--- reviewers: - mml - wojtek-t - jpbetz title: Operating etcd clusters for Kubernetes content_type: task weight: 270 --- {{< glossary_definition term_id="etcd" length="all" prepend="etcd is a ">}} ## {{% heading "prerequisites" %}} {{< include "task-tutorial-prereqs.md" >}} {{< version-check >}} ## Prerequisites * Run etcd as a cluster of odd members. * etcd is a leader-based distributed system. Ensure that the leader periodically send heartbeats on time to all followers to keep the cluster stable. * 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. * 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](https://etcd.io/docs/current/op-guide/hardware/). * The minimum recommended etcd versions to run in production are `3.4.22+` and `3.5.6+`. ## Resource requirements 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](https://etcd.io/docs/current/op-guide/hardware/#example-hardware-configurations). ## Starting etcd clusters This section covers starting a single-node and multi-node etcd cluster. ### Single-node etcd cluster Use a single-node etcd cluster only for testing purpose. 1. Run the following: ```sh 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=$PRIVATE_IP:2379`. Make sure `PRIVATE_IP` is set to your etcd client IP. ### Multi-node etcd cluster For durability and high availability, run etcd as a multi-node cluster in production and back it up periodically. A five-member cluster is recommended in production. For more information, see [FAQ documentation](https://etcd.io/docs/current/faq/#what-is-failure-tolerance). Configure an etcd cluster either by static member information or by dynamic discovery. For more information on clustering, see [etcd clustering documentation](https://etcd.io/docs/current/op-guide/clustering/). For an example, consider a five-member etcd cluster running with the following client URLs: `http://$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: ```shell 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,$IP5:2379`. Make sure the `IP` variables are set to your client IP addresses. ### Multi-node etcd cluster with load balancer To run a load balancing etcd cluster: 1. Set up an etcd cluster. 2. Configure a load balancer in front of the etcd cluster. For example, let the address of the load balancer be `$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](https://github.com/coreos/etcd/tree/master/hack/tls-setup) 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 etcd cluster to only the Kubernetes API servers. Use TLS authentication to do so. 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`. {{< note >}} etcd authentication is not currently supported by Kubernetes. For more information, see the related issue [Support Basic Auth for Etcd v2](https://github.com/kubernetes/kubernetes/issues/23398). {{< /note >}} ## 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`: ```shell etcdctl --endpoints=http://10.0.0.2,http://10.0.0.3 member list ``` The following message is displayed: ```console 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 ``` 1. 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. 1. If each Kubernetes API server communicates with a single etcd member, then stop the Kubernetes API server that communicates with the failed etcd. 1. Stop the etcd server on the broken node. It is possible that other clients besides the Kubernetes API server is causing traffic to etcd and it is desirable to stop all traffic to prevent writes to the data dir. 1. Remove the failed member: ```shell etcdctl member remove 8211f1d0f64f3269 ``` The following message is displayed: ```console Removed member 8211f1d0f64f3269 from cluster ``` 1. Add the new member: ```shell etcdctl member add member4 --peer-urls=http://10.0.0.4:2380 ``` The following message is displayed: ```console Member 2be1eb8f84b7f63e added to cluster ef37ad9dc622a7c4 ``` 1. Start the newly added member on a machine with the IP `10.0.0.4`: ```shell 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] ``` 1. 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. 1. 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](https://etcd.io/docs/current/op-guide/runtime-configuration/#remove-a-member). ## Backing up an etcd cluster All Kubernetes objects are stored on 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 states 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 taken from a live member with the `etcdctl snapshot save` command or by copying the `member/snap/db` file from an etcd [data directory](https://etcd.io/docs/current/op-guide/configuration/#--data-dir) that is not currently used by an etcd process. Taking the snapshot will not affect the performance of the member. Below is an example for taking a snapshot of the keyspace served by `$ENDPOINT` to the file `snapshotdb`: ```shell ETCDCTL_API=3 etcdctl --endpoints $ENDPOINT snapshot save snapshotdb ``` Verify the snapshot: ```shell ETCDCTL_API=3 etcdctl --write-out=table snapshot status snapshotdb ``` ```console +----------+----------+------------+------------+ | 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 taking a snapshot of the storage volume. ### Snapshot using etcdctl options We can also take the snapshot using various options given by etcdctl. For example ```shell ETCDCTL_API=3 etcdctl -h ``` will list various options available from etcdctl. For example, you can take a snapshot by specifying the endpoint, certificates etc as shown below: ```shell ETCDCTL_API=3 etcdctl --endpoints=https://127.0.0.1:2379 \ --cacert= --cert= --key= \ snapshot save ``` 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 highly 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](https://etcd.io/docs/current/op-guide/runtime-configuration/#remove-a-member) 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](http://semver.org/) version. Restoring a version from a different patch version of etcd also is 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](https://etcd.io/docs/current/op-guide/configuration/#--data-dir). Here is an example: ```shell ETCDCTL_API=3 etcdctl --endpoints 10.2.0.9:2379 snapshot restore snapshotdb ``` Another example for restoring using etcdctl options: ```shell ETCDCTL_API=3 etcdctl snapshot restore --data-dir snapshotdb ``` Yet another example would be to first export the environment variable ```shell export ETCDCTL_API=3 etcdctl snapshot restore --data-dir snapshotdb ``` For more information and examples on restoring a cluster from a snapshot file, see [etcd disaster recovery documentation](https://etcd.io/docs/current/op-guide/recovery/#restoring-a-cluster). If the access URLs of the restored cluster is 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. {{< note >}} If any API servers are running in your cluster, you should not attempt to restore instances of etcd. Instead, follow these steps to restore etcd: - stop *all* API server instances - restore state in all etcd instances - restart all API server instances We also recommend restarting any components (e.g. `kube-scheduler`, `kube-controller-manager`, `kubelet`) to ensure that they don't rely on some stale data. Note that in practice, the restore takes a bit of time. During the restoration, critical components will lose leader lock and restart themselves. {{< /note >}} ## Upgrading etcd clusters For more details on etcd upgrade, please refer to the [etcd upgrades](https://etcd.io/docs/latest/upgrades/) documentation. {{< note >}} Before you start an upgrade, please back up your etcd cluster first. {{< /note >}} ## Maintaining etcd clusters Fore more details on etcd maintenance, please refer to the [etcd maintenance](https://etcd.io/docs/latest/op-guide/maintenance/) documentation. {{% thirdparty-content single="true" %}} {{< note >}} Defragmentation is an expensive operation, so it should be executed as infrequent as possible. On the other hand, it's also necessary to make sure any etcd member will not run out of the storage quota. The Kubernetes project recommends that when you perform defragmentation, you use a tool such as [etcd-defrag](https://github.com/ahrtr/etcd-defrag). {{< /note >}}