Sensu Mutators

Reference documentation

What is Sensu mutators?

Sensu mutators are executable scripts or other programs that modify event data for Sensu event handlers which may expect additional or modified event data (e.g. custom attributes that are not provided by the default event data specification.

The Sensu mutator specification

  • Accept input/data via STDIN
  • Able to parse a JSON data payload (i.e. a event data)
  • Output JSON data (the modified event data) to STDOUT or STDERR
  • Produce an exit status code to indicate state:
    • 0 indicates OK
    • exit status codes other than 0 indicates a failure

When to use a mutator

Many Sensu event handlers will modify event data in the course of processing an event, and in many cases this is recommended because modifying the event data and performing some action in memory (in the same process) will result in better performance than executing a mutator and a handler (two separate processes). However, when multiple handlers require similar event data modifications, mutators provide the ability to avoid code duplication (DRY), and simplify event handler logic.

How do Sensu mutators work?

Sensu mutators are applied when event handlers are configured to use a mutator. Prior to executing a Handler, the Sensu server will execute the configured mutator. If the mutator is successfully executed, the modified event data is then provided to the handler and the handler will be executed. If the mutator fails to execute for any reason, an error will be logged and the handler will not be executed. The complete process may be described as follows:

  • When the Sensu server is processing an event, it will check for the definition of a mutator. Prior to executing each handler, the Sensu server will first execute the configured mutator (if any) for the handler
  • If the mutator is successfully executed (i.e. if it returns an exit status code of 0), the modified event data is provided to the handler and the handler will be executed.
  • If the mutator fails to execute (i.e. returns a non-zero exit status code, or does not complete execution within the configured timeout), an error will be logged and the handler will not be executed

Please refer to the Sensu event handler definition specification for more information about applying a mutator to an event handler (see the mutator attribute).

Mutator commands

What is a mutator command?

Each Sensu mutator definition defines a command to be executed. Mutator commands are literally executable commands which will be executed on a Sensu server, run as the sensu user. Most mutator commands are provided by Sensu plugins.

Mutator command arguments

Sensu mutator command attributes may include command line arguments for controlling the behavior of the command executable. Many Sensu mutator plugins provide support for command line arguments for reusability.

How and where are mutator commands executed?

As mentioned above, all mutator commands are executed by a Sensu server as the sensu user. Commands must be executable files that are discoverable on the Sensu server system (i.e. installed in a system $PATH directory).

NOTE: By default, the Sensu installer packages will modify the system $PATH for the Sensu processes to include /etc/sensu/plugins. As a result, executable scripts (e.g. plugins) located in /etc/sensu/plugins will be valid commands. This allows command attributes to use “relative paths” for Sensu plugin commands;

e.g.: "command": "check-http.rb -u"

Mutator configuration

Example mutator definition

The following is an example Sensu mutator definition, a JSON configuration file located at /etc/sensu/conf.d/example_mutator.json. This mutator definition uses an imaginary Sensu plugin called example_mutator.rb to modify event data prior to handling the event.

  "mutators": {
    "example_mutator": {
      "command": "example_mutator.rb"

Mutator definition specification

Mutator name(s)

Each mutator definition has a unique mutator name, used for the definition key. Every mutator definition is within the "mutators": {} definition scope.

  • A unique string used to name/identify the mutator
  • Cannot contain special characters or spaces
  • Validated with Ruby regex /^[\w\.-]+$/.match("mutator-name")

Mutator attributes

The mutator command to be executed. The event data is passed to the process via STDIN.
"command": "/etc/sensu/plugins/mutated.rb"
The mutator execution duration timeout in seconds (hard stop).
"timeout": 30

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