Run a Sensu cluster
To deploy Sensu for use outside of a local development environment, first decide whether you want to run a Sensu cluster.
A Sensu cluster is a group of at least three sensu-backend nodes, each connected to a shared database provided either by Sensu’s embedded etcd or an external etcd cluster. Creating a Sensu cluster ultimately configures an etcd cluster.
Clustering improves Sensu’s availability, reliability, and durability. It allows you to absorb the loss of a backend node, prevent data loss, and distribute the network load of agents. If you have a healthy clustered backend, you only need to make Sensu API calls to any one of the cluster members. The cluster protocol will replicate your changes to all cluster members.
Scaling a single backend to a cluster or migrating a cluster from cleartext HTTP to encrypted HTTPS without downtime can require a number of tedious steps. For this reason, we recommend that you decide whether your deployment will require clustering as part of your initial planning effort.
No matter whether you deploy a single backend or a clustered configuration, begin by securing Sensu with transport layer security (TLS). The first step in setting up TLS is to generate the certificates you need. Then, follow our Secure Sensu guide to make Sensu production-ready.
After you’ve secured Sensu, continue reading this document to set up and update a clustered configuration.
NOTE: We recommend using a load balancer to evenly distribute agent connections across a cluster.
Configure a cluster
The sensu-backend arguments for its store mirror the etcd configuration flags, but the Sensu flags are prefixed with etcd
.
For more detailed descriptions of the different arguments, see the etcd documentation or the Sensu backend reference.
You can configure a Sensu cluster in a couple different ways — we’ll show you a few below — but you should adhere to some etcd cluster guidelines as well:
The recommended etcd cluster size is 3, 5 or 7, which is decided by the fault tolerance requirement. A 7-member cluster can provide enough fault tolerance in most cases. While a larger cluster provides better fault tolerance, the write performance reduces since data needs to be replicated to more machines. It is recommended to have an odd number of members in a cluster. Having an odd cluster size doesn’t change the number needed for majority, but you gain a higher tolerance for failure by adding the extra member. etcd2 Admin Guide
We also recommend using stable platforms to support your etcd instances (see etcd’s supported platforms).
NOTE: If a cluster member is started before it is configured to join a cluster, the member will persist its prior configuration to disk.
For this reason, you must remove any previously started member’s etcd data by stopping sensu-backend and deleting the contents of /var/lib/sensu/sensu-backend/etcd
before proceeding with cluster configuration.
Docker
If you prefer to stand up your Sensu cluster within Docker containers, check out the Sensu Go Docker configuration. This configuration defines three sensu-backend containers and three sensu-agent containers.
Traditional computer instance
NOTE: The remainder of this guide describes on-disk configuration.
If you are using an ephemeral computer instance, you can use sensu-backend start --help
to see examples of etcd command line flags.
The configuration file entries in the rest of this guide translate to sensu-backend
flags.
Sensu backend configuration
WARNING: You must update the default configuration for Sensu’s embedded etcd with an explicit, non-default configuration to secure etcd communication in transit. If you do not properly configure secure etcd communication, your Sensu configuration will be vulnerable to unauthorized manipulation via etcd client connections.
The examples in this section are configuration snippets from /etc/sensu/backend.yml
using a three-node cluster.
The nodes are named backend-1
, backend-2
and backend-3
with IP addresses 10.0.0.1
, 10.0.0.2
and 10.0.0.3
, respectively.
NOTE: This backend configuration assumes you have set up and installed the sensu-backend on all the nodes used in your cluster. Follow the Install Sensu guide if you have not already done this.
Store configuration for backend-1/10.0.0.1
etcd-advertise-client-urls: "http://10.0.0.1:2379"
etcd-listen-client-urls: "http://10.0.0.1:2379"
etcd-listen-peer-urls: "http://0.0.0.0:2380"
etcd-initial-cluster: "backend-1=http://10.0.0.1:2380,backend-2=http://10.0.0.2:2380,backend-3=http://10.0.0.3:2380"
etcd-initial-advertise-peer-urls: "http://10.0.0.1:2380"
etcd-initial-cluster-state: "new"
etcd-initial-cluster-token: "unique_token_for_this_cluster"
etcd-name: "backend-1"
Store configuration for backend-2/10.0.0.2
etcd-advertise-client-urls: "http://10.0.0.2:2379"
etcd-listen-client-urls: "http://10.0.0.2:2379"
etcd-listen-peer-urls: "http://0.0.0.0:2380"
etcd-initial-cluster: "backend-1=http://10.0.0.1:2380,backend-2=http://10.0.0.2:2380,backend-3=http://10.0.0.3:2380"
etcd-initial-advertise-peer-urls: "http://10.0.0.2:2380"
etcd-initial-cluster-state: "new"
etcd-initial-cluster-token: "unique_token_for_this_cluster"
etcd-name: "backend-2"
Store configuration for backend-3/10.0.0.3
etcd-advertise-client-urls: "http://10.0.0.3:2379"
etcd-listen-client-urls: "http://10.0.0.3:2379"
etcd-listen-peer-urls: "http://0.0.0.0:2380"
etcd-initial-cluster: "backend-1=http://10.0.0.1:2380,backend-2=http://10.0.0.2:2380,backend-3=http://10.0.0.3:2380"
etcd-initial-advertise-peer-urls: "http://10.0.0.3:2380"
etcd-initial-cluster-state: "new"
etcd-initial-cluster-token: "unique_token_for_this_cluster"
etcd-name: "backend-3"
IMPORTANT: To properly secure etcd communication, replace the default URLs for etcd-advertise-client-urls
, etcd-listen-client-urls
, etcd-listen-peer-urls
, and etcd-initial-cluster
in the store configurations for your backends with non-default values.
