Add support of managing the raft cluster via the client (#258)

```
❯ _build/kvctl raft list peers

|---------|-----------------------|-----------|
| NODE ID |     NODE ADDRESS      | IS LEADER |
|---------|-----------------------|-----------|
|       1 | http://127.0.0.1:6001 | YES       |
|---------|-----------------------|-----------|

❯ _build/kvctl raft add peer 2 http://127.0.0.1:6002
Add node '2' with address 'http://127.0.0.1:6002' successfully

❯ _build/kvctl -H "http://127.0.0.1:9380" raft remove peer 2
Remove node '2' successfully

```

cmd/client/command/raft.go

@@ -0,0 +1,175 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ *
+ */
+
+package command
+
+import (
+	"errors"
+	"fmt"
+	"net/url"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/olekukonko/tablewriter"
+
+	"github.com/spf13/cobra"
+)
+
+const (
+	raftCommandList   = "list"
+	raftCommandAdd    = "add"
+	raftCommandRemove = "remove"
+)
+
+var RaftCommand = &cobra.Command{
+	Use:   "raft",
+	Short: "Raft operations",
+	Example: `
+# Display all memberships in the cluster
+kvctl raft list peers
+
+# Add a node to the cluster
+kvctl raft add peer <node_id> <node_address>
+
+# Remove a node from the cluster
+kvctl raft remove peer <node_id>
+`,
+	ValidArgs: []string{raftCommandList, raftCommandAdd, raftCommandRemove},
+	RunE: func(cmd *cobra.Command, args []string) error {
+		host, _ := cmd.Flags().GetString("host")
+		client := newClient(host)
+		switch strings.ToLower(args[0]) {
+		case raftCommandList:
+			if len(args) < 2 || args[1] != "peers" {
+				return fmt.Errorf("unsupported openeration: '%s' in raft command", args[1])
+			}
+			return listRaftPeers(client)
+		case raftCommandAdd, raftCommandRemove:
+			if len(args) < 2 {
+				return errors.New("missing 'peer' in raft command")
+			}
+			if args[1] != "peer" {
+				return fmt.Errorf("unsupported openeration: '%s' in raft command", args[1])
+			}
+			if len(args) < 3 {
+				return fmt.Errorf("missing node_id and node_address")
+			}
+			id, err := strconv.ParseUint(args[2], 10, 64)
+			if err != nil {
+				return fmt.Errorf("invalid node_id: %s", args[1])
+			}
+			if args[0] == raftCommandAdd {
+				if len(args) < 4 {
+					return fmt.Errorf("missing node_address")
+				}
+				address := args[3]
+				if _, err := url.Parse(address); err != nil {
+					return fmt.Errorf("invalid node_address: %s", address)
+				}
+				return addRaftPeer(client, id, address)
+			} else {
+				return removeRaftPeer(client, id)
+			}
+		default:
+			return fmt.Errorf("unsupported openeration: '%s' in raft command", args[0])
+		}
+	},
+	SilenceUsage:  true,
+	SilenceErrors: true,
+}
+
+func listRaftPeers(cli *client) error {
+	rsp, err := cli.restyCli.R().Get("/raft/peers")
+	if err != nil {
+		return err
+	}
+	if rsp.IsError() {
+		return unmarshalError(rsp.Body())
+	}
+
+	var result struct {
+		Leader uint64            `json:"leader"`
+		Peers  map[uint64]string `json:"peers"`
+	}
+	if err := unmarshalData(rsp.Body(), &result); err != nil {
+		return err
+	}
+	writer := tablewriter.NewWriter(os.Stdout)
+	printLine("")
+	writer.SetHeader([]string{"NODE_ID", "NODE_ADDRESS", "IS_LEADER"})
+	writer.SetCenterSeparator("|")
+	for id, addr := range result.Peers {
+		isLeader := "NO"
+		if id == result.Leader {
+			isLeader = "YES"
+		}
+		columns := []string{fmt.Sprintf("%d", id), addr, isLeader}
+		writer.Append(columns)
+	}
+	writer.Render()
+	return nil
+}
+
+func addRaftPeer(cli *client, id uint64, address string) error {
+	var request struct {
+		ID        uint64 `json:"id"`
+		Peer      string `json:"peer"`
+		Operation string `json:"operation"`
+	}
+	request.ID = id
+	request.Peer = address
+	request.Operation = "add"
+
+	rsp, err := cli.restyCli.R().
+		SetBody(&request).
+		Post("/raft/peers")
+	if err != nil {
+		return err
+	}
+	if rsp.IsError() {
+		return unmarshalError(rsp.Body())
+	}
+
+	printLine("Add node '%d' with address '%s' successfully", id, address)
+	return nil
+}
+
+func removeRaftPeer(cli *client, id uint64) error {
+	var request struct {
+		ID        uint64 `json:"id"`
+		Operation string `json:"operation"`
+	}
+	request.ID = id
+	request.Operation = "remove"
+
+	rsp, err := cli.restyCli.R().
+		SetBody(&request).
+		Post("/raft/peers")
+	if err != nil {
+		return err
+	}
+	if rsp.IsError() {
+		return unmarshalError(rsp.Body())
+	}
+
+	printLine("Remove node '%d' successfully", id)
+	return nil
+}

cmd/client/main.go

@@ -57,6 +57,7 @@ func init() {
 	rootCommand.AddCommand(command.ImportCommand)
 	rootCommand.AddCommand(command.MigrateCommand)
 	rootCommand.AddCommand(command.FailoverCommand)
+	rootCommand.AddCommand(command.RaftCommand)
 
 	rootCommand.SilenceUsage = true
 	rootCommand.SilenceErrors = true

server/api/raft.go

@@ -44,7 +44,7 @@ type RaftHandler struct{}
 type MemberRequest struct {
 	ID        uint64 `json:"id" validate:"required,gt=0"`
 	Operation string `json:"operation" validate:"required"`
-	Peer      string `json:"peer"`
+	Peer      string `json:"peer,omitempty"`
 }
 
 func (r *MemberRequest) validate() error {