Charm architecture

At its core, Content-cache-k8s is a NGINX cache application that helps another charm serving static content for it.

The charm design leverages the sidecar pattern to allow multiple containers in each pod with Pebble running as the workload container’s entrypoint.

Pebble is a lightweight, API-driven process supervisor that is responsible for configuring processes to run in a container and controlling those processes throughout the workload lifecycle.

Pebble services are configured through layers, and the following containers represent each one a layer forming the effective Pebble configuration, or plan:

1. An NGINX container with caching features, which can be used to efficiently serve static resources, as well as be the incoming point for all web traffic to the pod. This is the content cache container itself.
2. A NGINX Prometheus Exporter container that can be used to provide statistics on web traffic.

As a result, if you run a kubectl get pods on a namespace named for the Juju model you’ve deployed the Content-cache-k8s charm into, you’ll see something like the following:

NAME                             READY   STATUS    RESTARTS   AGE
content-cache-k8s-0              3/3     Running   0          97s

This shows there are three containers - the two named above, as well as a container for the charm code itself.

And if you run kubectl describe pod content-cache-k8s-0, all the containers will have as Command /charm/bin/pebble. That’s because Pebble is responsible for the processes startup as explained above.

Containers

Configuration files for the containers can be found in the files directory of the charm repository and in the templates directory of the charm repository.

C4Component
title Component diagram for Content Cache K8s Charm

Container_Boundary(content-cache-k8s, "Content Cache K8s") {
  Component(nginx-prometheus, "NGINX prometheus exporter", "", "Scrapes metrics from the NGINX of the Content Cache container")
  Component(charm, "Content Cache", "", "Caches static files using NGINX")

  Rel(nginx-prometheus, charm, "")
}

Content cache

This container is the entry point for all web traffic to the pod (on port 80). Serves static files directly.

The workload that this container is running is defined in the Content-cache rock in the charm repository.

Nginx pometheus exporter

This container runs the nginx/nginx-prometheus-exporter image.

The NGINX Prometheus Exporter is started with -nginx.scrape-uri=http://localhost:9080/stub_status so will scrape metrics from the NGINX container.

This has been configured in the NGINX container to return NGINX’s stub_status. The exporter listens on port 9113 and metrics about web traffic to the pod can be scraped by Prometheus there.

Docker images

The image defined in Content-cache dockerfile in the charm repository is published to Charmhub, the official repository of charms.

This is done by publishing a resource to Charmhub as described in the Juju SDK How-to guides.

Integrations

See relation endpoints.

The Content Cache K8s charm supports various integrations to provide additional functionality.

It is possible for a type of integration to be fulfilled by multiple types of charms. Below is a diagram of a basic Content Cache deployment with some sample charms to fulfill each integration.

C4Component
title Sample integration for Content Cache K8s Charm

Component(nginx-ingress, "NGINX Ingress", "", "")
Component(charm, "Content Cache K8s", "", "")
Component(grafana-agent-k8s, "Grafana Agent K8s", "", "")

Rel(charm, nginx-ingress,"nginx-route")
Rel(charm, grafana-agent-k8s, "grafana-dashboard, metric-endpoint, logging")

UpdateRelStyle(charm, nginx-ingress, $offsetX="-30", $offsetY="10")
UpdateRelStyle(charm, grafana-agent-k8s, $offsetX="-50", $offsetY="30")

Juju events

For this charm, the following events are observed:

1. <container name>_pebble_ready: fired on Kubernetes charms when the requested container is ready. Action: wait for the integrations, and configure the containers.
2. config_changed: usually fired in response to a configuration change using the CLI. Action: wait for the integrations, validate the configuration, update Ingress, and restart the containers.
3. report_visits_by_ip: fired when report-visits-by-ip action is executed. Action: Report the amount of visits grouped by IP that have visited the service ordered by amount of visits.

Charm code overview

The src/charm.py is the default entry point for a charm and has the ContentCacheCharm Python class which inherits from CharmBase.

CharmBase is the base class from which all Charms are formed, defined by Ops (Python framework for developing charms).

See more information in Charm.

The __init__ method guarantees that the charm observes all events relevant to its operation and handles them.

Take, for example, when a configuration is changed by using the CLI.

1. User runs the command:

juju config content-cache-k8s backend="http://mybackend.local:80"

2. A config-changed event is emitted.
3. In the __init__ method is defined how to handle this event like this:

self.framework.observe(self.on.config_changed, self._on_config_changed)

4. The method _on_config_changed, for its turn, will take the necessary actions such as waiting for all the relations to be ready and then configuring the containers.