一、Etcd
类比于Zookeeper，开源的、分布式的键值对数据存储系统，主要用来做配置共享、服务的注册和发现。高可用，强一致性。内部采用 Raft 协议作为一致性算法选举leader，保证集群数据不丢失
每个 etcd cluster 都是有若干个 member 组成的，每个 member 是一个独立运行的 etcd 实例，单台机器上可以运行多个 member。在正常运行的状态下，集群中会有一个 leader，其余的 member 都是 followers。leader 向 followers 同步日志，保证数据在各个 member 都有副本。leader 还会定时向所有的 member 发送心跳报文，如果在规定的时间里 follower 没有收到心跳，就会重新进行选举。客户端所有的请求都会先发送给 leader，leader 向所有的 followers 同步日志，等收到超过半数的确认后就把该日志存储到磁盘，并返回响应客户端。每个 etcd 服务有三大主要部分组成：raft 实现、WAL 日志存储、数据的存储和索引。WAL 会在本地磁盘（就是之前提到的 --data-dir）上存储日志内容（wal file）和快照（snapshot）
官方文档：https://coreos.com/etcd/
etcd-browser(etcd-webui)： https://github.com/henszey/etcd-browser
etckeeper：https://github.com/evildecay/etcdkeeper
参考文档：
etcd：从应用场景到实现原理的全方位解读
ETCD是什么
Etcd安装及使用详解
Java】用etcd做服务注册和发现
Etcd V3版本API
二、Mac安装
用brew安装非常方便，没安装的自行安装Homebrew
brew search etcd
可以看到候选安装包
brew install etcd
安装
#执行etcd即可启动服务
etcd
可以看到以下信息：
2018-11-26 17:35:02.659194 I | etcdmain: etcd Version: 3.3.10
2018-11-26 17:35:02.659310 I | etcdmain: Git SHA: GitNotFound
2018-11-26 17:35:02.659315 I | etcdmain: Go Version: go1.11.1
2018-11-26 17:35:02.659319 I | etcdmain: Go OS/Arch: darwin/amd64
2018-11-26 17:35:02.659324 I | etcdmain: setting maximum number of CPUs to 4, total number of available CPUs is 4
2018-11-26 17:35:02.659335 N | etcdmain: failed to detect default host (default host not supported on darwin_amd64)
2018-11-26 17:35:02.659345 W | etcdmain: no data-dir provided, using default data-dir ./default.etcd
2018-11-26 17:35:02.659758 N | etcdmain: the server is already initialized as member before, starting as etcd member...
2018-11-26 17:35:02.661503 I | embed: listening for peers on http://localhost:2380
2018-11-26 17:35:02.661918 I | embed: listening for client requests on localhost:2379
2018-11-26 17:35:02.666171 I | etcdserver: name = default
2018-11-26 17:35:02.666189 I | etcdserver: data dir = default.etcd
2018-11-26 17:35:02.666195 I | etcdserver: member dir = default.etcd/member
2018-11-26 17:35:02.666199 I | etcdserver: heartbeat = 100ms
2018-11-26 17:35:02.666203 I | etcdserver: election = 1000ms
2018-11-26 17:35:02.666207 I | etcdserver: snapshot count = 100000
2018-11-26 17:35:02.666220 I | etcdserver: advertise client URLs = http://localhost:2379
2018-11-26 17:35:02.668339 I | etcdserver: restarting member 8e9e05c52164694d in cluster cdf818194e3a8c32 at commit index 4
2018-11-26 17:35:02.668614 I | raft: 8e9e05c52164694d became follower at term 2
2018-11-26 17:35:02.668636 I | raft: newRaft 8e9e05c52164694d [peers: [], term: 2, commit: 4, applied: 0, lastindex: 4, lastterm: 2]
2018-11-26 17:35:02.671551 W | auth: simple token is not cryptographically signed
2018-11-26 17:35:02.672511 I | etcdserver: starting server... [version: 3.3.10, cluster version: to_be_decided]
2018-11-26 17:35:02.672700 E | etcdserver: cannot monitor file descriptor usage (cannot get FDUsage on darwin)
2018-11-26 17:35:02.673933 I | etcdserver/membership: added member 8e9e05c52164694d [http://localhost:2380] to cluster cdf818194e3a8c32
2018-11-26 17:35:02.674131 N | etcdserver/membership: set the initial cluster version to 3.3
2018-11-26 17:35:02.674257 I | etcdserver/api: enabled capabilities for version 3.3
2018-11-26 17:35:03.671575 I | raft: 8e9e05c52164694d is starting a new election at term 2
2018-11-26 17:35:03.671609 I | raft: 8e9e05c52164694d became candidate at term 3
2018-11-26 17:35:03.671916 I | raft: 8e9e05c52164694d received MsgVoteResp from 8e9e05c52164694d at term 3
2018-11-26 17:35:03.671942 I | raft: 8e9e05c52164694d became leader at term 3
2018-11-26 17:35:03.671951 I | raft: raft.node: 8e9e05c52164694d elected leader 8e9e05c52164694d at term 3
2018-11-26 17:35:03.672275 I | etcdserver: published {Name:default ClientURLs:[http://localhost:2379]} to cluster cdf818194e3a8c32
2018-11-26 17:35:03.672537 I | embed: ready to serve client requests
2018-11-26 17:35:03.682936 N | embed: serving insecure client requests on 127.0.0.1:2379, this is strongly discouraged!
etcdserver: name = default name表示节点名称，默认为default
data-dir保存日志和快照的目录，默认为当前工作目录default.etcd/
在http://localhost:2380和集群中其他节点通信
在http://localhost:2379提供HTTP API服务，供客户端交互。等会配置webui就是这个地址
etcdserver: heartbeat = 100ms leader发送心跳到followers的间隔时间
etcdserver: election = 1000ms 重新投票的超时时间，如果follow在该时间间隔没有收到心跳包，会触发重新投票，默认为1000ms
三、安装etcd webui
记得启动Etcd服务。
先安装node，git环境，然后clone
git clone https://github.com/henszey/etcd-browser.git
cd etcd-browser/
vim server.js
编辑server.js，修改内容如下：
var etcdHost = process.env.ETCD_HOST || '127.0.0.1'; # etcd 主机IP
var etcdPort = process.env.ETCD_PORT || 4001; # etcd 主机端口
var serverPort = process.env.SERVER_PORT || 8000; # etcd-browser 监听端口
然后启动
node server.js
访问：http://127.0.0.1:8000/
