choose switchover candidate based on lag and role

pkg/cluster/pod.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"math/rand"
 	"strconv"
+	"strings"
 	"time"
 
 	appsv1 "k8s.io/api/apps/v1"
@@ -459,8 +460,12 @@ func (c *Cluster) recreatePods(pods []v1.Pod, switchoverCandidates []spec.Namesp
 		// 1. we have not observed a new master pod when re-creating former replicas
 		// 2. we know possible switchover targets even when no replicas were recreated
 		if newMasterPod == nil && len(replicas) > 0 {
-			if err := c.Switchover(masterPod, masterCandidate(replicas)); err != nil {
+			masterCandidate, err := c.getSwitchoverCandidate(masterPod)
-				c.logger.Warningf("could not perform switch over: %v", err)
+			if err != nil {
+				return fmt.Errorf("skipping switchover: %v", err)
+			}
+			if err := c.Switchover(masterPod, masterCandidate); err != nil {
+				return fmt.Errorf("could not perform switch over: %v", err)
 			}
 		} else if newMasterPod == nil && len(replicas) == 0 {
 			c.logger.Warningf("cannot perform switch over before re-creating the pod: no replicas")
@@ -475,6 +480,57 @@ func (c *Cluster) recreatePods(pods []v1.Pod, switchoverCandidates []spec.Namesp
 	return nil
 }
 
+func (c *Cluster) getSwitchoverCandidate(master *v1.Pod) (spec.NamespacedName, error) {
+
+	var members []patroni.ClusterMember
+	candidates := make([]spec.NamespacedName, 0)
+	syncCandidates := make([]spec.NamespacedName, 0)
+	skipReasons := make([]string, 0)
+
+	err := retryutil.Retry(1*time.Second, 5*time.Second,
+		func() (bool, error) {
+			var err error
+			members, err = c.patroni.GetClusterMembers(master)
+
+			if err != nil {
+				return false, err
+			}
+			return true, nil
+		},
+	)
+	if err != nil {
+		return spec.NamespacedName{}, fmt.Errorf("failed to get Patroni cluster members: %s", err)
+	}
+
+	for _, member := range members {
+		if member.LagInMB > 0 {
+			skipReasons = append(skipReasons, fmt.Sprintf("%s lags behind by %d MB", member.Name, member.LagInMB))
+			continue
+		}
+		if PostgresRole(member.Role) != Leader && PostgresRole(member.Role) != StandbyLeader && member.State == "running" {
+			candidates = append(candidates, spec.NamespacedName{Namespace: master.Namespace, Name: member.Name})
+			if PostgresRole(member.Role) != SyncStandby {
+				syncCandidates = append(syncCandidates, spec.NamespacedName{Namespace: master.Namespace, Name: member.Name})
+			}
+		}
+	}
+
+	if len(syncCandidates) > 0 {
+		return candidates[rand.Intn(len(syncCandidates))], nil
+	}
+	if len(candidates) > 0 {
+		return candidates[rand.Intn(len(candidates))], nil
+	}
+
+	if len(skipReasons) > 0 {
+		err = fmt.Errorf("no replica suitable for switchover: %s", strings.Join(skipReasons, `','`))
+	} else {
+		err = fmt.Errorf("no replica running")
+	}
+
+	return spec.NamespacedName{}, err
+}
+
 func (c *Cluster) podIsEndOfLife(pod *v1.Pod) (bool, error) {
 	node, err := c.KubeClient.Nodes().Get(context.TODO(), pod.Spec.NodeName, metav1.GetOptions{})
 	if err != nil {

