最近在搭建kubernetes on Mesos，用到了etcd。etcd是一个分布式的，可靠的，key-value数据库。其项目主页在这里。
etcd的搭建很简单，有现成的binary可以使用。不过让我感到与其它项目不同的是，它会为某项服务同时启动两个端口。比如：
-listen-peer-urls
List of URLs to listen on for peer traffic. This flag tells the etcd to accept incoming requests from its peers on the specified scheme://IP:port combinations. Scheme can be either http or https.If 0.0.0.0 is specified as the IP, etcd listens to the given port on all interfaces. If an IP address is given as well as a port, etcd will listen on the given port and interface. Multiple URLs may be used to specify a number of addresses and ports to listen on. The etcd will respond to requests from any of the listed addresses and ports.
default: “http://localhost:2380,http://localhost:7001”
env variable: ETCDLISTENPEER_URLS
example: “http://10.0.0.1:2380”
invalid example: “http://example.com:2380” (domain name is invalid for binding)-listen-client-urls
List of URLs to listen on for client traffic. This flag tells the etcd to accept incoming requests from the clients on the specified scheme://IP:port combinations. Scheme can be either http or https. If 0.0.0.0 is specified as the IP, etcd listens to the given port on all interfaces. If an IP address is given as well as a port, etcd will listen on the given port and interface. Multiple URLs may be used to specify a number of addresses and ports to listen on. The etcd will respond to requests from any of the listed addresses and ports.
default: “http://localhost:2379,http://localhost:4001”
env variable: ETCDLISTENCLIENT_URLS
example: “http://10.0.0.1:2379”
invalid example: “http://example.com:2379” (domain name is invalid for binding)
-listen-peer-urls监听peer的请求，默认端口为2380和7001，而-listen-client-urls监听client的请求，默认端口为2379和4001。
另外一个有趣的配置项是-advertise-client-urls，它是把当前运行的etcd的client URL地址通知给cluster中的其它成员。
-advertise-client-urls
List of this member’s client URLs to advertise to the rest of the cluster. These URLs can contain domain names. default: “http://localhost:2379,http://localhost:4001”
env variable: ETCDADVERTISECLIENT_URLS
example: “http://example.com:2379, http://10.0.0.1:2379”
Be careful if you are advertising URLs such as http://localhost:2379 from a cluster member and are using the proxy feature of etcd. This will cause loops, because the proxy will be forwarding requests to itself until its resources (memory, file descriptors) are eventually depleted.
参考资料：
Configuration Flags。