diff --git a/weed/server/master_grpc_server_digest_test.go b/weed/server/master_grpc_server_digest_test.go
new file mode 100644
index 000000000..25847358e
--- /dev/null
+++ b/weed/server/master_grpc_server_digest_test.go
@@ -0,0 +1,90 @@
+package weed_server
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
+	"github.com/seaweedfs/seaweedfs/weed/sequence"
+	"github.com/seaweedfs/seaweedfs/weed/topology"
+)
+
+func digestTestCluster(t *testing.T) (*MasterServer, *topology.DataNode) {
+	t.Helper()
+	topo := topology.NewTopology("test", sequence.NewMemorySequencer(), 32*1024*1024*1024, 5, false)
+	dn := topo.GetOrCreateDataCenter("dc1").GetOrCreateRack("rack1").
+		GetOrCreateDataNode("127.0.0.1", 8080, 18080, "", "", map[string]uint32{"": 100})
+	return &MasterServer{Topo: topo}, dn
+}
+
+func digestTestVolumeMessage(id uint32) *master_pb.VolumeInformationMessage {
+	return &master_pb.VolumeInformationMessage{
+		Id: id, Size: 1024, Collection: "c", Version: 3,
+	}
+}
+
+// A volume server that predates the digest keeps sending its whole list and
+// must never be asked for anything, whatever the master computes. This is what
+// lets the two sides be upgraded in either order.
+func TestDigestCheckIgnoresServersThatReportNone(t *testing.T) {
+	ms, dn := digestTestCluster(t)
+	ms.Topo.SyncDataNodeRegistration([]*master_pb.VolumeInformationMessage{digestTestVolumeMessage(1)}, dn)
+
+	if ms.checkVolumeDigest(&master_pb.Heartbeat{
+		Volumes: []*master_pb.VolumeInformationMessage{digestTestVolumeMessage(1)},
+	}, dn) {
+		t.Error("a server reporting no digest was asked to resend")
+	}
+}
+
+func TestDigestCheckAcceptsAMatchingReport(t *testing.T) {
+	ms, dn := digestTestCluster(t)
+	volumes := []*master_pb.VolumeInformationMessage{digestTestVolumeMessage(1), digestTestVolumeMessage(2)}
+	ms.Topo.SyncDataNodeRegistration(volumes, dn)
+
+	digest := dn.VolumeDigest()
+	if ms.checkVolumeDigest(&master_pb.Heartbeat{Volumes: volumes, VolumeDigest: &digest}, dn) {
+		t.Error("a matching digest was asked to resend")
+	}
+}
+
+// A heartbeat that already carried the whole list has nothing further to give,
+// so a mismatch there is a genuine disagreement to report rather than something
+// to ask about again.
+func TestDigestCheckDoesNotReaskAfterAFullList(t *testing.T) {
+	ms, dn := digestTestCluster(t)
+	volumes := []*master_pb.VolumeInformationMessage{digestTestVolumeMessage(1)}
+	ms.Topo.SyncDataNodeRegistration(volumes, dn)
+
+	wrong := dn.VolumeDigest() ^ 1
+	if ms.checkVolumeDigest(&master_pb.Heartbeat{Volumes: volumes, VolumeDigest: &wrong}, dn) {
+		t.Error("a full volume list that still disagreed was asked to resend, which would repeat forever")
+	}
+}
+
+// The case the request exists for: a heartbeat carrying no list whose digest
+// disagrees means the master has drifted and needs the list back.
+func TestDigestCheckAsksForTheListWhenADeltaDisagrees(t *testing.T) {
+	ms, dn := digestTestCluster(t)
+	ms.Topo.SyncDataNodeRegistration([]*master_pb.VolumeInformationMessage{digestTestVolumeMessage(1)}, dn)
+
+	wrong := dn.VolumeDigest() ^ 1
+	if !ms.checkVolumeDigest(&master_pb.Heartbeat{VolumeDigest: &wrong}, dn) {
+		t.Error("a disagreeing digest with no list to fall back on was not asked to resend")
+	}
+}
+
+// A node reporting one volume id twice is stored once, so the digests cannot
+// agree however often the list is resent.
+func TestDigestCheckSkipsNodesWithDuplicateVolumeIds(t *testing.T) {
+	ms, dn := digestTestCluster(t)
+	duplicated := digestTestVolumeMessage(1)
+	duplicated.DiskId = 1
+	ms.Topo.SyncDataNodeRegistration([]*master_pb.VolumeInformationMessage{
+		digestTestVolumeMessage(1), duplicated,
+	}, dn)
+
+	wrong := dn.VolumeDigest() ^ 1
+	if ms.checkVolumeDigest(&master_pb.Heartbeat{VolumeDigest: &wrong}, dn) {
+		t.Error("a node the master cannot represent was asked to resend, which would repeat forever")
+	}
+}