pkg/cluster/pod_test.go

@@ -0,0 +1,80 @@
+package cluster
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	"github.com/zalando/postgres-operator/mocks"
+	acidv1 "github.com/zalando/postgres-operator/pkg/apis/acid.zalan.do/v1"
+	"github.com/zalando/postgres-operator/pkg/spec"
+	"github.com/zalando/postgres-operator/pkg/util/k8sutil"
+	"github.com/zalando/postgres-operator/pkg/util/patroni"
+)
+
+func TestGetSwitchoverCandidate(t *testing.T) {
+	testName := "test getting right switchover candidate"
+	namespace := "default"
+
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	var cluster = New(Config{}, k8sutil.KubernetesClient{}, acidv1.Postgresql{}, logger, eventRecorder)
+
+	// simulate different member scenarios
+	tests := []struct {
+		subtest           string
+		clusterJson       string
+		expectedCandidate spec.NamespacedName
+		expectedError     error
+	}{
+		{
+			subtest:           "choose sync_standby over replica",
+			clusterJson:       `{"members": [{"name": "acid-test-cluster-0", "role": "leader", "state": "running", "api_url": "http://192.168.100.1:8008/patroni", "host": "192.168.100.1", "port": 5432, "timeline": 1}, {"name": "acid-test-cluster-1", "role": "sync_standby", "state": "running", "api_url": "http://192.168.100.2:8008/patroni", "host": "192.168.100.2", "port": 5432, "timeline": 1, "lag": 0}, {"name": "acid-test-cluster-2", "role": "replica", "state": "running", "api_url": "http://192.168.100.3:8008/patroni", "host": "192.168.100.3", "port": 5432, "timeline": 1, "lag": 0}]}`,
+			expectedCandidate: spec.NamespacedName{Namespace: namespace, Name: "acid-test-cluster-1"},
+			expectedError:     nil,
+		},
+		{
+			subtest:           "choose replica without a lag",
+			clusterJson:       `{"members": [{"name": "acid-test-cluster-0", "role": "leader", "state": "running", "api_url": "http://192.168.100.1:8008/patroni", "host": "192.168.100.1", "port": 5432, "timeline": 1}, {"name": "acid-test-cluster-1", "role": "replica", "state": "running", "api_url": "http://192.168.100.2:8008/patroni", "host": "192.168.100.2", "port": 5432, "timeline": 1, "lag": 5}, {"name": "acid-test-cluster-2", "role": "replica", "state": "running", "api_url": "http://192.168.100.3:8008/patroni", "host": "192.168.100.3", "port": 5432, "timeline": 1, "lag": 0}]}`,
+			expectedCandidate: spec.NamespacedName{Namespace: namespace, Name: "acid-test-cluster-2"},
+			expectedError:     nil,
+		},
+		{
+			subtest:           "no suitable replica available",
+			clusterJson:       `{"members": [{"name": "acid-test-cluster-0", "role": "leader", "state": "running", "api_url": "http://192.168.100.1:8008/patroni", "host": "192.168.100.1", "port": 5432, "timeline": 1}, {"name": "acid-test-cluster-1", "role": "replica", "state": "running", "api_url": "http://192.168.100.2:8008/patroni", "host": "192.168.100.2", "port": 5432, "timeline": 1, "lag": 5}]}`,
+			expectedCandidate: spec.NamespacedName{},
+			expectedError:     fmt.Errorf("no replica suitable for switchover: acid-test-cluster-1 lags behind by 5 MB"),
+		},
+	}
+
+	for _, tt := range tests {
+		// mocking cluster members
+		r := ioutil.NopCloser(bytes.NewReader([]byte(tt.clusterJson)))
+
+		response := http.Response{
+			StatusCode: 200,
+			Body:       r,
+		}
+
+		mockClient := mocks.NewMockHTTPClient(ctrl)
+		mockClient.EXPECT().Get(gomock.Any()).Return(&response, nil).AnyTimes()
+
+		p := patroni.New(patroniLogger, mockClient)
+		cluster.patroni = p
+		mockMasterPod := newMockPod("192.168.100.1")
+		mockMasterPod.Namespace = namespace
+
+		candidate, err := cluster.getSwitchoverCandidate(mockMasterPod)
+		if err != nil && err.Error() != tt.expectedError.Error() {
+			t.Errorf("%s - %s: unexpected error, %v", testName, tt.subtest, err)
+		}
+
+		if candidate != tt.expectedCandidate {
+			t.Errorf("%s - %s: unexpect switchover candidate, got %s, expected %s", testName, tt.subtest, candidate, tt.expectedCandidate)
+		}
+	}
+}

pkg/cluster/types.go

@@ -14,11 +14,11 @@ import (
 type PostgresRole string
 
 const (
-	// Master role
 	Master        PostgresRole = "master"
-
-	// Replica role
 	Replica       PostgresRole = "replica"
+	Leader        PostgresRole = "leader"
+	StandbyLeader PostgresRole = "standby_leader"
+	SyncStandby   PostgresRole = "sync_standby"
 )
 
 // PodEventType represents the type of a pod-related event

pkg/cluster/util.go

@@ -6,7 +6,6 @@ import (
 	"encoding/gob"
 	"encoding/json"
 	"fmt"
-	"math/rand"
 	"reflect"
 	"sort"
 	"strings"
@@ -525,10 +524,6 @@ func (c *Cluster) credentialSecretNameForCluster(username string, clusterName st
 		"tprgroup", acidzalando.GroupName)
 }
 
-func masterCandidate(replicas []spec.NamespacedName) spec.NamespacedName {
-	return replicas[rand.Intn(len(replicas))]
-}
-
 func cloneSpec(from *acidv1.Postgresql) (*acidv1.Postgresql, error) {
 	var (
 		buf    bytes.Buffer

pkg/util/patroni/patroni.go

@@ -21,6 +21,7 @@ import (
 const (
 	failoverPath = "/failover"
 	configPath   = "/config"
+	clusterPath  = "/cluster"
 	statusPath   = "/patroni"
 	restartPath  = "/restart"
 	apiPort      = 8008
@@ -29,6 +30,7 @@ const (
 
