v3api: Add a flag to RangeRequest to return only the keys.

Currently the user can't list only the keys in a prefix search. In
order to support such operations the filtering will be done on the
server side to reduce the encoding and network transfer costs.

Documentation/dev-guide/apispec/swagger/rpc.swagger.json

@@ -935,6 +935,11 @@
           "format": "byte",
           "description": "key is the first key for the range. If range_end is not given, the request only looks up key."
         },
+        "keys_only": {
+          "type": "boolean",
+          "format": "boolean",
+          "description": "keys_only when set returns only the keys and not the values."
+        },
         "limit": {
           "type": "string",
           "format": "int64",

clientv3/kv.go

@@ -134,7 +134,14 @@ func (kv *kv) do(ctx context.Context, op Op) (OpResponse, error) {
 	// TODO: handle other ops
 	case tRange:
 		var resp *pb.RangeResponse
-		r := &pb.RangeRequest{Key: op.key, RangeEnd: op.end, Limit: op.limit, Revision: op.rev, Serializable: op.serializable}
+		r := &pb.RangeRequest{
+			Key:          op.key,
+			RangeEnd:     op.end,
+			Limit:        op.limit,
+			Revision:     op.rev,
+			Serializable: op.serializable,
+			KeysOnly:     op.keysOnly,
+		}
 		if op.sort != nil {
 			r.SortOrder = pb.RangeRequest_SortOrder(op.sort.Order)
 			r.SortTarget = pb.RangeRequest_SortTarget(op.sort.Target)

clientv3/op.go

@@ -41,6 +41,7 @@ type Op struct {
 	limit        int64
 	sort         *SortOption
 	serializable bool
+	keysOnly     bool
 
 	// for range, watch
 	rev int64
@@ -208,6 +209,12 @@ func WithSerializable() OpOption {
 	return func(op *Op) { op.serializable = true }
 }
 
+// WithKeysOnly makes the 'Get' request return only the keys and the corresponding
+// values will be omitted.
+func WithKeysOnly() OpOption {
+	return func(op *Op) { op.keysOnly = true }
+}
+
 // WithFirstCreate gets the key with the oldest creation revision in the request range.
 func WithFirstCreate() []OpOption { return withTop(SortByCreateRevision, SortAscend) }
 

e2e/ctl_v3_kv_test.go

@@ -34,8 +34,9 @@ func TestCtlV3GetPeerTLS(t *testing.T)       { testCtl(t, getTest, withCfg(confi
 func TestCtlV3GetTimeout(t *testing.T)       { testCtl(t, getTest, withDialTimeout(0)) }
 func TestCtlV3GetQuorum(t *testing.T)        { testCtl(t, getTest, withQuorum()) }
 
-func TestCtlV3GetFormat(t *testing.T) { testCtl(t, getFormatTest) }
-func TestCtlV3GetRev(t *testing.T)    { testCtl(t, getRevTest) }
+func TestCtlV3GetFormat(t *testing.T)   { testCtl(t, getFormatTest) }
+func TestCtlV3GetRev(t *testing.T)      { testCtl(t, getRevTest) }
+func TestCtlV3GetKeysOnly(t *testing.T) { testCtl(t, getKeysOnlyTest) }
 
 func TestCtlV3Del(t *testing.T)          { testCtl(t, delTest) }
 func TestCtlV3DelNoTLS(t *testing.T)     { testCtl(t, delTest, withCfg(configNoTLS)) }
@@ -145,6 +146,25 @@ func getRevTest(cx ctlCtx) {
 	}
 }
 
+func getKeysOnlyTest(cx ctlCtx) {
+	var (
+		kvs = []kv{{"key1", "val1"}}
+	)
+	for i := range kvs {
+		if err := ctlV3Put(cx, kvs[i].key, kvs[i].val, ""); err != nil {
+			cx.t.Fatalf("getKeysOnlyTest #%d: ctlV3Put error (%v)", i, err)
+		}
+	}
+
+	cmdArgs := append(cx.PrefixArgs(), "get")
+	cmdArgs = append(cmdArgs, []string{"--prefix", "--keys-only", "key"}...)
+
+	err := spawnWithExpects(cmdArgs, []string{"key1", ""}...)
+	if err != nil {
+		cx.t.Fatalf("getKeysOnlyTest : error (%v)", err)
+	}
+}
+
 func delTest(cx ctlCtx) {
 	tests := []struct {
 		puts []kv

etcdctl/ctlv3/command/get_command.go

@@ -30,6 +30,7 @@ var (
 	getPrefix      bool
 	getFromKey     bool
 	getRev         int64
+	getKeysOnly    bool
 )
 
 // NewGetCommand returns the cobra command for "get".
@@ -47,6 +48,7 @@ func NewGetCommand() *cobra.Command {
 	cmd.Flags().BoolVar(&getPrefix, "prefix", false, "get keys with matching prefix")
 	cmd.Flags().BoolVar(&getFromKey, "from-key", false, "get keys that are greater than or equal to the given key")
 	cmd.Flags().Int64Var(&getRev, "rev", 0, "specify the kv revision")
+	cmd.Flags().BoolVar(&getKeysOnly, "keys-only", false, "get only the keys")
 	return cmd
 }
 
@@ -144,5 +146,9 @@ func getGetOp(cmd *cobra.Command, args []string) (string, []clientv3.OpOption) {
 		opts = append(opts, clientv3.WithFromKey())
 	}
 
+	if getKeysOnly {
+		opts = append(opts, clientv3.WithKeysOnly())
+	}
+
 	return key, opts
 }

etcdserver/apply.go

@@ -270,6 +270,9 @@ func (a *applierV3backend) Range(txnID int64, r *pb.RangeRequest) (*pb.RangeResp
 
 	resp.Header.Revision = rev
 	for i := range kvs {
+		if r.KeysOnly {
+			kvs[i].Value = nil
+		}
 		resp.Kvs = append(resp.Kvs, &kvs[i])
 	}
 	return resp, nil

etcdserver/etcdserverpb/rpc.proto

@@ -364,6 +364,9 @@ message RangeRequest {
   // a serializable range request is served locally without needing to reach consensus
   // with other nodes in the cluster.
   bool serializable = 7;
+
+  // keys_only when set returns only the keys and not the values.
+  bool keys_only = 8;
 }
 
 message RangeResponse {