Ubernetes Lite: Volumes can dictate zone scheduling

For AWS EBS, a volume can only be attached to a node in the same AZ.
The scheduler must therefore detect if a volume is being attached to a
pod, and ensure that the pod is scheduled on a node in the same AZ as
the volume.

So that the scheduler need not query the cloud provider every time, and
to support decoupled operation (e.g. bare metal) we tag the volume with
our placement labels.  This is done automatically by means of an
admission controller on AWS when a PersistentVolume is created backed by
an EBS volume.

Support for tagging GCE PVs will follow.

Pods that specify a volume directly (i.e. without using a
PersistentVolumeClaim) will not currently be scheduled correctly (i.e.
they will be scheduled without zone-awareness).

pkg/cloudprovider/providers/aws/aws.go

@@ -45,6 +45,7 @@ import (
 	"k8s.io/kubernetes/pkg/util/sets"
 
 	"github.com/golang/glog"
+	"k8s.io/kubernetes/pkg/api/unversioned"
 )
 
 const ProviderName = "aws"
@@ -158,6 +159,9 @@ type Volumes interface {
 	// Create a volume with the specified options
 	CreateVolume(volumeOptions *VolumeOptions) (volumeName string, err error)
 	DeleteVolume(volumeName string) error
+
+	// Get labels to apply to volume on creation
+	GetVolumeLabels(volumeName string) (map[string]string, error)
 }
 
 // InstanceGroups is an interface for managing cloud-managed instance groups / autoscaling instance groups
@@ -510,6 +514,16 @@ func isRegionValid(region string) bool {
 	return false
 }
 
+// Derives the region from a valid az name.
+// Returns an error if the az is known invalid (empty)
+func azToRegion(az string) (string, error) {
+	if len(az) < 1 {
+		return "", fmt.Errorf("invalid (empty) AZ")
+	}
+	region := az[:len(az)-1]
+	return region, nil
+}
+
 // newAWSCloud creates a new instance of AWSCloud.
 // AWSProvider and instanceId are primarily for tests
 func newAWSCloud(config io.Reader, awsServices AWSServices) (*AWSCloud, error) {
@@ -527,7 +541,10 @@ func newAWSCloud(config io.Reader, awsServices AWSServices) (*AWSCloud, error) {
 	if len(zone) <= 1 {
 		return nil, fmt.Errorf("invalid AWS zone in config file: %s", zone)
 	}
-	regionName := zone[:len(zone)-1]
+	regionName, err := azToRegion(zone)
+	if err != nil {
+		return nil, err
+	}
 
 	valid := isRegionValid(regionName)
 	if !valid {
@@ -1274,6 +1291,32 @@ func (aws *AWSCloud) DeleteVolume(volumeName string) error {
 	return awsDisk.deleteVolume()
 }
 
+// Implements Volumes.GetVolumeLabels
+func (c *AWSCloud) GetVolumeLabels(volumeName string) (map[string]string, error) {
+	awsDisk, err := newAWSDisk(c, volumeName)
+	if err != nil {
+		return nil, err
+	}
+	info, err := awsDisk.getInfo()
+	if err != nil {
+		return nil, err
+	}
+	labels := make(map[string]string)
+	az := aws.StringValue(info.AvailabilityZone)
+	if az == "" {
+		return nil, fmt.Errorf("volume did not have AZ information: %q", info.VolumeId)
+	}
+
+	labels[unversioned.LabelZoneFailureDomain] = az
+	region, err := azToRegion(az)
+	if err != nil {
+		return nil, err
+	}
+	labels[unversioned.LabelZoneRegion] = region
+
+	return labels, nil
+}
+
 func (v *AWSCloud) Configure(name string, spec *api.NodeSpec) error {
 	return nil
 }