Using the Alarm package

The /whisk.system/alarms package can be used to fire a trigger at a specified frequency. This is useful for setting up recurring jobs or tasks, such as invoking a system backup action every hour.

The package includes the following feed.

Entity	Type	Parameters	Description
/whisk.system/alarms	package	-	Alarms and periodic utility
/whisk.system/alarms/alarm	feed	cron, trigger_payload, maxTriggers	Fire trigger event periodically

Firing a trigger event periodically

The /whisk.system/alarms/alarm feed configures the Alarm service to fire a trigger event at a specified frequency. The parameters are as follows:

• cron: A string, based on the UNIX crontab syntax, that indicates when to fire the trigger in Coordinated Universal Time (UTC). The string is a sequence of five fields that are separated by spaces: X X X X X. For more details about using cron syntax, see: http://crontab.org. Following are some examples of the frequency that is indicated by the string:

  • * * * * *: top of every minute.
  • 0 * * * *: top of every hour.
  • 0 */2 * * *: every 2 hours (i.e. 02:00:00, 04:00:00, ...)
  • 0 9 8 * *: at 9:00:00AM (UTC) on the eighth day of every month

• trigger_payload: The value of this parameter becomes the content of the trigger every time the trigger is fired.

• maxTriggers: Stop firing triggers when this limit is reached. Defaults to 1,000,000. You can set it to infinite (-1).

The following is an example of creating a trigger that will be fired once every 2 minutes with name and place values in the trigger event.

wsk trigger create periodic \
  --feed /whisk.system/alarms/alarm \
  --param cron "*/2 * * * *" \
  --param trigger_payload "{\"name\":\"Odin\",\"place\":\"Asgard\"}"

Each generated event will include as parameters the properties specified in the trigger_payload value. In this case, each trigger event will have parameters name=Odin and place=Asgard.

Note: The parameter cron also supports a custom syntax of six fields, where the first field represents seconds. For more details about using this custom cron syntax, see: https://github.com/ncb000gt/node-cron. Here is an example using six fields notation:

• */30 * * * * *: every thirty seconds.