环境 Redhat 7.3 / ambari 2.7 / hadoop 3.1
使用ambaar完成i部署hadoop 集群没有报错，但发现有几个服务不能启动，查看日志提示都显示指向某一台host connection refuse, 如：
Connection failed: [Errno 111] Connection refused to ……
Failed connect to hadoop-app1.novalocal:50070; Connection refused
网上找了一圈，这个错误很常见，一般是防火墙相关设置的问题。
所以先查看一下firewalld
[[email protected] ~]# systemctl status firewalld
● firewalld.service - firewalld - dynamic firewall daemon
Loaded: loaded (/usr/lib/systemd/system/firewalld.service; disabled; vendor preset: enabled)
Active: inactive (dead)
Docs: man:firewalld(1)
显示防火墙已经禁用，试着重启agent问题依然存在。
又创仔细重新阅读官方文档，逐步核对环境准备各步骤。
一步一步重新试了，也重新装了NTP，并且根据要求 systemctl enable ntpd, 依然不行。。
查看了下不同host各服务有哪些不同。
connection refuse 的host ntpd status --
[[email protected] ~]# systemctl status ntpd
● ntpd.service - Network Time Service
Loaded: loaded (/usr/lib/systemd/system/ntpd.service; enabled; vendor preset: disabled)
Active: inactive (dead)
对比其他未报错host
[[email protected] ~]# systemctl status ntpd
● ntpd.service - Network Time Service
Loaded: loaded (/usr/lib/systemd/system/ntpd.service; enabled; vendor preset: disabled)
Active: active (running) since Thu 2019-01-24 10:39:05 CST; 1 weeks 0 days ago
Main PID: 569 (ntpd)
CGroup: /system.slice/ntpd.service
└─569 /usr/sbin/ntpd -u ntp:ntp -g
原来虽然enable 了，但不知道为啥，没有启动，运行命令 systemctl start ntpd 启动，查看ntpd status 与其他host一样 active,再手动启动hadoop 相关service connection refuse 错误解决
*****************************
看NTP的简介便可以发现它在集群互信过程中起着重要作用。。
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in current use. NTPwas designed by David L. Mills of the University of Delawar...