new snapd_refresh config to control snapd refresh frequency (#141)

* add snapd_refresh config and handlers to k8s-master and -worker

* lint readmes

* add snapd_refresh doc to the readme; make "max" less specific

* adjust wording to note snapd_refresh only affects store snaps

cluster/juju/layers/kubernetes-master/README.md

@@ -40,6 +40,38 @@ firewall rules may not be able to contact the Charm Store. In these network
 restricted  environments the resource can be uploaded to the model by the Juju
 operator.
 
+#### Snap Refresh
+
+The kubernetes resources used by this charm are snap packages. When not
+specified during deployment, these resources come from the public store. By
+default, the `snapd` daemon will refresh all snaps installed from the store
+four (4) times per day. A charm configuration option is provided for operators
+to control this refresh frequency.
+
+>NOTE: this is a global configuration option and will affect the refresh
+time for all snaps installed on a system.
+
+Examples:
+
+```sh
+## refresh kubernetes-master snaps every tuesday
+juju config kubernetes-master snapd_refresh="tue"
+
+## refresh snaps at 11pm on the last (5th) friday of the month
+juju config kubernetes-master snapd_refresh="fri5,23:00"
+
+## delay the refresh as long as possible
+juju config kubernetes-master snapd_refresh="max"
+
+## use the system default refresh timer
+juju config kubernetes-master snapd_refresh=""
+```
+
+For more information on the possible values for `snapd_refresh`, see the
+*refresh.timer* section in the [system options][] documentation.
+
+[system options]: https://forum.snapcraft.io/t/system-options/87
+
 # Configuration
 
 This charm supports some configuration options to set up a Kubernetes cluster

cluster/juju/layers/kubernetes-master/config.yaml

@@ -147,6 +147,15 @@ options:
     default: true
     description: |
       If true the metrics server for Kubernetes will be deployed onto the cluster.
+  snapd_refresh:
+    default: "max"
+    type: string
+    description: |
+      How often snapd handles updates for installed snaps. Setting an empty
+      string will check 4x per day. Set to "max" to delay the refresh as long
+      as possible. You may also set a custom string as described in the
+      'refresh.timer' section here:
+        https://forum.snapcraft.io/t/system-options/87
   default-storage:
     type: string
     default: "auto"

cluster/juju/layers/kubernetes-master/reactive/kubernetes_master.py

@@ -427,6 +427,38 @@ def set_app_version():
     hookenv.application_version_set(version.split(b' v')[-1].rstrip())
 
 
+@when('kubernetes-master.snaps.installed')
+@when('snap.refresh.set')
+@when('leadership.is_leader')
+def process_snapd_timer():
+    ''' Set the snapd refresh timer on the leader so all cluster members
+    (present and future) will refresh near the same time. '''
+    # Get the current snapd refresh timer; we know layer-snap has set this
+    # when the 'snap.refresh.set' flag is present.
+    timer = snap.get(snapname='core', key='refresh.timer').decode('utf-8')
+
+    # The first time through, data_changed will be true. Subsequent calls
+    # should only update leader data if something changed.
+    if data_changed('master_snapd_refresh', timer):
+        hookenv.log('setting snapd_refresh timer to: {}'.format(timer))
+        leader_set({'snapd_refresh': timer})
+
+
+@when('kubernetes-master.snaps.installed')
+@when('snap.refresh.set')
+@when('leadership.changed.snapd_refresh')
+@when_not('leadership.is_leader')
+def set_snapd_timer():
+    ''' Set the snapd refresh.timer on non-leader cluster members. '''
+    # NB: This method should only be run when 'snap.refresh.set' is present.
+    # Layer-snap will always set a core refresh.timer, which may not be the
+    # same as our leader. Gating with 'snap.refresh.set' ensures layer-snap
+    # has finished and we are free to set our config to the leader's timer.
+    timer = leader_get('snapd_refresh')
+    hookenv.log('setting snapd_refresh timer to: {}'.format(timer))
+    snap.set_refresh_timer(timer)
+
+
 @hookenv.atexit
 def set_final_status():
     ''' Set the final status of the charm as we leave hook execution '''

cluster/juju/layers/kubernetes-worker/README.md

@@ -27,6 +27,38 @@ To add additional compute capacity to your Kubernetes workers, you may
 join any related kubernetes-master, and enlist themselves as ready once the
 deployment is complete.
 
