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Proxy

etcd can now run as a transparent proxy. Running etcd as a proxy allows for easy 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 participate 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. Including URLs in the -advertise-client-urls flag that point to the proxy itself, e.g. http://localhost:2379, is even more problematic as it will cause loops, because the proxy keeps trying to forward requests to itself until its resources (memory, file descriptors) are eventually depleted. 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.

Using an etcd proxy

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.

Start an etcd proxy with a static configuration

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

Start an etcd proxy with the discovery service

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

Fallback to proxy mode with discovery service

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.

Promote a proxy to a member of etcd cluster

A Proxy is in the part of etcd cluster that does not participate in consensus. A proxy will not promote itself to an etcd member that participates in consensus automtically in any case.

If you want to promote a proxy to an etcd member, there are four steps you need to follow:

use etcdctl to add the proxy node as an etcd member into the existing cluster
stop the etcd proxy process or service
remove the existing proxy data directory
restart the etcd process with new member configuration

Example

We assume you have a one member etcd cluster with one proxy. The cluster information is listed below:

Name	Address
infra0	10.0.1.10
proxy0	10.0.1.11

This example walks you through a case that you promote one proxy to an etcd member. The cluster will become a two member cluster after finishing the four steps.

Add a new member into the existing cluster

First, use etcdctl to add the member to the cluster, which will output the environment variables need to correctly configure the new member:

$ etcdctl -endpoint http://10.0.1.10:2379 member add infra1 http://10.0.1.11:2380
added member 9bf1b35fc7761a23 to cluster

ETCD_NAME="infra1"
ETCD_INITIAL_CLUSTER="infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380"
ETCD_INITIAL_CLUSTER_STATE=existing

Stop the proxy process

Stop the existing proxy so we can wipe it's state on disk and reload it with the new configuration:

px aux | grep etcd
kill %etcd_proxy_pid%

or (if you are running etcd proxy as etcd service under systemd)

sudo systemctl stop etcd

Remove the existing proxy data dir

rm -rf %data_dir%/proxy

Start etcd as a new member

Finally, start the reconfigured member and make sure it joins the cluster correctly:

$ export ETCD_NAME="infra1"
$ export ETCD_INITIAL_CLUSTER="infra0=http://10.0.1.10:2380,infra1=http://10.0.1.11:2380"
$ export ETCD_INITIAL_CLUSTER_STATE=existing
$ etcd -listen-client-urls http://10.0.1.11:2379 \
-advertise-client-urls http://10.0.1.11:2379 \
-listen-peer-urls http://10.0.1.11:2380 \
-initial-advertise-peer-urls http://10.0.1.11:2380 \
-data-dir %data_dir%

If you are running etcd under systemd, you should modify the service file with correct configuration and restart the service:

sudo systemd restart etcd

If you see an error, you can read the add member troubleshooting doc.

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