Add KV datastore

Signed-off-by: Emile Vauge <emile@vauge.com>

cluster/leadership.go

@@ -0,0 +1,56 @@
+package cluster
+
+import (
+	log "github.com/Sirupsen/logrus"
+	"github.com/cenkalti/backoff"
+	"github.com/containous/traefik/safe"
+	"github.com/containous/traefik/types"
+	"github.com/docker/leadership"
+	"time"
+)
+
+// Leadership allows leadership election using a KV store
+type Leadership struct {
+	types.Cluster
+	candidate *leadership.Candidate
+}
+
+// Participate tries to be a leader
+func (l *Leadership) Participate(pool *safe.Pool, isElected func(bool)) {
+	pool.Go(func(stop chan bool) {
+		l.candidate = leadership.NewCandidate(l.Store, l.Store.Prefix+"/leader", l.Node, 30*time.Second)
+		backOff := backoff.NewExponentialBackOff()
+		operation := func() error {
+			return l.run(l.candidate, stop, isElected)
+		}
+
+		notify := func(err error, time time.Duration) {
+			log.Errorf("Leadership election error %+v, retrying in %s", err, time)
+		}
+		err := backoff.RetryNotify(operation, backOff, notify)
+		if err != nil {
+			log.Errorf("Cannot elect leadership %+v", err)
+		}
+	})
+}
+
+// Resign resigns from being a leader
+func (l *Leadership) Resign() {
+	if l.candidate != nil {
+		l.candidate.Resign()
+	}
+}
+
+func (l *Leadership) run(candidate *leadership.Candidate, stop chan bool, isElected func(bool)) error {
+	electedCh, errCh := candidate.RunForElection()
+	for {
+		select {
+		case elected := <-electedCh:
+			isElected(elected)
+		case err := <-errCh:
+			return err
+		case <-stop:
+			return nil
+		}
+	}
+}