+## Snap Configuration
+
+The kubernetes resources used by this charm are snap packages. When not
+specified during deployment, these resources come from the public store. By
+default, the `snapd` daemon will refresh all snaps installed from the store
+four (4) times per day. A charm configuration option is provided for operators
+to control this refresh frequency.
+
+>NOTE: this is a global configuration option and will affect the refresh
+time for all snaps installed on a system.
+
+Examples:
+
+```sh
+## refresh kubernetes-worker snaps every tuesday
+juju config kubernetes-worker snapd_refresh="tue"
+
+## refresh snaps at 11pm on the last (5th) friday of the month
+juju config kubernetes-worker snapd_refresh="fri5,23:00"
+
+## delay the refresh as long as possible
+juju config kubernetes-worker snapd_refresh="max"
+
+## use the system default refresh timer
+juju config kubernetes-worker snapd_refresh=""
+```
+
+For more information on the possible values for `snapd_refresh`, see the
+*refresh.timer* section in the [system options][] documentation.
+
+[system options]: https://forum.snapcraft.io/t/system-options/87
+
 ## Operational actions
 
 The kubernetes-worker charm supports the following Operational Actions:
@@ -97,4 +129,3 @@ follow the same steps inversely.
 ```
 juju run --application kubernetes-worker close-port 30510
 ```
-

cluster/juju/layers/kubernetes-worker/config.yaml

@@ -80,3 +80,12 @@ options:
     description: |
       Docker image to use for the default backend. Auto will select an image
       based on architecture.
+  snapd_refresh:
+    default: "max"
+    type: string
+    description: |
+      How often snapd handles updates for installed snaps. Setting an empty
+      string will check 4x per day. Set to "max" to delay the refresh as long
+      as possible. You may also set a custom string as described in the
+      'refresh.timer' section here:
+        https://forum.snapcraft.io/t/system-options/87

cluster/juju/layers/kubernetes-worker/reactive/kubernetes_worker.py

@@ -22,6 +22,8 @@ import shutil
 import subprocess
 import time
 
+from charms.leadership import leader_get, leader_set
+
 from pathlib import Path
 from shlex import split
 from subprocess import check_call, check_output
@@ -289,6 +291,38 @@ def set_app_version():
     hookenv.application_version_set(version.split(b' v')[-1].rstrip())
 
 
+@when('kubernetes-worker.snaps.installed')
+@when('snap.refresh.set')
+@when('leadership.is_leader')
+def process_snapd_timer():
+    ''' Set the snapd refresh timer on the leader so all cluster members
+    (present and future) will refresh near the same time. '''
+    # Get the current snapd refresh timer; we know layer-snap has set this
+    # when the 'snap.refresh.set' flag is present.
+    timer = snap.get(snapname='core', key='refresh.timer').decode('utf-8')
+
+    # The first time through, data_changed will be true. Subsequent calls
+    # should only update leader data if something changed.
+    if data_changed('worker_snapd_refresh', timer):
+        hookenv.log('setting snapd_refresh timer to: {}'.format(timer))
+        leader_set({'snapd_refresh': timer})
+
+
+@when('kubernetes-worker.snaps.installed')
+@when('snap.refresh.set')
+@when('leadership.changed.snapd_refresh')
+@when_not('leadership.is_leader')
+def set_snapd_timer():
+    ''' Set the snapd refresh.timer on non-leader cluster members. '''
+    # NB: This method should only be run when 'snap.refresh.set' is present.
+    # Layer-snap will always set a core refresh.timer, which may not be the
+    # same as our leader. Gating with 'snap.refresh.set' ensures layer-snap
+    # has finished and we are free to set our config to the leader's timer.
+    timer = leader_get('snapd_refresh')
+    hookenv.log('setting snapd_refresh timer to: {}'.format(timer))
+    snap.set_refresh_timer(timer)
+
+
 @when('kubernetes-worker.snaps.installed')
 @when_not('kube-control.dns.available')
 def notify_user_transient_status():