kubeadm: enable etcd's learner mode when joining etcd members

cmd/kubeadm/app/features/features.go

@@ -33,12 +33,15 @@ const (
 	PublicKeysECDSA = "PublicKeysECDSA"
 	// RootlessControlPlane is expected to be in alpha in v1.22
 	RootlessControlPlane = "RootlessControlPlane"
+	// EtcdLearnerMode is expected to be in alpha in v1.27
+	EtcdLearnerMode = "EtcdLearnerMode"
 )
 
 // InitFeatureGates are the default feature gates for the init command
 var InitFeatureGates = FeatureList{
 	PublicKeysECDSA:      {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
 	RootlessControlPlane: {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
+	EtcdLearnerMode:      {FeatureSpec: featuregate.FeatureSpec{Default: false, PreRelease: featuregate.Alpha}},
 }
 
 // Feature represents a feature being gated

cmd/kubeadm/app/phases/etcd/local.go

@@ -149,7 +149,11 @@ func CreateStackedEtcdStaticPodManifestFile(client clientset.Interface, manifest
 		fmt.Printf("[etcd] Would add etcd member: %s\n", etcdPeerAddress)
 	} else {
 		klog.V(1).Infof("[etcd] Adding etcd member: %s", etcdPeerAddress)
-		cluster, err = etcdClient.AddMember(nodeName, etcdPeerAddress)
+		if features.Enabled(cfg.FeatureGates, features.EtcdLearnerMode) {
+			cluster, err = etcdClient.AddMemberAsLeanerAndPromote(nodeName, etcdPeerAddress)
+		} else {
+			cluster, err = etcdClient.AddMember(nodeName, etcdPeerAddress)
+		}
 		if err != nil {
 			return err
 		}

cmd/kubeadm/app/phases/upgrade/staticpods_test.go

@@ -251,6 +251,10 @@ func (c fakeTLSEtcdClient) ListMembers() ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }
 
+func (c fakeTLSEtcdClient) AddMemberAsLeanerAndPromote(name string, peerAddrs string) ([]etcdutil.Member, error) {
+	return []etcdutil.Member{}, nil
+}
+
 func (c fakeTLSEtcdClient) AddMember(name string, peerAddrs string) ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }
@@ -286,6 +290,10 @@ func (c fakePodManifestEtcdClient) ListMembers() ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }
 
+func (c fakePodManifestEtcdClient) AddMemberAsLeanerAndPromote(name string, peerAddrs string) ([]etcdutil.Member, error) {
+	return []etcdutil.Member{}, nil
+}
+
 func (c fakePodManifestEtcdClient) AddMember(name string, peerAddrs string) ([]etcdutil.Member, error) {
 	return []etcdutil.Member{}, nil
 }

cmd/kubeadm/app/util/etcd/etcd.go

@@ -60,6 +60,7 @@ type ClusterInterrogator interface {
 	Sync() error
 	ListMembers() ([]Member, error)
 	AddMember(name string, peerAddrs string) ([]Member, error)
+	AddMemberAsLeanerAndPromote(name string, peerAddrs string) ([]Member, error)
 	GetMemberID(peerURL string) (uint64, error)
 	RemoveMember(id uint64) ([]Member, error)
 }
@@ -341,10 +342,20 @@ func (c *Client) RemoveMember(id uint64) ([]Member, error) {
 	return ret, nil
 }
 
-// AddMember notifies an existing etcd cluster that a new member is joining, and
+// AddMember adds a new member into the etcd cluster
+func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
+	return c.addMember(name, peerAddrs, false)
+}
+
+// AddMemberAsLeanerAndPromote adds a new learner member into the etcd cluster and promotes it to a voting member
+func (c *Client) AddMemberAsLeanerAndPromote(name string, peerAddrs string) ([]Member, error) {
+	return c.addMember(name, peerAddrs, true)
+}
+
+// addMember notifies an existing etcd cluster that a new member is joining, and
 // return the updated list of members. If the member has already been added to the
 // cluster, this will return the existing list of etcd members.
-func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
+func (c *Client) addMember(name string, peerAddrs string, isLearner bool) ([]Member, error) {
 	// Parse the peer address, required to add the client URL later to the list
 	// of endpoints for this client. Parsing as a first operation to make sure that
 	// if this fails no member addition is performed on the etcd cluster.
@@ -357,6 +368,7 @@ func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
 	var (
 		lastError   error
 		respMembers []*etcdserverpb.Member
+		learnerID   uint64
 	)
 	err = wait.ExponentialBackoff(etcdBackoff, func() (bool, error) {
 		cli, err := clientv3.New(clientv3.Config{
@@ -376,6 +388,26 @@ func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
 		ctx, cancel := context.WithTimeout(context.Background(), etcdTimeout)
 		defer cancel()
 		var resp *clientv3.MemberAddResponse
+		if isLearner {
+			// if learnerID is set, it means the etcd member is already added successfully.
+			if learnerID == 0 {
+				klog.V(1).Infof("[etcd] Adding etcd member as learner: %016x", peerAddrs)
+				resp, err = cli.MemberAddAsLearner(ctx, []string{peerAddrs})
+				if err != nil {
+					lastError = err
+					return false, nil
+				}
+				learnerID = resp.Member.ID
+			}
+			err = memberPromote(ctx, cli, learnerID)
+			if err != nil {
+				lastError = err
+				return false, nil
+			}
+			respMembers = resp.Members
+			return true, nil
+		}
+
 		resp, err = cli.MemberAdd(ctx, []string{peerAddrs})
 		if err == nil {
 			respMembers = resp.Members
@@ -427,6 +459,23 @@ func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
 	return ret, nil
 }
 
+func memberPromote(ctx context.Context, cli *clientv3.Client, learnerID uint64) error {
+	klog.V(1).Infof("[etcd] Promoting a learner as a voting member: %016x", learnerID)
+	// TODO: warning logs from etcd client should be removed.
+	// The warning logs are printed by etcd client code for several reasons, including
+	// 1. can not promote yet(no synced)
+	// 2. context deadline exceeded
+	// 3. peer URLs already exists
+	// Once the client provides a way to check if the etcd learner is ready to promote, the retry logic can be revisited.
+	_, err := cli.MemberPromote(ctx, learnerID)
+	if err == nil {
+		klog.V(1).Infof("[etcd] The learner was promoted as a voting member: %016x", learnerID)
+		return nil
+	}
+	klog.V(5).Infof("[etcd] Promoting the learner %016x failed: %v", learnerID, err)
+	return err
+}
+
 // CheckClusterHealth returns nil for status Up or error for status Down
 func (c *Client) CheckClusterHealth() error {
 	_, err := c.getClusterStatus()