What is etcd?
- Docker & Kubernetes course
- Expert
- Lesson 1/47
- 4 min
Learn that etcd stores cluster state.
What is etcd?
etcd is a distributed, consistent key-value database holding the whole state of the cluster (the desired and actual state of every object). It ensures consistency with the Raft consensus algorithm: a write becomes final when a majority (quorum) of members confirm it. That's why etcd is run with an odd number of members (3, 5).
3 members -> quorum 2 -> survives the loss of 1 member
5 members -> quorum 3 -> survives the loss of 2 members
4 members -> quorum 3 -> still survives the loss of only 1Only kube-apiserver connects to etcd directly; all other components talk through the API server.
Looking inside etcd
Kubernetes objects are stored in etcd under the keys /registry/<type>/<namespace>/<name>. They can be inspected with etcdctl (on a control-plane node, with certificates):
ETCDCTL_API=3 etcdctl --endpoints=https://127.0.0.1:2379 \
--cacert=/etc/kubernetes/pki/etcd/ca.crt \
--cert=/etc/kubernetes/pki/etcd/server.crt \
--key=/etc/kubernetes/pki/etcd/server.key \
get /registry/deployments/default --prefix --keys-only
/registry/deployments/default/web
/registry/deployments/default/apiValues are encoded with Protobuf; Secrets are unreadable only if encryption has been configured.
Practice etcd health
Write two commands (with the certificate flags from the card, each on one line): query the health with etcdctl ... endpoint health; list the members with etcdctl ... member list -w table. Flags: --endpoints=https://127.0.0.1:2379, --cacert=/etc/kubernetes/pki/etcd/ca.crt, --cert=/etc/kubernetes/pki/etcd/server.crt, --key=/etc/kubernetes/pki/etcd/server.key.
Starter code
# buraya yazSample solution
etcdctl --endpoints=https://127.0.0.1:2379 --cacert=/etc/kubernetes/pki/etcd/ca.crt --cert=/etc/kubernetes/pki/etcd/server.crt --key=/etc/kubernetes/pki/etcd/server.key endpoint health
etcdctl --endpoints=https://127.0.0.1:2379 --cacert=/etc/kubernetes/pki/etcd/ca.crt --cert=/etc/kubernetes/pki/etcd/server.crt --key=/etc/kubernetes/pki/etcd/server.key member list -w tableExpected output
https://127.0.0.1:2379 is healthy: successfully committed proposal: took = 4.2ms +------------------+---------+-------+---------------------------+ | ID | STATUS | NAME | PEER ADDRS | +------------------+---------+-------+---------------------------+ | 8e9e05c52164694d | started | cp-1 | https://192.168.49.2:2380 | +------------------+---------+-------+---------------------------+
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What happens if 2 members are lost in a 3-member etcd cluster?
- Quorum is lost; the cluster accepts no writes Correct answer
- It keeps working normally
- The remaining member becomes leader and writes
- The data is deleted
Explanation: Existing workloads keep running but no changes can be made.
2 List only the keys.
etcdctl get /registry/pods/default --prefix ____etcdctl get /registry/pods/default --prefix --keys-onlyExplanation: --keys-only doesn't print the values.
3 Match.
| Term | Meaning |
|---|---|
etcd |
Cluster state |
Raft |
Consensus algorithm |
quorum |
Majority |
/registry/ |
Key prefix |