Operating etcd clusters for Kubernetes

etcd is a consistent and highly-available key value store used as Kubernetes' backing store for all cluster data.

If your Kubernetes cluster uses etcd as its backing store, make sure you have a back up plan for the data.

You can find in-depth information about etcd in the official documentation.

Before you begin

Before you follow steps in this page to deploy, manage, back up or restore etcd, you need to understand the typical expectations for operating an etcd cluster. Refer to the etcd documentation for more context.

Key details include:

• The minimum recommended etcd versions to run in production are 3.4.22+ and 3.5.6+.

• etcd is a leader-based distributed system. Ensure that the leader periodically send heartbeats on time to all followers to keep the cluster stable.

• You should run etcd as a cluster with an odd number of members.

• Aim to ensure that no resource starvation occurs.

  Performance and stability of the cluster is sensitive to network and disk I/O. Any resource starvation can lead to heartbeat timeout, causing instability of the cluster. An unstable etcd indicates that no leader is elected. Under such circumstances, a cluster cannot make any changes to its current state, which implies no new pods can be scheduled.

Resource requirements for etcd

Operating etcd with limited resources is suitable only for testing purposes. For deploying in production, advanced hardware configuration is required. Before deploying etcd in production, see resource requirement reference.

Keeping etcd clusters stable is critical to the stability of Kubernetes clusters. Therefore, run etcd clusters on dedicated machines or isolated environments for guaranteed resource requirements.

Tools

Depending on which specific outcome you're working on, you will need the etcdctl tool or the etcdutl tool (you may need both).

Understanding etcdctl and etcdutl

etcdctl and etcdutl are command-line tools used to interact with etcd clusters, but they serve different purposes:

• etcdctl: This is the primary command-line client for interacting with etcd over a network. It is used for day-to-day operations such as managing keys and values, administering the cluster, checking health, and more.

• etcdutl: This is an administration utility designed to operate directly on etcd data files, including migrating data between etcd versions, defragmenting the database, restoring snapshots, and validating data consistency. For network operations, etcdctl should be used.

For more information on etcdutl, you can refer to the etcd recovery documentation.

Starting etcd clusters

This section covers starting a single-node and multi-node etcd cluster.

This guide assumes that etcd is already installed.

Single-node etcd cluster

Use a single-node etcd cluster only for testing purposes.

1. Run the following:

   etcd --listen-client-urls=http://$PRIVATE_IP:2379 \
      --advertise-client-urls=http://$PRIVATE_IP:2379
   

2. Start the Kubernetes API server with the flag --etcd-servers=$PRIVATE_IP:2379.

   Make sure PRIVATE_IP is set to your etcd client IP.

Multi-node etcd cluster

For durability and high availability, run etcd as a multi-node cluster in production and back it up periodically. A five-member cluster is recommended in production. For more information, see FAQ documentation.

As you're using Kubernetes, you have the option to run etcd as a container inside one or more Pods. The kubeadm tool sets up etcd static pods by default, or you can deploy a separate cluster and instruct kubeadm to use that etcd cluster as the control plane's backing store.

You configure an etcd cluster either by static member information or by dynamic discovery. For more information on clustering, see etcd clustering documentation.

For an example, consider a five-member etcd cluster running with the following client URLs: http://$IP1:2379, http://$IP2:2379, http://$IP3:2379, http://$IP4:2379, and http://$IP5:2379. To start a Kubernetes API server:

1. Run the following:

   etcd --listen-client-urls=http://$IP1:2379,http://$IP2:2379,http://$IP3:2379,http://$IP4:2379,http://$IP5:2379 --advertise-client-urls=http://$IP1:2379,http://$IP2:2379,http://$IP3:2379,http://$IP4:2379,http://$IP5:2379
   

2. Start the Kubernetes API servers with the flag --etcd-servers=$IP1:2379,$IP2:2379,$IP3:2379,$IP4:2379,$IP5:2379.

   Make sure the IP<n> variables are set to your client IP addresses.

Multi-node etcd cluster with load balancer

To run a load balancing etcd cluster:

1. Set up an etcd cluster.
2. Configure a load balancer in front of the etcd cluster. For example, let the address of the load balancer be $LB.
3. Start Kubernetes API Servers with the flag --etcd-servers=$LB:2379.

Securing etcd clusters

Access to etcd is equivalent to root permission in the cluster so ideally only the API server should have access to it. Considering the sensitivity of the data, it is recommended to grant permission to only those nodes that require access to etcd clusters.

To secure etcd, either set up firewall rules or use the security features provided by etcd. etcd security features depend on x509 Public Key Infrastructure (PKI). To begin, establish secure communication channels by generating a key and certificate pair. For example, use key pairs peer.key and peer.cert for securing communication between etcd members, and client.key and client.cert for securing communication between etcd and its clients. See the example scripts provided by the etcd project to generate key pairs and CA files for client authentication.

