一、简介
etcd是一个高可用的分布式键值(key-value)数据库。etcd内部采用raft协议作为一致性算法，etcd基于Go语言实现。
提供配置共享和服务发现的系统比较多，其中最为大家熟知的是[Zookeeper]（后文简称ZK），而ETCD可以算得上是后起之秀了。在项目实现，一致性协议易理解性，运维，安全等多个维度上，ETCD相比Zookeeper都占据优势。
etcd是一个服务发现系统，具备以下的特点：
简单：安装配置简单，而且提供了HTTP API进行交互，使用也很简单
安全：支持SSL证书验证
快速：根据官方提供的benchmark数据，单实例支持每秒2k+读操作
可靠：采用raft算法，实现分布式系统数据的可用性和一致性
二、应用场景
和ZK类似，ETCD有很多使用场景，包括：
配置管理
服务注册于发现
选主
应用调度
分布式队列
分布式锁
安装
本文所使用的系统为： ubuntu-14.04.6-server-amd64
wget https://github.com/etcd-io/etcd/releases/download/v3.3.10/etcd-v3.3.10-linux-amd64.tar.gz
tar zxvf etcd-v3.3.10-linux-amd64.tar.gz
mv etcd-v3.3.10-linux-amd64 /opt/etcd-v3.3.10
解压后的文件如下所示：
root@ubuntu:cd /opt/etcd-v3.3.10
root@ubuntu:/opt/etcd-v3.3.10# ls
default.etcd Documentation etcd etcdctl README-etcdctl.md README.md READMEv2-etcdctl.md
其中etcd是server端，etcdctl是客户端，操作之后会生成一个default.etcd，主要用来存储etct数据。
启动一个单节点的etcd服务，只需要运行etcd命令就行。不过有可能会出现以下问题：
root@ubuntu:/opt/etcd-v3.3.10# ./etcd
bash: ./etcd: 权限不够
如果出现权限，采用如下方法：
root@ubuntu:/opt/etcd-v3.3.10# chmod 755 etcd
启动成功后会出现如下内容：
root@ubuntu:/opt/etcd-v3.3.10# ./etcd
2020-04-24 10:22:44.631034 I | etcdmain: etcd Version: 3.3.10
2020-04-24 10:22:44.631114 I | etcdmain: Git SHA: 27fc7e2
2020-04-24 10:22:44.631122 I | etcdmain: Go Version: go1.10.4
2020-04-24 10:22:44.631128 I | etcdmain: Go OS/Arch: linux/amd64
2020-04-24 10:22:44.631134 I | etcdmain: setting maximum number of CPUs to 2, total number of available CPUs is 2
2020-04-24 10:22:44.631142 W | etcdmain: no data-dir provided, using default data-dir ./default.etcd
2020-04-24 10:22:44.631734 I | embed: listening for peers on http://localhost:2380
2020-04-24 10:22:44.631820 I | embed: listening for client requests on localhost:2379
2020-04-24 10:22:44.633281 I | etcdserver: name = default
2020-04-24 10:22:44.633323 I | etcdserver: data dir = default.etcd
2020-04-24 10:22:44.633356 I | etcdserver: member dir = default.etcd/member
2020-04-24 10:22:44.633366 I | etcdserver: heartbeat = 100ms
2020-04-24 10:22:44.633372 I | etcdserver: election = 1000ms
2020-04-24 10:22:44.633377 I | etcdserver: snapshot count = 100000
2020-04-24 10:22:44.633391 I | etcdserver: advertise client URLs = http://localhost:2379
2020-04-24 10:22:44.633400 I | etcdserver: initial advertise peer URLs = http://localhost:2380
2020-04-24 10:22:44.633411 I | etcdserver: initial cluster = default=http://localhost:2380
2020-04-24 10:22:44.636967 I | etcdserver: starting member 8e9e05c52164694d in cluster cdf818194e3a8c32
2020-04-24 10:22:44.637013 I | raft: 8e9e05c52164694d became follower at term 0
2020-04-24 10:22:44.637025 I | raft: newRaft 8e9e05c52164694d [peers: [], term: 0, commit: 0, applied: 0, lastindex: 0, lastterm: 0]
2020-04-24 10:22:44.637031 I | raft: 8e9e05c52164694d became follower at term 1
2020-04-24 10:22:44.640746 W | auth: simple token is not cryptographically signed
2020-04-24 10:22:44.641951 I | etcdserver: starting server... [version: 3.3.10, cluster version: to_be_decided]
2020-04-24 10:22:44.642508 I | etcdserver: 8e9e05c52164694d as single-node; fast-forwarding 9 ticks (election ticks 10)
2020-04-24 10:22:44.642868 I | etcdserver/membership: added member 8e9e05c52164694d [http://localhost:2380] to cluster cdf818194e3a8c32
2020-04-24 10:22:45.537814 I | raft: 8e9e05c52164694d is starting a new election at term 1
2020-04-24 10:22:45.537903 I | raft: 8e9e05c52164694d became candidate at term 2
2020-04-24 10:22:45.537934 I | raft: 8e9e05c52164694d received MsgVoteResp from 8e9e05c52164694d at term 2
2020-04-24 10:22:45.537951 I | raft: 8e9e05c52164694d became leader at term 2
2020-04-24 10:22:45.538000 I | raft: raft.node: 8e9e05c52164694d elected leader 8e9e05c52164694d at term 2
2020-04-24 10:22:45.539042 I | etcdserver: published {Name:default ClientURLs:[http://localhost:2379]} to cluster cdf818194e3a8c32
2020-04-24 10:22:45.539650 I | etcdserver: setting up the initial cluster version to 3.3
2020-04-24 10:22:45.540372 N | etcdserver/membership: set the initial cluster version to 3.3
2020-04-24 10:22:45.540701 I | embed: ready to serve client requests
2020-04-24 10:22:45.541113 I | etcdserver/api: enabled capabilities for version 3.3
2020-04-24 10:22:45.542200 N | embed: serving insecure client requests on 127.0.0.1:2379, this is strongly discouraged!
从上面的输出中，我们可以看到很多信息。以下是几个比较重要的信息：
2020-04-24 10:22:44.633281 I | etcdserver: name = default
name表示节点名称，默认为default。
2020-04-24 10:22:44.633323 I | etcdserver: data dir = default.etcd
data-dir 保存日志和快照的目录，默认为当前工作目录default.etcd/目录下。
etcdserver: advertise client URLs = http://localhost:2379
在http://localhost:2379提供HTTP API服务，供客户端交互。
etcdserver/membership: added member 8e9e05c52164694d [http://localhost:2380] to cluster cdf818194e3a8c32
在http://localhost:2380和集群中其他节点通信。
2020-04-24 10:22:44.633366 I | etcdserver: heartbeat = 100ms
heartbeat为100ms，该参数的作用是leader多久发送一次心跳到followers，默认值是100ms。
2020-04-24 10:22:44.633372 I | etcdserver: election = 1000ms
election为1000ms，该参数的作用是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。
2020-04-24 10:22:44.633377 I | etcdserver: snapshot count = 100000
snapshot count为10000，该参数的作用是指定有多少事务被提交时，触发截取快照保存到磁盘。
集群和每个节点都会生成一个uuid。
启动的时候会运行raft，选举出leader。
采用这种方式启动的etcd只是一个程序，如果启动etcd的窗口被关闭的话则etcd便会被关闭
，所以如果要长期使用的话最好是为etcd开启一个服务。
使用如下指令：
root@ubuntu:/opt/etcd-v3.3.10# ./etcd &
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