitName}-worker-key": None}

    # Update the relation data on the relation.
    relation_set(relation_id(), relation_settings=relation_data) 

The master hasn’t done anything else on the host itself, so its duties are complete.

Inspecting the relation will show that the data for worker/1 is gone:

juju run --unit worker/0 'relation-get -r master:1 - master/0'
worker/0-worker-key: 5914

Departing – as it happens on the worker

On the worker side of the relation, the worker didn’t set any relation data, so it doesn’t have to do anything to clean up in its relation data.

But, the worker should remove the WORKERKEY.file that it created on the host as part of joining the relation.

This cleanup procedure is placed in the ‘relation-broken’ hook.


    # Remove the WORKERKEY.file

The relation-broken hook is the final state when unit is completely cut-off from the other side of the relation, as if the relation was never there. It is last in the relation life-cycle and is a good place to do cleanup related to the host or underlying service deployed by the charm.

If the relation-broken is being executed, you can be sure that no remote units are currently known locally. So, on the master, this hook is not ran until there are no more workers.

Keep in mind that the name of the hook “-broken” has nothing to do with that the relation is “bogus/error”. Its just that the relation is “cut”.

Lets finish up by removing all the relations:

juju remove-relation master worker.

Inspect the relations and look for the file WORKERKEY.file on the remaining worker units (they are gone!).

You will also see in the juju debug-log that the master has finally ran its “relation-broken” hook.

Congratulations, you have completed the tutorial on juju relations!

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