 // Interface describe patroni methods
 type Interface interface {
+	GetClusterMembers(master *v1.Pod) ([]ClusterMember, error)
 	Switchover(master *v1.Pod, candidate string) error
 	SetPostgresParameters(server *v1.Pod, options map[string]string) error
 	GetMemberData(server *v1.Pod) (MemberData, error)
@@ -175,6 +177,20 @@ func (p *Patroni) SetConfig(server *v1.Pod, config map[string]interface{}) error
 	return p.httpPostOrPatch(http.MethodPatch, apiURLString+configPath, buf)
 }
 
+// ClusterMembers array of cluster members from Patroni API
+type ClusterMembers struct {
+	Members []ClusterMember `json:"members"`
+}
+
+// ClusterMember cluster member data from Patroni API
+type ClusterMember struct {
+	Name     string `json:"name"`
+	Role     string `json:"role"`
+	State    string `json:"state"`
+	Timeline int    `json:"timeline"`
+	LagInMB  int    `json:"lag"`
+}
+
 // MemberDataPatroni child element
 type MemberDataPatroni struct {
 	Version string `json:"version"`
@@ -246,6 +262,27 @@ func (p *Patroni) Restart(server *v1.Pod) error {
 	return nil
 }
 
+// GetClusterMembers read cluster data from patroni API
+func (p *Patroni) GetClusterMembers(server *v1.Pod) ([]ClusterMember, error) {
+
+	apiURLString, err := apiURL(server)
+	if err != nil {
+		return []ClusterMember{}, err
+	}
+	body, err := p.httpGet(apiURLString + clusterPath)
+	if err != nil {
+		return []ClusterMember{}, err
+	}
+
+	data := ClusterMembers{}
+	err = json.Unmarshal([]byte(body), &data)
+	if err != nil {
+		return []ClusterMember{}, err
+	}
+
+	return data.Members, nil
+}
+
 // GetMemberData read member data from patroni API
 func (p *Patroni) GetMemberData(server *v1.Pod) (MemberData, error) {
 

pkg/util/patroni/patroni_test.go

@@ -85,6 +85,55 @@ func TestApiURL(t *testing.T) {
 	}
 }
 
+func TestGetClusterMembers(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	expectedClusterMemberData := []ClusterMember{
+		{
+			Name:     "acid-test-cluster-0",
+			Role:     "leader",
+			State:    "running",
+			Timeline: 1,
+			LagInMB:  0,
+		}, {
+			Name:     "acid-test-cluster-1",
+			Role:     "sync_standby",
+			State:    "running",
+			Timeline: 1,
+			LagInMB:  0,
+		}, {
+			Name:     "acid-test-cluster-2",
+			Role:     "replica",
+			State:    "running",
+			Timeline: 1,
+			LagInMB:  0,
+		}}
+
+	json := `{"members": [{"name": "acid-test-cluster-0", "role": "leader", "state": "running", "api_url": "http://192.168.100.1:8008/patroni", "host": "192.168.100.1", "port": 5432, "timeline": 1}, {"name": "acid-test-cluster-1", "role": "sync_standby", "state": "running", "api_url": "http://192.168.100.2:8008/patroni", "host": "192.168.100.2", "port": 5432, "timeline": 1, "lag": 0}, {"name": "acid-test-cluster-2", "role": "replica", "state": "running", "api_url": "http://192.168.100.3:8008/patroni", "host": "192.168.100.3", "port": 5432, "timeline": 1, "lag": 0}]}`
+	r := ioutil.NopCloser(bytes.NewReader([]byte(json)))
+
+	response := http.Response{
+		StatusCode: 200,
+		Body:       r,
+	}
+
+	mockClient := mocks.NewMockHTTPClient(ctrl)
+	mockClient.EXPECT().Get(gomock.Any()).Return(&response, nil)
+
+	p := New(logger, mockClient)
+
+	clusterMemberData, err := p.GetClusterMembers(newMockPod("192.168.100.1"))
+
+	if !reflect.DeepEqual(expectedClusterMemberData, clusterMemberData) {
+		t.Errorf("Patroni cluster members differ: expected: %#v, got: %#v", expectedClusterMemberData, clusterMemberData)
+	}
+
+	if err != nil {
+		t.Errorf("Could not read Patroni data: %v", err)
+	}
+}
+
 func TestGetMemberData(t *testing.T) {
 	ctrl := gomock.NewController(t)
 	defer ctrl.Finish()