1. 简介
ETCD 是一个高可用的分布式键值数据库，可用于服务发现。ETCD 采用 raft 一致性算法，基于 Go 语言实现。
特点
简单：安装配置使用简单，提供 HTTP API
安全：支持 SSL 证书
可靠：采用 raft 算法，实现分布式系统数据的可用性和一致性
2. 用法
通过 etcdctl 可以使用 ETCD。假设使用 v3 版本，且集群设置如下：
export ETCDCTL_API=3
HOST_1=10.240.0.17
HOST_2=10.240.0.18
HOST_3=10.240.0.19
ENDPOINTS=$HOST_1:2379,$HOST_2:2379,$HOST_3:2379
etcdctl --endpoints=$ENDPOINTS member list版本设置
ETCD v2 和 v3 两个版本不兼容，且 v2 已经停止支持。如果一个系统上同时按照了两个版本，需要通过环境变量来切换：
export ETCDCTL_API=3读写数据
put 命令写数据：
etcdctl --endpoints=$ENDPOINTS put foo "Hello World!"get 命令读数据：
etcdctl --endpoints=$ENDPOINTS get foo
etcdctl --endpoints=$ENDPOINTS --write-out="json" get foo通过 --write-out="json" 选项读数据时，会读包括元数据在内的所有数据，并以 JSON 格式输出。
通过 prefix 选项读数据
etcdctl --endpoints=$ENDPOINTS put web1 value1
etcdctl --endpoints=$ENDPOINTS put web2 value2
etcdctl --endpoints=$ENDPOINTS put web3 value3
etcdctl --endpoints=$ENDPOINTS get web --prefix删除数据
etcdctl --endpoints=$ENDPOINTS put key myvalue
etcdctl --endpoints=$ENDPOINTS del key
etcdctl --endpoints=$ENDPOINTS put k1 value1
etcdctl --endpoints=$ENDPOINTS put k2 value2
etcdctl --endpoints=$ENDPOINTS del k --prefix事务性写入 txn
通过 txn 可以将多个请求放入一个事务：
etcdctl --endpoints=$ENDPOINTS put user1 bad
etcdctl --endpoints=$ENDPOINTS txn --interactive
compares:
value("user1") = "bad"
success requests (get, put, delete):
del user1
failure requests (get, put, delete):
put user1 good监控数据变化 watch
通过 watch 选项，可以在将来发生改变时获得通知：
etcdctl --endpoints=$ENDPOINTS watch stock1
etcdctl --endpoints=$ENDPOINTS put stock1 1000
etcdctl --endpoints=$ENDPOINTS watch stock --prefix
etcdctl --endpoints=$ENDPOINTS put stock1 10
etcdctl --endpoints=$ENDPOINTS put stock2 20安全写入 lease
lease 选项可以通过 TTL 写入数据：
etcdctl --endpoints=$ENDPOINTS lease grant 300
# lease 2be7547fbc6a5afa granted with TTL(300s)
etcdctl --endpoints=$ENDPOINTS put sample value --lease=2be7547fbc6a5afa
etcdctl --endpoints=$ENDPOINTS get sample
etcdctl --endpoints=$ENDPOINTS lease keep-alive 2be7547fbc6a5afa
etcdctl --endpoints=$ENDPOINTS lease revoke 2be7547fbc6a5afa
# or after 300 seconds
etcdctl --endpoints=$ENDPOINTS get sample分布式锁 lock
etcdctl --endpoints=$ENDPOINTS lock mutex1
# another client with the same name blocks
etcdctl --endpoints=$ENDPOINTS lock mutex1集群状态
为每台机器指定初始群集配置：
etcdctl --write-out=table --endpoints=$ENDPOINTS endpoint status
+------------------+------------------+---------+---------+-----------+-----------+------------+
| ENDPOINT | ID | VERSION | DB SIZE | IS LEADER | RAFT TERM | RAFT INDEX |
+------------------+------------------+---------+---------+-----------+-----------+------------+
| 10.240.0.17:2379 | 4917a7ab173fabe7 | 3.0.0 | 45 kB | true | 4 | 16726 |
| 10.240.0.18:2379 | 59796ba9cd1bcd72 | 3.0.0 | 45 kB | false | 4 | 16726 |
| 10.240.0.19:2379 | 94df724b66343e6c | 3.0.0 | 45 kB | false | 4 | 16726 |
+------------------+------------------+---------+---------+-----------+-----------+------------+
etcdctl --endpoints=$ENDPOINTS endpoint health
10.240.0.17:2379 is healthy: successfully committed proposal: took = 3.345431ms
10.240.0.19:2379 is healthy: successfully committed proposal: took = 3.767967ms
10.240.0.18:2379 is healthy: successfully committed proposal: took = 4.025451msMember
添加，删除，更新成员。