diff --git a/internal/controller/etcdcluster_controller.go b/internal/controller/etcdcluster_controller.go
index d9e47ad..dd7b960 100644
--- a/internal/controller/etcdcluster_controller.go
+++ b/internal/controller/etcdcluster_controller.go
@@ -113,11 +113,40 @@ func (r *EtcdClusterReconciler) Reconcile(ctx context.Context, req ctrl.Request)
 	}
 	state.endpointsFound = clusterClient != nil && singleClients != nil
 
+	if clusterClient != nil {
+		state.endpoints = clusterClient.Endpoints()
+	}
+
+	// fetch PVCs
+	state.pvcs, err = factory.PVCs(ctx, instance, r.Client)
+	if err != nil {
+		return ctrl.Result{}, err
+	}
+
 	if !state.endpointsFound {
 		if !state.stsExists {
-			// TODO: happy path for new cluster creation
-			log.Debug(ctx, "happy path for new cluster creation (not yet implemented)")
+			return r.createClusterFromScratch(ctx, &state) // TODO: needs implementing
+		}
+		// else try reconciling the sts
+		existingSts := state.statefulSet.DeepCopy()
+		desiredSts := factory.TemplateStatefulSet() // TODO: needs implementing
+		existingSts.Spec.Template.Spec = desiredSts.Spec.Template.Spec
+		err := r.patchOrCreateObject(ctx, existingSts)
+		if err != nil {
+			return ctrl.Result{}, err
+		}
+		state.statefulSet = *existingSts
+		if existingSts.Status.ReadyReplicas != *existingSts.Spec.Replicas { // TODO: this check might not be the best to check for a ready sts
+			return ctrl.Result{}, fmt.Errorf("waiting for statefulset to become ready")
+		}
+		if *existingSts.Spec.Replicas > 0 {
+			return ctrl.Result{}, fmt.Errorf("reached an impossible state (no endpoints, but active pods)")
+		}
+		if *instance.Spec.Replicas == 0 {
+			// cluster successfully scaled down to zero
+			return ctrl.Result{}, nil
 		}
+		return r.scaleUpFromZero(ctx, &state) // TODO: needs implementing
 	}
 
 	// get status of every endpoint and member list from every endpoint
diff --git a/internal/controller/observables.go b/internal/controller/observables.go
index 4d080f4..66e5cc5 100644
--- a/internal/controller/observables.go
+++ b/internal/controller/observables.go
@@ -2,12 +2,12 @@ package controller
 
 import (
 	"context"
-	// "strconv"
-	// "strings"
+	"strconv"
+	"strings"
 	"sync"
 
 	"github.com/aenix-io/etcd-operator/api/v1alpha1"
-	// "github.com/aenix-io/etcd-operator/pkg/set"
+	"github.com/aenix-io/etcd-operator/pkg/set"
 	clientv3 "go.etcd.io/etcd/client/v3"
 	appsv1 "k8s.io/api/apps/v1"
 	corev1 "k8s.io/api/core/v1"
@@ -49,15 +49,43 @@ func (o *observables) setClusterID() {
 
 // inSplitbrain compares clusterID field with clusterIDs in etcdStatuses.
 // If more than one unique ID is reported, cluster is in splitbrain.
+// Also if members have different opinions on the list of members, this is
+// also a splitbrain.
 func (o *observables) inSplitbrain() bool {
+	return o.clusterIDsAllEqual() && o.memberListsAllEqual()
+}
+
+func (o *observables) clusterIDsAllEqual() bool {
+	ids := set.New[uint64]()
 	for i := range o.etcdStatuses {
 		if o.etcdStatuses[i].endpointStatus != nil {
-			if o.clusterID != o.etcdStatuses[i].endpointStatus.Header.ClusterId {
-				return true
+			ids.Add(o.etcdStatuses[i].endpointStatus.Header.ClusterId)
+		}
+	}
+	return len(ids) <= 1
+}
+
+func (o *observables) memberListsAllEqual() bool {
+	type m struct {
+		Name string
+		ID   uint64
+	}
+	memberLists := make([]set.Set[m], 0, len(o.etcdStatuses))
+	for i := range o.etcdStatuses {
+		if o.etcdStatuses[i].memberList != nil {
+			memberSet := set.New[m]()
+			for _, member := range o.etcdStatuses[i].memberList.Members {
+				memberSet.Add(m{member.Name, member.ID})
 			}
+			memberLists = append(memberLists, memberSet)
+		}
+	}
+	for i := range memberLists {
+		if !memberLists[0].Equals(memberLists[i]) {
+			return false
 		}
 	}
-	return false
+	return true
 }
 
 // fill takes a single-endpoint client and populates the fields of etcdStatus
@@ -73,15 +101,75 @@ func (s *etcdStatus) fill(ctx context.Context, c *clientv3.Client) {
 	wg.Wait()
 }
 
-// TODO: make a real function
+func (o *observables) pvcMaxIndex() (max int) {
+	max = -1
+	for i := range o.pvcs {
+		tokens := strings.Split(o.pvcs[i].Name, "-")
+		index, err := strconv.Atoi(tokens[len(tokens)-1])
+		if err != nil {
+			continue
+		}
+		if index > max {
+			max = index
+		}
+	}
+	return max
+}
+
+func (o *observables) endpointMaxIndex() (max int) {
+	for i := range o.endpoints {
+		tokens := strings.Split(o.endpoints[i], ":")
+		if len(tokens) < 2 {
+			continue
+		}
+		tokens = strings.Split(tokens[len(tokens)-2], "-")
+		index, err := strconv.Atoi(tokens[len(tokens)-1])
+		if err != nil {
+			continue
+		}
+		if index > max {
+			max = index
+		}
+	}
+	return max
+}
+
+// TODO: make a real function to determine the right number of replicas.
+// Hint: if ClientURL in the member list is absent, the member has not yet
+// started, but if the name field is populated, this is a member of the
+// initial cluster. If the name field is empty, this member has just been
+// added with etcdctl member add (or equivalent API call).
 // nolint:unused
-func (o *observables) desiredReplicas() int {
+func (o *observables) desiredReplicas() (max int) {
+	max = -1
 	if o.etcdStatuses != nil {
 		for i := range o.etcdStatuses {
 			if o.etcdStatuses[i].memberList != nil {
-				return len(o.etcdStatuses[i].memberList.Members)
+				for j := range o.etcdStatuses[i].memberList.Members {
+					tokens := strings.Split(o.etcdStatuses[i].memberList.Members[j].Name, "-")
+					index, err := strconv.Atoi(tokens[len(tokens)-1])
+					if err != nil {
+						continue
+					}
+					if index > max {
+						max = index
+					}
+				}
 			}
 		}
 	}
-	return 0
+	if max > -1 {
+		return max + 1
+	}
+
+	if epMax := o.endpointMaxIndex(); epMax > max {
+		max = epMax
+	}
+	if pvcMax := o.pvcMaxIndex(); pvcMax > max {
+		max = pvcMax
+	}
+	if max == -1 {
+		return int(*o.instance.Spec.Replicas)
+	}
+	return max + 1
 }