Added HEALTHZ command

Returns 'ok' if the server is the leader or a follower with
a 'caught up' log.

This is mainly for HTTP connections that are using an
orchestration environment like kubernetes, but will work as a
general RESP command.

For HTTP a '200 OK' for 'caught up' and
'500 Internal Server Error' otherwise.

See #608

internal/server/server.go

@@ -744,7 +744,8 @@ func (server *Server) handleInputCommand(client *Client, msg *Message) error {
 			return WriteWebSocketMessage(client, []byte(res))
 		case HTTP:
 			status := "200 OK"
-			if server.http500Errors && !gjson.Get(res, "ok").Bool() {
+			if (server.http500Errors || msg._command == "healthz") &&
+				!gjson.Get(res, "ok").Bool() {
 				status = "500 Internal Server Error"
 			}
 			_, err := fmt.Fprintf(client, "HTTP/1.1 %s\r\n"+
@@ -881,7 +882,7 @@ func (server *Server) handleInputCommand(client *Client, msg *Message) error {
 		}
 	case "get", "keys", "scan", "nearby", "within", "intersects", "hooks",
 		"chans", "search", "ttl", "bounds", "server", "info", "type", "jget",
-		"evalro", "evalrosha":
+		"evalro", "evalrosha", "healthz":
 		// read operations
 
 		server.mu.RLock()
@@ -1067,6 +1068,8 @@ func (server *Server) command(msg *Message, client *Client) (
 		res, err = server.cmdStats(msg)
 	case "server":
 		res, err = server.cmdServer(msg)
+	case "healthz":
+		res, err = server.cmdHealthz(msg)
 	case "info":
 		res, err = server.cmdInfo(msg)
 	case "scan":

internal/server/stats.go

@@ -3,6 +3,7 @@ package server
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"runtime"
@@ -94,6 +95,24 @@ func (s *Server) cmdStats(msg *Message) (res resp.Value, err error) {
 	return res, nil
 }
 
+func (s *Server) cmdHealthz(msg *Message) (res resp.Value, err error) {
+	start := time.Now()
+	// if s.config.followHost() != "" {
+	m := make(map[string]interface{})
+	s.basicStats(m)
+	if fmt.Sprintf("%v\n", m["caught_up"]) != "true" {
+		return NOMessage, errors.New("not caught up")
+	}
+	// }
+	switch msg.OutputType {
+	case JSON:
+		res = resp.StringValue(`{"ok":true,"elapsed":"` + time.Since(start).String() + "\"}")
+	case RESP:
+		res = resp.SimpleStringValue("OK")
+	}
+	return res, nil
+}
+
 func (s *Server) cmdServer(msg *Message) (res resp.Value, err error) {
 	start := time.Now()
 	m := make(map[string]interface{})