Securing communication

To configure etcd with secure peer communication, specify flags --peer-key-file=peer.key and --peer-cert-file=peer.cert, and use HTTPS as the URL schema.

Similarly, to configure etcd with secure client communication, specify flags --key-file=k8sclient.key and --cert-file=k8sclient.cert, and use HTTPS as the URL schema. Here is an example on a client command that uses secure communication:

ETCDCTL_API=3 etcdctl --endpoints 10.2.0.9:2379 \
  --cert=/etc/kubernetes/pki/etcd/server.crt \
  --key=/etc/kubernetes/pki/etcd/server.key \
  --cacert=/etc/kubernetes/pki/etcd/ca.crt \
  member list

Limiting access of etcd clusters

After configuring secure communication, restrict the access of the etcd cluster to only the Kubernetes API servers using TLS authentication.

For example, consider key pairs k8sclient.key and k8sclient.cert that are trusted by the CA etcd.ca. When etcd is configured with --client-cert-auth along with TLS, it verifies the certificates from clients by using system CAs or the CA passed in by --trusted-ca-file flag. Specifying flags --client-cert-auth=true and --trusted-ca-file=etcd.ca will restrict the access to clients with the certificate k8sclient.cert.

Once etcd is configured correctly, only clients with valid certificates can access it. To give Kubernetes API servers the access, configure them with the flags --etcd-certfile=k8sclient.cert, --etcd-keyfile=k8sclient.key and --etcd-cafile=ca.cert.

Replacing a failed etcd member

etcd cluster achieves high availability by tolerating minor member failures. However, to improve the overall health of the cluster, replace failed members immediately. When multiple members fail, replace them one by one. Replacing a failed member involves two steps: removing the failed member and adding a new member.

Though etcd keeps unique member IDs internally, it is recommended to use a unique name for each member to avoid human errors. For example, consider a three-member etcd cluster. Let the URLs be, member1=http://10.0.0.1, member2=http://10.0.0.2, and member3=http://10.0.0.3. When member1 fails, replace it with member4=http://10.0.0.4.

1. Get the member ID of the failed member1:

   etcdctl --endpoints=http://10.0.0.2,http://10.0.0.3 member list
   

   The following message is displayed:

   8211f1d0f64f3269, started, member1, http://10.0.0.1:2380, http://10.0.0.1:2379
   91bc3c398fb3c146, started, member2, http://10.0.0.2:2380, http://10.0.0.2:2379
   fd422379fda50e48, started, member3, http://10.0.0.3:2380, http://10.0.0.3:2379
   

2. Do either of the following:

   1. If each Kubernetes API server is configured to communicate with all etcd members, remove the failed member from the --etcd-servers flag, then restart each Kubernetes API server.
   2. If each Kubernetes API server communicates with a single etcd member, then stop the Kubernetes API server that communicates with the failed etcd.

3. Stop the etcd server on the broken node. It is possible that other clients besides the Kubernetes API server are causing traffic to etcd and it is desirable to stop all traffic to prevent writes to the data directory.

4. Remove the failed member:

   etcdctl member remove 8211f1d0f64f3269
   

   The following message is displayed:

   Removed member 8211f1d0f64f3269 from cluster
   

5. Add the new member:

   etcdctl member add member4 --peer-urls=http://10.0.0.4:2380
   

   The following message is displayed:

   Member 2be1eb8f84b7f63e added to cluster ef37ad9dc622a7c4
   

6. Start the newly added member on a machine with the IP 10.0.0.4:

   export ETCD_NAME="member4"
   export ETCD_INITIAL_CLUSTER="member2=http://10.0.0.2:2380,member3=http://10.0.0.3:2380,member4=http://10.0.0.4:2380"
   export ETCD_INITIAL_CLUSTER_STATE=existing
   etcd [flags]
   

7. Do either of the following:

   1. If each Kubernetes API server is configured to communicate with all etcd members, add the newly added member to the --etcd-servers flag, then restart each Kubernetes API server.
   2. If each Kubernetes API server communicates with a single etcd member, start the Kubernetes API server that was stopped in step 2. Then configure Kubernetes API server clients to again route requests to the Kubernetes API server that was stopped. This can often be done by configuring a load balancer.

For more information on cluster reconfiguration, see etcd reconfiguration documentation.

Backing up an etcd cluster

All Kubernetes objects are stored in etcd. Periodically backing up the etcd cluster data is important to recover Kubernetes clusters under disaster scenarios, such as losing all control plane nodes. The snapshot file contains all the Kubernetes state and critical information. In order to keep the sensitive Kubernetes data safe, encrypt the snapshot files.

