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Learning to speak Juju

This article is more than 6 years old.

Learning to speak Juju

One of my favorite quotes is that there are two hard problems in tech, cache invalidation and naming things. Naming things is fun and you quickly realize that in any tech community there’s a vocabulary you need to understand in order to participate. Programming languages, technical tools, and even just communities all build entire languages you need to understand to participate. Juju is no different here. When you look at Juju you’ll find there’s a small vocabulary that it really helps to understand.

“There are only two hard things in Computer Science: cache invalidation and naming things. -- Phil Karlton”

The Juju Client

$ juju --version

We start with Juju itself. Now honestly, there’s a few layers of Juju you end up working with but let’s start with the command line client. When you invoke Juju from the CLI you’re running a local client that communicates with APIs that perform the real work. Clients are available on all major systems and you can even think of things like the Juju GUI as a web based client. You might be asked what version of Juju you’re running and the best place to start is with the version of your client.


$ juju clouds

Cloud        Regions  Default        Type        Description
aws               14  us-east-1      ec2         Amazon Web Services
azure             26  centralus      azure       Microsoft Azure
google             8  us-east1       gce         Google Cloud Platform
localhost          1  localhost      lxd         LXD Container Hypervisor
guimaas            0                 maas        Metal As A Service

Just what is the Cloud? In our ecosystem a Cloud is any API that will provision machines for running software on. There are a number of public clouds and each has an APIs, such as AWS, GCE, and Azure. There are private clouds that you can operate software on, such as OpenStack or MAAS. There’s even a local cloud using the LXD API to provide a cloud experience on your local system controlling LXD containers. The key thing is that Juju is primarily intended to abstract away the details of the various cloud APIs so that the operations work you need to perform is repeatable and consistent regardless on which cloud you choose to use.


$ juju controllers

Controller   Model        User               Access     Cloud/Region   Models  Machines    HA  Version
guimaas      -            admin              superuser  guimaas             2         1  none  2.2.4
jaas*        k8-test      rharding@external  (unknown)                      -         -     -  2.2.2
jujuteamops  teamwebsite  admin              superuser  aws/us-east-1       2         1  none  2.2.4

A controller is the main brain of Juju. It holds the database of what is deployed, what configuration is expected, what is the current status of everything that’s been and being deployed and running over time. The controller houses the users and permissions and it’s the API endpoint that the Juju Client communicates with. The controller runs on the same cloud as the workloads that will be operated and can be scaled for HA purposes as it’s important infrastructure in the Juju system. You can see, in the above snippet, I’m running controllers on my local MAAS, AWS, and using the JAAS hosted controller.


juju models

Controller: jujuteamops

Model            Cloud/Region   Status     Machines  Cores  Access  Last connection
ci-everything    aws/us-east-1  available         0      -  admin   never connected
ricks-test-case  aws/us-east-1  available         3      3  admin   2017-09-28
teamwebsite*     aws/us-east-1  available         1      1  admin   2017-09-28

In Juju we talk a lot about modeling and models. Models are namespaces that work is done within. A single Controller can manage many many Models. Each Model tracks the state of what’s going on inside of it, it’s able to be ACL’d using the users known to the Juju Controller, and you can dump out the contents of a Model into a reusable format. In the above snippet you can see I have several Models running on a Controller in AWS right now. One is handling some CI work, another I’m using to test out a bug fix, and a third is the Model where we operate the software required to provide our website.

Users are able to switch between models and it really helps provide a great level of focus on what a user is working on at any given moment in time.


$ juju deploy mysql

Located charm "cs:mysql-57".
Deploying charm "cs:mysql-57".

A Charm is the component that users use to deploy software. Each software component is deployed using a ZIP file that contains all of the scripts, configuration information, event handling, and more that might be required to operate the software over time. The URL of the Charm that’s been found breaks down like so:

  • cs: = “charm store” (vs local file on disk)
  • mysql = name of the charm
  • 57 = what revision of the charm did we find. (latest when not specified)

Charms are just chunks of software themselves. They are the set of YAML, scripts, and templates that makes it possible to operate things. In the above case the Charm provides all the software required to operate MySQL over time. This includes configuration information, backup scripts, and code that handles the provisioning of databases to other software in the Model.

