etcd is a strong, consistent, and highly-available key value store which Kubernetes uses for persistent storage of all of its API objects. This documentation provides specific instruction on operating, upgrading, and rolling back etcd clusters for Kubernetes. For in-depth information on etcd, see [etcd documentation](https://github.com/coreos/etcd/blob/master/Documentation/docs.md).
Performance and stability of the cluster is sensitive to network and disk IO. 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 stable etcd clusters is critical to the stability of Kubernetes clusters. Therefore, run etcd clusters on dedicated machines or isolated environments for [guaranteed resource requirements](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/hardware.md#hardware-recommendations).
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 documentation](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/hardware.md#example-hardware-configurations).
## Starting Kubernetes API server
This section covers starting a Kubernetes API server with an etcd cluster in the deployment.
### Single-node etcd cluster
Use a single-node etcd cluster only for testing purpose.
2. Start Kubernetes API server with the flag `--etcd-servers=$PRIVATE_IP:2379`.
Replace `PRIVATE_IP` with your etcd client IP.
### Multi-node etcd cluster
For durability and high availability, run etcd as a multi-node cluster in production and back it up periodically. A five-member cluster is recommended in production. For more information, see [FAQ Documentation](https://github.com/coreos/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance).
Configure an etcd cluster either by static member information or by dynamic discovery. For more information on clustering, see [etcd Clustering Documentation](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/clustering.md).
For an example, consider a five-member etcd cluster running with the following client URLs: `http://$IP1:2379`, `http://$IP2:2379`, `http://$IP3:2379`, `http://$IP4:2379`, and `http://$IP5:2379`. To start a Kubernetes API server:
2. Start Kubernetes API servers with the flag `--etcd-servers=$IP1:2379, $IP2:2379, $IP3:2379, $IP4:2379, $IP5:2379`.
Replace `IP` with your client IP addresses.
### Multi-node etcd cluster with load balancer
To run a load balancing etcd cluster:
1. Set up an etcd cluster.
2. Configure a load balancer in front of the etcd cluster.
For example, let the address of the load balancer be `$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 URL schema.
Similarly, to configure etcd with secure client communication, specify flags `--key-file=peer.key` and `--cert-file=peer.cert`, and use https as URL schema.
### Limiting access of etcd clusters
After configuring secure communication, restrict the access of etcd cluster to only the Kubernetes API server. 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 `--trust-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 server the access, configure it with the flags `--etcd-certfile=k8sclient.cert` and `--etcd-keyfile=k8sclient.key`.
**Note**: etcd authentication is not currently supported by Kubernetes. For more information, see see the related issue [Support Basic Auth for Etcd v2](https://github.com/kubernetes/kubernetes/issues/23398).
## 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. Update its `--etcd-servers` flag to make Kubernetes aware of the configuration changes, then restart the Kubernetes API server.
2. Update the load balancer configuration if a load balancer is used in the deployment.
For more information on cluster reconfiguration, see [etcd Reconfiguration Documentation](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/runtime-configuration.md#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 master 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, so backing up an etcd cluster is easy. 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://github.com/coreos/etcd/blob/master/Documentation/op-guide/configuration.md#--data-dir) that is not currently used by an etcd process. `datadir` is located at `$DATA_DIR/member/snap/db`. Taking the snapshot will normally 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`:
```sh
ETCDCTL_API=3 etcdctl --endpoints $ENDPOINT snapshot save snapshotdb
# exit 0
# verify the snapshot
ETCDCTL_API=3 etcdctl --write-out=table snapshot status snapshotdb
+----------+----------+------------+------------+
| HASH | REVISION | TOTAL KEYS | TOTAL SIZE |
+----------+----------+------------+------------+
| fe01cf57 | 10 | 7 | 2.1 MB |
+----------+----------+------------+------------+
```
### Volume snapshot
If etcd is running on a storage volume that supports backup, such as Amazon Elastic Block Store, back up etcd data by taking a snapshot of the storage volume.
## Scaling up etcd clusters
Scaling up etcd clusters increases availability by trading off performance. Scaling does not increase cluster performance nor capability. A general rule is not to scale up or down etcd clusters. Do not configure any auto scaling groups for etcd clusters. It is highly recommended to always run a static five-member etcd cluster for production Kubernetes clusters at any officially supported scale.
A reasonable scaling is to upgrade a three-member cluster to a five-member one, when more reliability is desired. See [etcd Reconfiguration Documentation](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/runtime-configuration.md#remove-a-member) for information on how to add members into an existing cluster.
## Restoring an etcd cluster
etcd supports restoring from snapshots that are taken from an etcd process of the [major.minor](http://semver.org/) version. Restoring a version from a different patch version of etcd also is supported. A restore operation is employed to recover the data of a failed cluster.
Before starting the restore operation, a snapshot file must be present. It can either be a snapshot file from a previous backup operation, or from a remaining [data directory](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/configuration.md#--data-dir). `datadir` is located at `$DATA_DIR/member/snap/db`. For more information and examples on restoring a cluster from a snapshot file, see [etcd disaster recovery documentation](https://github.com/coreos/etcd/blob/master/Documentation/op-guide/recovery.md#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 server 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 server to fix the issue.
## Upgrading and rolling back etcd clusters
### Important assumptions
The upgrade procedure described in this document assumes that either:
1. The etcd cluster has only a single node
2. The etcd cluster has multiple nodes.
In this case, the upgrade procedure requires shutting down the
etcd cluster. During the time the etcd cluster is shut down, the Kubernetes API Server will be read only.
**Warning**: Deviations from the assumptions are untested by continuous
integration, and deviations might create undesirable consequences. Additional information about operating an etcd cluster is available [from the etcd maintainers](https://github.com/coreos/etcd/tree/master/Documentation).