Backing up an etcd cluster can be accomplished in two ways: etcd built-in snapshot and volume snapshot.

Built-in snapshot

etcd supports built-in snapshot. A snapshot may either be created from a live member with the etcdctl snapshot save command or by copying the member/snap/db file from an etcd data directory that is not currently used by an etcd process. Creating the snapshot will not affect the performance of the member.

Below is an example for creating a snapshot of the keyspace served by $ENDPOINT to the file snapshot.db:

ETCDCTL_API=3 etcdctl --endpoints $ENDPOINT snapshot save snapshot.db

Verify the snapshot:

• Use etcdutl
• Use etcdctl (Deprecated)

etcdutl --write-out=table snapshot status snapshot.db 

+----------+----------+------------+------------+
|   HASH   | REVISION | TOTAL KEYS | TOTAL SIZE |
+----------+----------+------------+------------+
| fe01cf57 |       10 |          7 | 2.1 MB     |
+----------+----------+------------+------------+

export ETCDCTL_API=3
etcdctl --write-out=table snapshot status snapshot.db

Deprecated: Use `etcdutl snapshot status` instead.

+----------+----------+------------+------------+
|   HASH   | REVISION | TOTAL KEYS | TOTAL SIZE |
+----------+----------+------------+------------+
| fe01cf57 |       10 |          7 | 2.1 MB     |
+----------+----------+------------+------------+

Volume snapshot

If etcd is running on a storage volume that supports backup, such as Amazon Elastic Block Store, back up etcd data by creating a snapshot of the storage volume.

Snapshot using etcdctl options

We can also create the snapshot using various options given by etcdctl. For example:

ETCDCTL_API=3 etcdctl -h 

will list various options available from etcdctl. For example, you can create a snapshot by specifying the endpoint, certificates and key as shown below:

ETCDCTL_API=3 etcdctl --endpoints=https://127.0.0.1:2379 \
  --cacert=<trusted-ca-file> --cert=<cert-file> --key=<key-file> \
  snapshot save <backup-file-location>

where trusted-ca-file, cert-file and key-file can be obtained from the description of the etcd Pod.

Scaling out etcd clusters

Scaling out etcd clusters increases availability by trading off performance. Scaling does not increase cluster performance nor capability. A general rule is not to scale out or in etcd clusters. Do not configure any auto scaling groups for etcd clusters. It is strongly recommended to always run a static five-member etcd cluster for production Kubernetes clusters at any officially supported scale.

A reasonable scaling is to upgrade a three-member cluster to a five-member one, when more reliability is desired. See etcd reconfiguration documentation for information on how to add members into an existing cluster.

Restoring an etcd cluster

etcd supports restoring from snapshots that are taken from an etcd process of the major.minor version. Restoring a version from a different patch version of etcd is also supported. A restore operation is employed to recover the data of a failed cluster.

Before starting the restore operation, a snapshot file must be present. It can either be a snapshot file from a previous backup operation, or from a remaining data directory.

• Use etcdutl
• Use etcdctl (Deprecated)

etcdutl --data-dir <data-dir-location> snapshot restore snapshot.db

export ETCDCTL_API=3
etcdctl --data-dir <data-dir-location> snapshot restore snapshot.db

For more information and examples on restoring a cluster from a snapshot file, see etcd disaster recovery documentation.

If the access URLs of the restored cluster are changed from the previous cluster, the Kubernetes API server must be reconfigured accordingly. In this case, restart Kubernetes API servers with the flag --etcd-servers=$NEW_ETCD_CLUSTER instead of the flag --etcd-servers=$OLD_ETCD_CLUSTER. Replace $NEW_ETCD_CLUSTER and $OLD_ETCD_CLUSTER with the respective IP addresses. If a load balancer is used in front of an etcd cluster, you might need to update the load balancer instead.

If the majority of etcd members have permanently failed, the etcd cluster is considered failed. In this scenario, Kubernetes cannot make any changes to its current state. Although the scheduled pods might continue to run, no new pods can be scheduled. In such cases, recover the etcd cluster and potentially reconfigure Kubernetes API servers to fix the issue.

Upgrading etcd clusters

For details on etcd upgrade, refer to the etcd upgrades documentation.

Maintaining etcd clusters

For more details on etcd maintenance, please refer to the etcd maintenance documentation.

Cluster defragmentation

Defragmentation is an expensive operation, so it should be executed as infrequently as possible. On the other hand, it's also necessary to make sure any etcd member will not exceed the storage quota. The Kubernetes project recommends that when you perform defragmentation, you use a tool such as etcd-defrag.

You can also run the defragmentation tool as a Kubernetes CronJob, to make sure that defragmentation happens regularly. See etcd-defrag-cronjob.yaml for details.