Application, Unit, and Machine

$ juju status mysql

App    Version  Status   Scale  Charm  Store       Rev  OS      Notes
mysql           waiting    0/1  mysql  jujucharms   57  ubuntu

Unit     Workload  Agent       Machine  Public address  Ports  Message
mysql/0  waiting   allocating  1                               waiting for machine

Machine  State    DNS  Inst id              Series  AZ          Message
1        pending       i-0524bda18a2bb0cb9  xenial  us-east-1b  Start instance attempt 1

When you deploy MySQL you are asking that a single Machine is provisioned by the Controller using the Cloud API and that the Charm is placed on that machine and, once there, it’s executed. However, once that Charm added to the Model, we call MySQL an Application. The Application is an abstraction that is used to control how we operate our MySQL. This includes adding additional MySQL servers and building a MySQL cluster. If we state that the data for MySQL should be in /srv vs /opt we want that to be set on the Application and then each MySQL in the cluster will update and follow suit. Each member of the cluster is a Unit of the MySQL Application. We can talk to each one individually when necessary, but really we want them to treat them as a single Application in our Model.

In this way we say that an Application consists of one or more Units that are located on a Machine. The Machine is the actual hardware or VM allocated for the Unit to run on. If we were to remove the Machine #1 in the above example, we’d also lose our unit mysql/0 but the Application would still be in the model. The state details are still in the model and any new Units added would pick up where the first one left off.


juju relate mysql:db gypsy-danger

Having MySQL servers running is great but I have some Python software that would love to store some data into that MySQL cluster. In order to do this I need to write out the MySQL DSN to a config file that my application knows how to read. To facilitate this I’ll create a Charm for my gypsy-danger Application and in my Charm I’ll declare that I know how to speak the “db” language. Funny enough, the MySQL charm already knows how to speak the “db” language as well. This means that I can create a relation in the model that states that MySQL and my python project gypsy-danger can communicate using the “db” language.

This allows Juju to facilitate a conversation between Units of both Applications. gypsy-danger will ask MySQL for a database. MySQL will then create a fresh new database with a unique username and password and send those details back to the gypsy-danger Application. The scripts contained in the Charm will then make sure each Unit of the Application gypsy-danger updates their Python configuration files with those MySQL connection details.

As an operator, I don’t want to get into the business of specifying things that are transient or different from one Model to another. That just reduces the reusability of what I’ve built. Relations allow Applications to communicate and self-coordinate to perform actions needed to go from independent software components into a working collection of software that performs a useful function. What’s neat is that other Charms can claim to speak “db” and so one MySQL cluster can be used to server out databases to many other Applications in a Model. You can even have different servers provide those databases, such as Percona or MariaDB. By speaking a common protocol Relations provide a great way for Models to stay agile.


    charm: "cs:mysql-57"
    num_units: 1
    charm: "cs:~rharding/gypsy-danger-5"
    num_units: 1
  - - "mysql:db"
    - "gypsy-danger:db"
$ juju add-model testbundle
$ juju deploy mybundle.yaml

A bundle is basically a static dump of a Model. It takes all the Applications that are running, how many Units are there, what configuration is specified, what Relations exist in the Model, and dumps it to a clean reusable form. In this way, you can replicate this Model in a new Model, or on another Cloud, or just save it for later. It deploys just like a single Charm would, except that it is going to be performing a lot more work. We have bundles of all shapes and sizes and when you see examples of easily deploying a Kubernetes cluster or an OpenStack installation it’s typically done using a reusable Bundle.

There is our crash course in the language of Juju. Every project has a language, and we all get really creative naming things in the tech world. Juju is no exception. Hopefully this will help you better understand how the parts fit together as you dive into using Juju to operate your own software.

Terms to remember:

  • Juju (and the client)
  • Cloud
  • Controller
  • Model
  • Charm
  • Application
  • Unit
  • Machine
  • Relation
  • Bundle

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