add versioning to the cluster runtime data in etcd

This change allows us to introduce changes to the inner workings of the
cluster that break backwards compatibility, allowing a smooth transition
to the new format.

api.go

@@ -12,14 +12,26 @@ import (
 	"time"
 )
 
-var (
-	MasterElectionPath = "/icecast/cluster/master"
-	MountPrefix        = "/icecast/mounts/"
-	NodePrefix         = "/icecast/nodes/"
+const (
+	// Path to the mount configuration in etcd. This should never
+	// change, upgrades to the configuration format should be
+	// backwards-compatible.
+	MountPrefix = "/icecast/mounts/"
+
+	// Path for the cluster runtime data. Whenever the format of
+	// this data changes, the ABIVersion should be increased. A
+	// rolling restart of the cluster will then seamlessly cause a
+	// transition to the new consensus (the cluster will be
+	// partitioned in the meantime).
+	ABIVersion         = "2"
+	MasterElectionPath = "/icecast/" + ABIVersion + "/cluster/master"
+	NodePrefix         = "/icecast/" + ABIVersion + "/nodes/"
 
 	IcecastPort        = 8000
 	IcecastMountPrefix = "/_stream"
+)
 
+var (
 	ErrIsDirectory = errors.New("key is a directory")
 	ErrIsFile      = errors.New("key is a file")
 )