Avoid thundering herd on etcd on masters upgrade

staging/src/k8s.io/client-go/tools/cache/reflector.go

@@ -85,7 +85,12 @@ type Reflector struct {
 	// lastSyncResourceVersionMutex guards read/write access to lastSyncResourceVersion
 	lastSyncResourceVersionMutex sync.RWMutex
 	// WatchListPageSize is the requested chunk size of initial and resync watch lists.
-	// Defaults to pager.PageSize.
+	// If unset, for consistent reads (RV="") or reads that opt-into arbitrarily old data
+	// (RV="0") it will default to pager.PageSize, for the rest (RV != "" && RV != "0")
+	// it will turn off pagination to allow serving them from watch cache.
+	// NOTE: It should be used carefully as paginated lists are always served directly from
+	// etcd, which is significantly less efficient and may lead to serious performance and
+	// scalability problems.
 	WatchListPageSize int64
 }
 
@@ -220,6 +225,21 @@ func (r *Reflector) ListAndWatch(stopCh <-chan struct{}) error {
 			}))
 			if r.WatchListPageSize != 0 {
 				pager.PageSize = r.WatchListPageSize
+			} else {
+				// User didn't explicitly request pagination.
+				if options.ResourceVersion != "" && options.ResourceVersion != "0" {
+					// We also don't turn off pagination for ResourceVersion="0", since watch cache
+					// is ignoring Limit in that case anyway, and if watchcache is not enabled we
+					// don't introduce regression.
+
+					// With ResourceVersion != "", we have a possibility to list from watch cache,
+					// but we do that (for ResourceVersion != "0") only if Limit is unset.
+					// To avoid thundering herd on etcd (e.g. on master upgrades), we explicitly
+					// switch off pagination to force listing from watch cache (if enabled).
+					// With the existing semantic of RV (result is at least as fresh as provided RV),
+					// this is correct and doesn't lead to going back in time.
+					pager.PageSize = 0
+				}
 			}
 
 			list, err = pager.List(context.Background(), options)
@@ -320,7 +340,9 @@ func (r *Reflector) ListAndWatch(stopCh <-chan struct{}) error {
 		if err != nil {
 			switch {
 			case isExpiredError(err):
-				r.setIsLastSyncResourceVersionExpired(true)
+				// Don't set LastSyncResourceVersionExpired - LIST call with ResourceVersion=RV already
+				// has a semantic that it returns data at least as fresh as provided RV.
+				// So first try to LIST with setting RV to resource version of last observed object.
 				klog.V(4).Infof("%s: watch of %v closed with: %v", r.name, r.expectedTypeName, err)
 			case err == io.EOF:
 				// watch closed normally
@@ -344,8 +366,10 @@ func (r *Reflector) ListAndWatch(stopCh <-chan struct{}) error {
 			if err != errorStopRequested {
 				switch {
 				case isExpiredError(err):
-					r.setIsLastSyncResourceVersionExpired(true)
+					// Don't set LastSyncResourceVersionExpired - LIST call with ResourceVersion=RV already
-					klog.V(4).Infof("%s: watch of %v ended with: %v", r.name, r.expectedTypeName, err)
+					// has a semantic that it returns data at least as fresh as provided RV.
+					// So first try to LIST with setting RV to resource version of last observed object.
+					klog.V(4).Infof("%s: watch of %v closed with: %v", r.name, r.expectedTypeName, err)
 				default:
 					klog.Warningf("%s: watch of %v ended with: %v", r.name, r.expectedTypeName, err)
 				}

staging/src/k8s.io/client-go/tools/cache/reflector_test.go

@@ -425,6 +425,8 @@ func TestReflectorWatchListPageSize(t *testing.T) {
 		},
 	}
 	r := NewReflector(lw, &v1.Pod{}, s, 0)
+	// Set resource version to test pagination also for not consistent reads.
+	r.setLastSyncResourceVersion("10")
 	// Set the reflector to paginate the list request in 4 item chunks.
 	r.WatchListPageSize = 4
 	r.ListAndWatch(stopCh)
@@ -435,6 +437,41 @@ func TestReflectorWatchListPageSize(t *testing.T) {
 	}
 }
 
+func TestReflectorNotPaginatingNotConsistentReads(t *testing.T) {
+	stopCh := make(chan struct{})
+	s := NewStore(MetaNamespaceKeyFunc)
+
+	lw := &testLW{
+		WatchFunc: func(options metav1.ListOptions) (watch.Interface, error) {
+			// Stop once the reflector begins watching since we're only interested in the list.
+			close(stopCh)
+			fw := watch.NewFake()
+			return fw, nil
+		},
+		ListFunc: func(options metav1.ListOptions) (runtime.Object, error) {
+			if options.ResourceVersion != "10" {
+				t.Fatalf("Expected ResourceVersion: \"10\", got: %s", options.ResourceVersion)
+			}
+			if options.Limit != 0 {
+				t.Fatalf("Expected list Limit of 0 but got %d", options.Limit)
+			}
+			pods := make([]v1.Pod, 10)
+			for i := 0; i < 10; i++ {
+				pods[i] = v1.Pod{ObjectMeta: metav1.ObjectMeta{Name: fmt.Sprintf("pod-%d", i), ResourceVersion: fmt.Sprintf("%d", i)}}
+			}
+			return &v1.PodList{ListMeta: metav1.ListMeta{ResourceVersion: "10"}, Items: pods}, nil
+		},
+	}
+	r := NewReflector(lw, &v1.Pod{}, s, 0)
+	r.setLastSyncResourceVersion("10")
+	r.ListAndWatch(stopCh)
+
+	results := s.List()
+	if len(results) != 10 {
+		t.Errorf("Expected 10 results, got %d", len(results))
+	}
+}
+
 // TestReflectorResyncWithResourceVersion ensures that a reflector keeps track of the ResourceVersion and sends
 // it in relist requests to prevent the reflector from traveling back in time if the relist is to a api-server or
 // etcd that is partitioned and serving older data than the reflector has already processed.