Application containers running on worker nodes with etcd in proxy mode can read and write to an etcd cluster.
Very common etcd use cases are as follows: storing database connection settings, cache settings, and shared settings. For example, the Vulcand proxy server (http://vulcanproxy.com/) uses etcd to store web host connection details, and it becomes available for all cluster-connected worker machines. Another example could be to store a database password for MySQL and retrieve it when running an application container.
We will cover more details about cluster setup, central services, and worker role machines in the upcoming chapters.