Specify the same etcd-initial-cluster-token
value for all three backends.
This allows etcd to generate unique cluster IDs and member IDs even for clusters that have otherwise identical configurations and prevents cross-cluster-interaction.
After you configure each node as described in these examples, start each sensu-backend:
sudo systemctl start sensu-backend
Add Sensu agents to clusters
Each Sensu agent should have the following entries in /etc/sensu/agent.yml
to ensure the agent is aware of all cluster members.
This allows the agent to reconnect to a working backend if the backend it is currently connected to goes into an unhealthy state.
Here is an exmaple backend-url configuration for all agents connecting to the cluster over WebSocket:
backend-url:
- "ws://10.0.0.1:8081"
- "ws://10.0.0.2:8081"
- "ws://10.0.0.3:8081"
You should now have a highly available Sensu cluster! Confirm cluster health and try other cluster management commands with sensuctl.
Manage and monitor clusters with sensuctl
Sensuctl includes several commands to help you manage and monitor your cluster.
Run sensuctl cluster -h
for additional help information.
Get cluster health status
Get cluster health status and etcd alarm information:
sensuctl cluster health
The cluster health response will list the health status for each cluster member, similar to this example:
ID Name Error Healthy
────────────────── ─────────── ─────────────────────────────────────────────────── ─────────
a32e8f613b529ad4 backend-1 true
c3d9f4b8d0dd1ac9 backend-2 dial tcp 10.0.0.2:2379: connect: connection refused false
c8f63ae435a5e6bf backend-3 true
Add a cluster member
To add a new member node to an existing cluster:
-
Configure the new node’s store in its
/etc/sensu/backend.yml
configuration file. For the new nodebackend-4
with IP address10.0.0.4
:etcd-advertise-client-urls: "http://10.0.0.4:2379" etcd-listen-client-urls: "http://10.0.0.4:2379" etcd-listen-peer-urls: "http://0.0.0.0:2380" etcd-initial-cluster: "backend-1=http://10.0.0.1:2380,backend-2=http://10.0.0.2:2380,backend-3=http://10.0.0.3:2380,backend-4=https://10.0.0.4:2380" etcd-initial-advertise-peer-urls: "http://10.0.0.4:2380" etcd-initial-cluster-state: "existing" etcd-initial-cluster-token: "unique_token_for_this_cluster" etcd-name: "backend-4"
NOTE: To make sure the new member is added to the correct cluster, specify the same
etcd-initial-cluster-token
value that you used for the other members in the cluster. -
Run the sensuctl command to add the new cluster member:
sensuctl cluster member-add backend-4 https://10.0.0.4:2380
You will receive a sensuctl response to confirm that the new member was added:
added member 2f7ae42c315f8c2d to cluster
-
Start the new backend:
sudo systemctl start sensu-backend
-
Add the new cluster member’s WebSocket backend-url in
/etc/sensu/agent.yml
for all agents that connect to the cluster over WebSocket:backend-url: - "ws://10.0.0.1:8081" - "ws://10.0.0.2:8081" - "ws://10.0.0.3:8081" - "ws://10.0.0.4:8081"
List cluster members
List the ID, name, peer URLs, and client URLs of all nodes in a cluster:
sensuctl cluster member-list
You will receive a sensuctl response that lists all cluster members:
ID Name Peer URLs Client URLs
────────────────── ─────────── ───────────────────────── ─────────────────────────
a32e8f613b529ad4 backend-1 https://10.0.0.1:2380 https://10.0.0.1:2379
c3d9f4b8d0dd1ac9 backend-2 https://10.0.0.2:2380 https://10.0.0.2:2379
c8f63ae435a5e6bf backend-3 https://10.0.0.3:2380 https://10.0.0.3:2379
2f7ae42c315f8c2d backend-4 https://10.0.0.4:2380 https://10.0.0.4:2379
Remove a cluster member
Remove a faulty or decommissioned member node from a cluster:
sensuctl cluster member-remove 2f7ae42c315f8c2d
You will receive a sensuctl response to confirm that the cluster member was removed:
Removed member 2f7ae42c315f8c2d from cluster
Replace a faulty cluster member
To replace a faulty cluster member to restore a cluster’s health, start by running sensuctl cluster health
to identify the faulty cluster member.
