Send Slack alerts with handlers

Sensu event handlers are actions the Sensu backend executes on events. You can use handlers to send an email alert, create or resolve incidents (in PagerDuty, for example), or store metrics in a time-series database like InfluxDB.

This guide will help you send alerts to Slack in the channel monitoring by configuring a handler named slack to a check named check_cpu. If you don’t already have this check in place, follow Monitor server resources to add it.

Before you start, follow the RHEL/CentOS install instructions to install and configure the Sensu backend, the Sensu agent, and sensuctl.

Configure a Sensu entity

Every Sensu agent has a defined set of subscriptions that determine which checks the agent will execute. For an agent to execute a specific check, you must specify the same subscription in the agent configuration and the check definition. To run the check_cpu check, you’ll need a Sensu entity with the subscription system.

First, find your entity name:

sensuctl entity list

The ID in the response is the name of your entity.

Replace <entity_name> with the name of your entity in the sensuctl command below. Then run the command to add the system subscription to your entity:

sensuctl entity update <entity_name>
  • For Entity Class, press enter.
  • For Subscriptions, type system and press enter.

Confirm both Sensu services are running:

systemctl status sensu-backend && systemctl status sensu-agent

The response should indicate active (running) for both the Sensu backend and agent.

Register the dynamic runtime asset

Dynamic runtime assets are shareable, reusable packages that help you deploy Sensu plugins. In this guide, you’ll use the Sensu Slack Handler dynamic runtime asset to power a slack handler.

Use sensuctl asset add to register the Sensu Slack Handler dynamic runtime asset:

sensuctl asset add sensu/sensu-slack-handler:1.0.3 -r sensu-slack-handler

The response will indicate that the asset was added:

fetching bonsai asset: sensu/sensu-slack-handler:1.0.3
added asset: sensu/sensu-slack-handler:1.0.3

You have successfully added the Sensu asset resource, but the asset will not get downloaded until
it's invoked by another Sensu resource (ex. check). To add this runtime asset to the appropriate
resource, populate the "runtime_assets" field with ["sensu-slack-handler"].

This example uses the -r (rename) flag to specify a shorter name for the dynamic runtime asset: sensu-slack-handler.

You can also download the latest dynamic runtime asset definition for your platform from Bonsai and register the asset with sensuctl create --file filename.yml or sensuctl create --file filename.json.

You should receive a confirmation message from sensuctl:


NOTE: Sensu does not download and install dynamic runtime asset builds onto the system until they are needed for command execution. Read the asset reference for more information about dynamic runtime asset builds.

Get a Slack webhook

If you’re already the admin of a Slack, visit and follow the steps to add the Incoming WebHooks integration, choose a channel, and save the settings. If you’re not yet a Slack admin, create a new workspace and then create your webhook.

After saving, you can find your webhook URL under Integration Settings.

Create a handler

Use sensuctl to create a handler called slack that pipes observation data (events) to Slack using the sensu-slack-handler dynamic runtime asset. Edit the sensuctl command below to include your Slack webhook URL and the channel where you want to receive observation event data. For more information about customizing your Slack alerts, read the Sensu Slack Handler page in Bonsai.

sensuctl handler create slack \
--type pipe \
--env-vars "SLACK_WEBHOOK_URL=" \
--command "sensu-slack-handler --channel '#monitoring'" \
--filters not_silenced \
--runtime-assets sensu-slack-handler

You should receive a confirmation message:


The sensuctl handler create slack command creates a handler resource. To view the slack handler definition, run:

sensuctl handler info slack --format yaml
sensuctl handler info slack --format wrapped-json

The slack handler resource definition will be similar to this example:

type: Handler
api_version: core/v2
  name: slack
  command: sensu-slack-handler --channel '#monitoring'
  - not_silenced
  handlers: null
  - sensu-slack-handler
  secrets: null
  timeout: 0
  type: pipe
  "type": "Handler",
  "api_version": "core/v2",
  "metadata": {
    "name": "slack"
  "spec": {
    "command": "sensu-slack-handler --channel '#monitoring'",
    "env_vars": [
    "filters": [
    "handlers": null,
    "runtime_assets": [
    "secrets": null,
    "timeout": 0,
    "type": "pipe"

You can share and reuse this handler like code — save it to a file and start building a monitoring as code repository.

Assign the handler to a check

With the slack handler created, you can assign it to a check. To continue this example, use the check_cpu check created in Monitor server resources.

Assign your slack handler to the check_cpu check to receive Slack alerts when the CPU usage of your systems reaches the specific thresholds set in the check command:

sensuctl check set-handlers check_cpu slack

To view the updated check_cpu resource definition, run:

sensuctl check info check_cpu --format yaml
sensuctl check info check_cpu --format wrapped-json

The updated check definition will be similar to this example:

type: CheckConfig
api_version: core/v2
  name: check_cpu
  check_hooks: null
  command: check-cpu-usage -w 75 -c 90
  env_vars: null
  - slack
  high_flap_threshold: 0
  interval: 60
  low_flap_threshold: 0
  output_metric_format: ""
  output_metric_handlers: null
  proxy_entity_name: ""
  publish: true
  round_robin: false
  - check-cpu-usage
  secrets: null
  stdin: false
  subdue: null
  - system
  timeout: 0
  ttl: 0
  "type": "CheckConfig",
  "api_version": "core/v2",
  "metadata": {
    "created_by": "admin",
    "name": "check_cpu",
    "namespace": "default"
  "spec": {
    "check_hooks": null,
    "command": "check-cpu-usage -w 75 -c 90",
    "env_vars": null,
    "handlers": [
    "high_flap_threshold": 0,
    "interval": 60,
    "low_flap_threshold": 0,
    "output_metric_format": "",
    "output_metric_handlers": null,
    "proxy_entity_name": "",
    "publish": true,
    "round_robin": false,
    "runtime_assets": [
    "secrets": null,
    "stdin": false,
    "subdue": null,
    "subscriptions": [
    "timeout": 0,
    "ttl": 0

Validate the handler

It might take a few moments after you assign the handler to the check for the check to be scheduled on the entities and the result sent back to Sensu backend. After an event is handled, you should receive the following message in Slack:

Example Slack message

Verify the proper behavior of this handler with sensu-backend logs. Read Troubleshoot Sensu for log locations by platform.

Whenever an event is being handled, a log entry is added with the message "handler":"slack","level":"debug","msg":"sending event to handler", followed by a second log entry with the message "msg":"event pipe handler executed","output":"","status":0.

Next steps

Now that you know how to apply a handler to a check and take action on events, read the handlers reference for in-depth handler documentation and check out the Reduce alert fatigue guide.

Follow Send PagerDuty alerts with Sensu to configure a check that generates status events and a handler that sends Sensu alerts to PagerDuty for non-OK events.