Add storage-backend configuration option to kubernetes-master charm.

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

@@ -80,3 +80,9 @@ options:
     description: |
       When true, master nodes will not be upgraded until the user triggers
       it manually by running the upgrade action.
+  storage-backend:
+    type: string
+    default: "auto"
+    description: |
+      The storage backend for kube-apiserver persistence. Can be "etcd2", "etcd3", or
+      "auto". Auto mode will select etcd3 on new installations, or etcd2 on upgrades.

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

@@ -112,7 +112,12 @@ def check_for_upgrade_needed():
         # we take no risk and forcibly upgrade the snaps.
         # Forcibly means we do not prompt the user to call the upgrade action.
         set_upgrade_needed(forced=True)
-    upgrade_for_etcd()
+
+    # Set the auto storage backend to etcd2.
+    auto_storage_backend = leader_get('auto_storage_backend')
+    is_leader = is_state('leadership.is_leader')
+    if not auto_storage_backend and is_leader:
+        leader_set(auto_storage_backend='etcd2')
 
 
 def snap_resources_changed():
@@ -138,14 +143,6 @@ def snap_resources_changed():
         return 'unknown'
 
 
-def upgrade_for_etcd():
-    # we are upgrading the charm.
-    # If this is an old deployment etcd_version is not set
-    # so if we are the leader we need to set it to v2
-    if not leader_get('etcd_version') and is_state('leadership.is_leader'):
-        leader_set(etcd_version='etcd2')
-
-
 def add_rbac_roles():
     '''Update the known_tokens file with proper groups.'''
 
@@ -279,6 +276,11 @@ def password_changed():
     set_state('client.password.initialised')
 
 
+@when('config.changed.storage-backend')
+def storage_backend_changed():
+    remove_state('kubernetes-master.components.started')
+
+
 @when('cni.connected')
 @when_not('cni.configured')
 def configure_cni(cni):
@@ -432,7 +434,7 @@ def master_services_down():
 
 @when('etcd.available', 'tls_client.server.certificate.saved',
       'authentication.setup')
-@when('leadership.set.etcd_version')
+@when('leadership.set.auto_storage_backend')
 @when_not('kubernetes-master.components.started')
 def start_master(etcd):
     '''Run the Kubernetes master components.'''
@@ -450,8 +452,7 @@ def start_master(etcd):
     handle_etcd_relation(etcd)
 
     # Add CLI options to all components
-    leader_etcd_version = leader_get('etcd_version')
-    configure_apiserver(etcd.get_connection_string(), leader_etcd_version)
+    configure_apiserver(etcd.get_connection_string(), getStorageBackend())
     configure_controller_manager()
     configure_scheduler()
     set_state('kubernetes-master.components.started')
@@ -473,13 +474,15 @@ def etcd_data_change(etcd):
     if data_changed('etcd-connect', connection_string):
         remove_state('kubernetes-master.components.started')
 
-    # We are the leader and the etcd_version is not set meaning
+    # We are the leader and the auto_storage_backend is not set meaning
     # this is the first time we connect to etcd.
-    if is_state('leadership.is_leader') and not leader_get('etcd_version'):
+    auto_storage_backend = leader_get('auto_storage_backend')
+    is_leader = is_state('leadership.is_leader')
+    if is_leader and not auto_storage_backend:
         if etcd.get_version().startswith('3.'):
-            leader_set(etcd_version='etcd3')
+            leader_set(auto_storage_backend='etcd3')
         else:
-            leader_set(etcd_version='etcd2')
+            leader_set(auto_storage_backend='etcd2')
 
 
 @when('kube-control.connected')
@@ -747,7 +750,7 @@ def ceph_storage(ceph_admin):
         cmd = ['kubectl', 'apply', '-f', '/tmp/ceph-secret.yaml']
         check_call(cmd)
         os.remove('/tmp/ceph-secret.yaml')
-    except: # NOQA
+    except:  # NOQA
         # the enlistment in kubernetes failed, return and prepare for re-exec
         return
 
@@ -835,11 +838,11 @@ def on_config_allow_privileged_change():
 
 @when('config.changed.api-extra-args')
 @when('kubernetes-master.components.started')
-@when('leadership.set.etcd_version')
+@when('leadership.set.auto_storage_backend')
 @when('etcd.available')
 def on_config_api_extra_args_change(etcd):
     configure_apiserver(etcd.get_connection_string(),
-                        leader_get('etcd_version'))
+                        getStorageBackend())
 
 
 @when('config.changed.controller-manager-extra-args')
@@ -1280,3 +1283,10 @@ def touch(fname):
         os.utime(fname, None)
     except OSError:
         open(fname, 'a').close()
+
+
+def getStorageBackend():
+    storage_backend = hookenv.config('storage-backend')
+    if storage_backend == 'auto':
+        storage_backend = leader_get('auto_storage_backend')
+    return storage_backend