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  1. Blog
  2. Article

Charles Butler
on 22 October 2015

Editors Note: This post is one of many in a series covering the new patterns in charming. This first post will be information heavy and cover a walkthrough of the techonologies at play. Video content and additional tutorials will follow.

It’s been an exciting couple of months for the Juju Charmers. If you’ve been following the Juju mailing list, you’ve undoubtedly seen some mention of the newest features landing for writing charms which takes aim at easing the maintenance burden for charm authors by exposing a pattern of layering and reacting to charm events. This packs a one two punch at complexity and has literally re-defined charming in a very short span of time.

If you’ve written charms before now, you’ll know that it requires a fairly deep knowledge of the hook environment, how these hooks are run, and what sequence they may be run in. This hook execution pattern is deeply tied to how we model services in Charms and lends itself to a barrier to entry for new charm authors. There’s not inherently wrong with the pattern, it was just a steep learning curve to ask newcomers to learn. The newest framework – The Reactive Pattern – allows charm authors to perform actions in familar hook contexts, or to surface events – such as a database coming online, and then subscribe to those events with decorators on plain ole methods.

Setting the Stage, the Players

Each is a stand alone component, and almost certainly should be used in tandem. Like the unix philosophy, this tooling/library has been written in mind to perform only its job, and to do it well. I’d also like to take a moment to point out these features were developed by Ben Saller and Cory Johns of the Juju ecosystems team. If you see them in IRC or on the Mailing list, be sure to give them a friendly hi5 for their awesome work.

Charm Layers

Charms can be seen as a build-time artifact on the workstation machine. As a developer of many charms, its surfaced that when charming in a particular domain, say PHP website creation, I constantly need to setup apache (or nginx) and the php5-fpm daemon. Tuning these can be application specific, but normally I just set them up, and start modeling the application deployment. But I cannot do anything until this plumbing is complete.

Charm Layers solve for the complexity of abstracting away these common tasks into their own modules, which can then be called, included, and overridden in complimentary layers. I know this is an exercise in spacial thinking. So lets take a look at how this fits together, first by looking at how we can eliminate boiler plate. (this has a minor overlap with Reactive Charming, as this example will illustrate layering a reactive based charm)

includes: ['layer:basic']

Fig 1.1 – layer.yaml

By having a directory with only a layer.yaml, we can now build a charm that does nothing, however we’ve elminated the need for charm create and editing boilerplate code. This allows us a far simpler declarative syntax to write charms.

To assemble this noop boilerplate charm, we simply run charm build -o path/to/juju_repo

builder: Continuing with known changes to target layer. Changes will be overwritten
builder: Processing layer: layer:basic
builder: Processing layer: test

Fig 1.2 – Charm Builder Output

We now see that it pulled in the layer:basic charm layer (or in other words boilerplate) and its applied our test layer on top. Pretty neat! Using a domain that I am commonly involved with, Charming with Docker, I placed a diagram in the Documentation that helps illustrate exactly how this looks. Included below in Fig 1.3.

Fig 1.3 – Layer Diagram

We see that there are dependent, and extensive layers. This is the best representation I could put together to show how the nginx container layer
extends the “middle” docker layer. To explore how these concepts work, we need to look to our next utility – the Reactive Pattern

Inspecting Assembled Charms

By running charm layers in the directory of the assembled charm, you will be presented with a color coded map of where the files being included came from. This is particulary handy when inspecting a charm to see which layers were included, and how it impacted the overall architecture of the charm.

Payload Management

This is a new feature as of juju 1.25 beta2 and is not currently in any of the PPA branches for distribution (stable, or devel). This should be landing soon for mass consumption.

The Juju Core Moonstone team has been working dilligently to deliver a new feature that is exciting to me as a charmer of dockery type things. Payload
Management is a suite of hook utilities to register launched payloads, such as:

  • LXD Containers
  • Docker Containers
  • KVM Guests
  • Tomcat WAR files

And a myriad of other arbitrary payload classes in charms. While the charm surfaces a service to the topology, its often important to me as an admin to
know what exactly is running on the machine underneath this service abstraction. Perhaps 4 complete microservices make up a single service endpoint in my topology.

juju list-payloads

[Unit Payloads]
unit-idlerpg-0 1       slackbot      running docker 6dd92ba0
unit-idlerpg-0 1       application   running docker 5cfde4b5
unit-idlerpg-0 1       monitoring    running docker db3as541

In this particular example, there are two services comprising the IdleRPG service, launched on machine 1. The Slackbot container, and an NGINX application container serving up the rules and the map, and finally a monitoring process – CADVISOR listening to the docker socket and tracking resource usage.

I now have at a glance statistics of whats happening with my delivered payloads. Pair this in an update-status hook to provide constant updates on the service to know if the payload has stopped, is stopping (in error cycle state) and more.

The Moonstone team has assembled a good documentation resource for the feature:

The new payload management feature allows charmers to more accurately define large and complex deployments by registering different payloads, such as
lxc, kvm, and docker, with Juju. This lets the operator better understand the purpose and function of these payloads on a given machine.

You define these payloads in the metadata.yaml of the charm under the payloads section. You create a class for the payload, “monitoring” or “kvm-guest”, and
assign the type.

        type: docker
       type: kvm

From your charm hook you can manage your payload with the following commands:

payload-register <type> <class> <ID> [tags]
   payload-unregister <type> <class> <ID>
   payload-status-set <type> <class> <ID> <starting, running, stopping, stopped>

From the Juju command line you can view your payloads like this:

juju list-payloads <filter>

For more information run:

juju help payloads

But Wait, there’s more!

If you’re still reading, and cant wait to see the rest of the outlined features in the video, part 2 will be published tomorrow. You can grab an early peek

Stay tuned for a video overview of charming with layers. And if you’ve got any questions be sure to send them to the mailing list or to drop by on IRC in #juju on

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