etcd can now run as a transparent proxy. Running etcd as a proxy allows for easily discovery of etcd within your infrastructure, since it can run on each machine as a local service. In this mode, etcd acts as a reverse proxy and forwards client requests to an active etcd cluster. The etcd proxy does not participant in the consensus replication of the etcd cluster, thus it neither increases the resilience nor decreases the write performance of the etcd cluster.
etcd currently supports two proxy modes: readwrite and readonly. The default mode is readwrite, which forwards both read and write requests to the etcd cluster. A readonly etcd proxy only forwards read requests to the etcd cluster, and returns HTTP 501 to all write requests.
The proxy will shuffle the list of cluster members periodically to avoid sending all connections to a single member.
The member list used by proxy consists of all client URLs advertised within the cluster, as specified in each members' -advertise-client-urls flag. If this flag is set incorrectly, requests sent to the proxy are forwarded to wrong addresses and then fail. The fix for this problem is to restart etcd member with correct -advertise-client-urls flag. After client URLs list in proxy is recalculated, which happens every 30 seconds, requests will be forwarded correctly.
To start etcd in proxy mode, you need to provide three flags: proxy, listen-client-urls, and initial-cluster (or discovery).
To start a readwrite proxy, set -proxy on; To start a readonly proxy, set -proxy readonly.
The proxy will be listening on listen-client-urls and forward requests to the etcd cluster discovered from in initial-cluster or discovery url.
To start a proxy that will connect to a statically defined etcd cluster, specify the initial-cluster flag:
etcd -proxy on -listen-client-urls http://127.0.0.1:8080 -initial-cluster infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380,infra2=http://10.0.1.12:2380
If you bootstrap an etcd cluster using the discovery service, you can also start the proxy with the same discovery.
To start a proxy using the discovery service, specify the discovery flag. The proxy will wait until the etcd cluster defined at the discovery url finishes bootstrapping, and then start to forward the requests.
etcd -proxy on -listen-client-urls http://127.0.0.1:8080 -discovery https://discovery.etcd.io/3e86b59982e49066c5d813af1c2e2579cbf573de
If you bootstrap a etcd cluster using discovery service with more than the expected number of etcd members, the extra etcd processes will fall back to being readwrite proxies by default. They will forward the requests to the cluster as described above. For example, if you create a discovery url with size=5, and start ten etcd processes using that same discovery url, the result will be a cluster with five etcd members and five proxies. Note that this behaviour can be disabled with the proxy-fallback flag.