Support "preventSinglePointFailure" option in linear controller

README.md

@@ -66,25 +66,32 @@ data:
       "coresPerReplica": 2,
       "nodesPerReplica": 1,
       "min": 1,
-      "max": 100
+      "max": 100,
+      "preventSinglePointFailure": true
     }
 ```
 
 The equation of linear control mode as below:
 ```
 replicas = max( ceil( cores * 1/coresPerReplica ) , ceil( nodes * 1/nodesPerReplica ) )
+replicas = min(replicas, max)
+replicas = max(replicas, min)
 ```
 
-Notice that both `coresPerReplica` and `nodesPerReplica` are float.
+When `preventSinglePointFailure` is set to `true`, controller ensures at least 2 replicas
+if there are more than one node.
 
 For instance, given a cluster has 4 nodes and 13 cores. With above parameters, each replica could take care of 1 node.
 So we need `4 / 1 = 4` replicas to take care of all 4 nodes. And each replica could take care of 2 cores. We need `ceil(13 / 2) = 7`
 replicas to take care of all 13 cores. Controller will choose the greater one, which is `7` here, as the result.
 
-Either one of the `coresPerReplica` or `nodesPerReplica` could be omitted. Both `min` and `max ` could be omitted.
-If not set, `min` would be default to 1.
+Either one of the `coresPerReplica` or `nodesPerReplica` could be omitted. All of  `min`, `max` and
+`preventSinglePointFailure` is optional. If not set, `min` would be default to `1`,
+`preventSinglePointFailure` will be default to `false`.
 
-The lowest number of replicas is set to 1.
+Side notes:
+- Both `coresPerReplica` and `nodesPerReplica` are float.
+- The lowest replicas will be set to 1 when `min` is less than 1.
 
 ### Ladder Mode
 

pkg/autoscaler/autoscaler_server.go

@@ -91,16 +91,16 @@ func (s *AutoScaler) pollAPIServer() {
 
 	// Sync autoscaler ConfigMap with apiserver
 	configMap, err := s.syncConfigWithServer()
-	if err != nil {
+	if err != nil || configMap == nil {
 		glog.Errorf("Error syncing configMap with apiserver: %v", err)
 		return
 	}
 
-	// Only sync updated ConfigMap
-	if configMap.ObjectMeta.ResourceVersion != s.controller.GetParamsVersion() {
-		// Ensure corresponding controller type and scaling params
+	// Only sync updated ConfigMap or before controller is set.
+	if s.controller == nil || configMap.ObjectMeta.ResourceVersion != s.controller.GetParamsVersion() {
+		// Ensure corresponding controller type and scaling params.
 		s.controller, err = plugin.EnsureController(s.controller, configMap)
-		if err != nil {
+		if err != nil || s.controller == nil {
 			glog.Errorf("Error ensuring controller: %v", err)
 			return
 		}

pkg/autoscaler/controller/linearcontroller/linear_controller.go

@@ -48,10 +48,11 @@ func NewLinearController() controller.Controller {
 }
 
 type linearParams struct {
-	CoresPerReplica float64 `json:"coresPerReplica"`
-	NodesPerReplica float64 `json:"nodesPerReplica"`
-	Min             int     `json:"min"`
-	Max             int     `json:"max"`
+	CoresPerReplica           float64 `json:"coresPerReplica"`
+	NodesPerReplica           float64 `json:"nodesPerReplica"`
+	Min                       int     `json:"min"`
+	Max                       int     `json:"max"`
+	PreventSinglePointFailure bool    `json:"preventSinglePointFailure"`
 }
 
 func (c *LinearController) SyncConfig(configMap *apiv1.ConfigMap) error {
@@ -107,6 +108,13 @@ func (c *LinearController) GetExpectedReplicas(status *k8sclient.ClusterStatus)
 func (c *LinearController) getExpectedReplicasFromParams(schedulableNodes, schedulableCores int) int {
 	replicasFromCore := c.getExpectedReplicasFromParam(schedulableCores, c.params.CoresPerReplica)
 	replicasFromNode := c.getExpectedReplicasFromParam(schedulableNodes, c.params.NodesPerReplica)
+	// Prevent single point of failure by having at least 2 replicas when
+	// there are more than one node.
+	if c.params.PreventSinglePointFailure &&
+		schedulableNodes > 1 &&
+		replicasFromNode < 2 {
+		replicasFromNode = 2
+	}
 
 	// Returns the results which yields the most replicas
 	if replicasFromCore > replicasFromNode {

pkg/autoscaler/controller/linearcontroller/linear_controller_test.go

@@ -42,14 +42,16 @@ func TestControllerParser(t *testing.T) {
 		      "coresPerReplica": 2,
 		      "nodesPerReplica": 1,
 		      "min": 1,
-		      "max": 100
+		      "max": 100,
+		      "preventSinglePointFailure": true
 		    }`,
 			false,
 			&linearParams{
 				CoresPerReplica: 2,
 				NodesPerReplica: 1,
 				Min:             1,
 				Max:             100,
+				PreventSinglePointFailure: true,
 			},
 		},
 		{ // Invalid JSON
@@ -84,6 +86,18 @@ func TestControllerParser(t *testing.T) {
 			true,
 			&linearParams{},
 		},
+		// Wrong input for PreventSinglePointFailure.
+		{
+			`{
+		      "coresPerReplica": 2,
+		      "nodesPerReplica": 1,
+		      "min": 1,
+		      "max": 100,
+		      "preventSinglePointFailure": invalid,
+		    }`,
+			true,
+			&linearParams{},
+		},
 	}
 
 	for _, tc := range testCases {
@@ -145,16 +159,17 @@ func TestScaleFromMultipleParams(t *testing.T) {
 	testController.params = &linearParams{
 		CoresPerReplica: 2,
 		NodesPerReplica: 2.5,
-		Min:             2,
+		Min:             1,
 		Max:             100,
+		PreventSinglePointFailure: true,
 	}
 
 	testCases := []struct {
 		numCores    int
 		numNodes    int
 		expReplicas int
 	}{
-		{0, 0, 2},
+		{0, 0, 1},
 		{1, 2, 2},
 		{2, 3, 2},
 		{3, 4, 2},

pkg/autoscaler/k8sclient/k8sclient.go

@@ -141,9 +141,10 @@ func (k *k8sClient) GetClusterStatus() (clusterStatus *ClusterStatus, err error)
 	opt := api.ListOptions{Watch: false}
 
 	nodes, err := k.clientset.CoreClient.Nodes().List(opt)
-	if err != nil {
+	if err != nil || nodes == nil {
 		return nil, err
 	}
+	clusterStatus = &ClusterStatus{}
 	clusterStatus.TotalNodes = int32(len(nodes.Items))
 	var tc resource.Quantity
 	var sc resource.Quantity