Snap coherence

The core Kubernetes components used in Charmed Kubernetes are installed as snap packages. These include:

• kube-apiserver
• kube-controller-manager
• kube-scheduler
• kube-proxy
• kubelet
• kubectl

By default, the snapd daemon periodically checks installed snaps for updates and will automatically refresh upgradeable packages to ensure the software is current. If desired, an administrator can setup a Snap Store Proxy to have more control over the snap revisions used in a deployment.

When configured, snaps used by kubernetes-control-plane and kubernetes-worker charms will track revisions defined on the proxy rather than those available in the upstream Snap Store.

Deploy a Snap Store Proxy

The installation guide covers manual installation of a Snap Store Proxy and is best suited for administrators that wish to use existing infrastructure for the proxy.

A bundle is also available for those that wish to Juju deploy a full snap proxy environment. See the Snap Store Proxy charm documentation for details on this method of deployment.

Once deployed, the proxy will need to be registered. This process generates a snap store id that will be used by Charmed Kubernetes charms. Registration is described in the documentation available at the given links depending on your chosen method of deployment.

The remainder of this guide assumes a snap store proxy is available in your environment in a snap-store-proxy/0 unit.

Configure charms to use the proxy

Charm applications must be configured to use the proxied snap store. This is done by acknowledging the signed assertion that allows snapd to trust the proxy, followed by setting the core snap proxy to the new store id:

curl -s http://<domain>/v2/auth/store/assertions | sudo snap ack /dev/stdin
sudo snap set core proxy.store=<STORE_ID>

Relevant commands will be shown in juju status output when the proxy is deployed with the snap-store-proxy charm:

$ juju status snap-store-proxy/0
...
snap-store-proxy/0*  active    idle   11       18.144.52.58    80/tcp  \
  curl -s http://18.144.52.58/v2/auth/store/assertions | sudo snap ack /dev/stdin ; \
  sudo snap set core proxy.store=<STORE_ID>

Run the required commands on kubernetes-control-plane and kubernetes-worker units:

juju exec --application kubernetes-control-plane \
  'curl -s http://18.144.52.58/v2/auth/store/assertions | sudo snap ack /dev/stdin ; \
  sudo snap set core proxy.store=<STORE_ID>'
juju exec --application kubernetes-worker \
  'curl -s http://18.144.52.58/v2/auth/store/assertions | sudo snap ack /dev/stdin ; \
  sudo snap set core proxy.store=<STORE_ID>'

Override snap revisions

Use snap-proxy override to lock a snap channel to a particular revision. For example:

$ juju exec --unit snap-store-proxy/0 'sudo snap-proxy override kubectl 1.16/stable=1342'
kubectl 1.16/stable amd64 1342
$ juju exec --unit snap-store-proxy/0 'sudo snap-proxy list-overrides kubectl'
kubectl 1.16/stable amd64 1342 (upstream 1309)

Any unit that uses the proxy to install kubectl 1.16/stable will now receive revision 1342, even though the upstream snap store has this channel revision defined as 1309. Repeat the above process to override channel revisions for any desired snap.

Every subsequent hook will check for new snap revisions from the snap store proxy. When updates are available, the relevant snaps will first be refreshed across all kubernetes-control-plane units. If the update is successful, the kubernetes-worker units will then be refreshed.