For a faulty cluster member, the Error
column will include an error message and the Healthy
column will list false
.
In this example, the response indicates that cluster member backend-4
is faulty:
ID Name Error Healthy
────────────────── ─────────── ─────────────────────────────────────────────────── ─────────
a32e8f613b529ad4 backend-1 true
c3d9f4b8d0dd1ac9 backend-2 true
c8f63ae435a5e6bf backend-3 true
2f7ae42c315f8c2d backend-4 dial tcp 10.0.0.4:2379: connect: connection refused false
Then, delete the faulty cluster member.
To continue this example, you will delete cluster member backend-4
using its ID:
sensuctl cluster member-remove 2f7ae42c315f8c2d
The response should indicate that the cluster member was removed:
Removed member 2f7ae42c315f8c2d from cluster
Finally, add a newly created member to the cluster.
You can use the same name and IP address as the faulty member you deleted, with one change to the configuration: specify the etcd-initial-cluster-state
as existing
.
etcd-advertise-client-urls: "http://10.0.0.4:2379"
etcd-listen-client-urls: "http://10.0.0.4:2379"
etcd-listen-peer-urls: "http://0.0.0.0:2380"
etcd-initial-cluster: "backend-1=http://10.0.0.1:2380,backend-2=http://10.0.0.2:2380,backend-3=http://10.0.0.3:2380,backend-4=http://10.0.0.4:2380"
etcd-initial-advertise-peer-urls: "http://10.0.0.4:2380"
etcd-initial-cluster-state: "existing"
etcd-initial-cluster-token: ""
etcd-name: "backend-4"
If replacing the faulty cluster member does not resolve the problem, see the etcd operations guide for more information.
Update a cluster member
Update the peer URLs of a member in a cluster:
sensuctl cluster member-update c8f63ae435a5e6bf https://10.0.0.4:2380
You will receive a sensuctl response to confirm that the cluster member was updated:
Updated member with ID c8f63ae435a5e6bf in cluster
Cluster security
See Secure Sensu for information about cluster security.
Use an external etcd cluster
WARNING: You must update the example configuration for external etcd with an explicit, non-default configuration to secure etcd communication in transit. If you do not properly configure secure etcd communication, your Sensu configuration will be vulnerable to unauthorized manipulation via etcd client connections.
To use Sensu with an external etcd cluster, you must have etcd 3.3.2 or newer.
To stand up an external etcd cluster, follow etcd’s clustering guide using the same store configuration.
Do not configure external etcd in Sensu via backend command line flags or the backend configuration file (/etc/sensu/backend.yml
).
In this example, you will enable client-to-server and peer communication authentication using self-signed TLS certificates.
To start etcd for backend-1
based on the three-node configuration example:
etcd \
--listen-client-urls "https://10.0.0.1:2379" \
--advertise-client-urls "https://10.0.0.1:2379" \
--listen-peer-urls "https://10.0.0.1:2380" \
--initial-cluster "backend-1=https://10.0.0.1:2380,backend-2=https://10.0.0.2:2380,backend-3=https://10.0.0.3:2380" \
--initial-advertise-peer-urls "https://10.0.0.1:2380" \
--initial-cluster-state "new" \
--name "backend-1" \
--trusted-ca-file=./ca.pem \
--cert-file=./backend-1.pem \
--key-file=./backend-1-key.pem \
--client-cert-auth \
--peer-trusted-ca-file=./ca.pem \
--peer-cert-file=./backend-1.pem \
--peer-key-file=./backend-1-key.pem \
--peer-client-cert-auth \
--auto-compaction-mode revision \
--auto-compaction-retention 2
NOTE: The auto-compaction-mode
and auto-compaction-retention
flags are important.
Without these settings, your database may quickly reach etcd’s maximum database size limit.
To tell Sensu to use this external etcd data source, add the sensu-backend
flag --no-embed-etcd
to the original configuration, along with the path to a client certificate created using your CA:
sensu-backend start \
--etcd-trusted-ca-file=./ca.pem \
--etcd-cert-file=./client.pem \
--etcd-key-file=./client-key.pem \
--etcd-client-urls=https://10.0.0.1:2379 https://10.0.0.2:2379 https://10.0.0.3:2379 \
--no-embed-etcd
NOTE: The etcd-client-urls
value must be a space-delimited list or a YAML array.
Troubleshoot clusters
Failure modes
See the etcd failure modes documentation for information about cluster failure modes.
Disaster recovery
See the etcd recovery guide for disaster recovery information.