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Creating triggers and rules

OpenWhisk triggers and rules bring event-driven capabilities to the platform. Events from external and internal event sources are channeled through a trigger, and rules allow your actions to react to these events.

Creating triggers

Triggers are a named channel for a class of events. The following are examples of triggers:

  • A trigger of location update events.
  • A trigger of document uploads to a website.
  • A trigger of incoming emails.

Triggers can be fired (activated) by using a dictionary of key-value pairs. Sometimes this dictionary is referred to as the event. As with actions, each firing of a trigger results in an activation ID.

Triggers can be explicitly fired by a user or fired on behalf of a user by an external event source. A feed is a convenient way to configure an external event source to fire trigger events that can be consumed by OpenWhisk. Examples of feeds include the following:

  • Cloudant data change feed that fires a trigger event each time a document in a database is added or modified.
  • A Git feed that fires a trigger event for every commit to a Git repository.

Using rules

A rule associates one trigger with one action, with every firing of the trigger causing the corresponding action to be invoked with the trigger event as input.

With the appropriate set of rules, it's possible for a single trigger event to invoke multiple actions, or for an action to be invoked as a response to events from multiple triggers.

For example, consider a system with the following actions:

  • classifyImage action that detects the objects in an image and classifies them.
  • thumbnailImage action that creates a thumbnail version of an image.

Also, suppose that there are two event sources that are firing the following triggers:

  • newTweet trigger that is fired when a new tweet is posted.
  • imageUpload trigger that is fired when an image is uploaded to a website.

You can set up rules so that a single trigger event invokes multiple actions, and have multiple triggers invoke the same action:

  • newTweet -> classifyImage rule.
  • imageUpload -> classifyImage rule.
  • imageUpload -> thumbnailImage rule.

The three rules establish the following behavior: images in both tweets and uploaded images are classified, uploaded images are classified, and a thumbnail version is generated.

Creating and firing triggers

Triggers can be fired when certain events occur, or can be fired manually.

As an example, create a trigger to send user location updates, and manually fire the trigger.

  1. Enter the following command to create the trigger:
$ wsk trigger create locationUpdate

ok: created trigger locationUpdate
  1. Check that you created the trigger by listing the set of triggers.
$ wsk trigger list

triggers
/someNamespace/locationUpdate                            private

So far you've created a named "channel" to which events can be fired.

  1. Next, fire a trigger event by specifying the trigger name and parameters:
$ wsk trigger fire locationUpdate --param name Donald --param place "Washington, D.C."

ok: triggered locationUpdate with id fa495d1223a2408b999c3e0ca73b2677

A trigger that is fired without an accompanying rule to match against has no visible effect. Triggers cannot be created inside a package; they must be created directly under a namespace.

Associating triggers and actions by using rules

Rules are used to associate a trigger with an action. Each time a trigger event is fired, the action is invoked with the event parameters.

As an example, create a rule that calls the hello action whenever a location update is posted.

  1. Create a 'hello.js' file with the action code we will use:
function main(params) {
   return {payload:  'Hello, ' + params.name + ' from ' + params.place};
}
  1. Make sure that the trigger and action exist.
$ wsk trigger update locationUpdate

$ wsk action update hello hello.js
  1. Create the rule. Note that the rule will be enabled upon creation, meaning that it will be immediately available to respond to activations of your trigger. The three parameters are the name of the rule, the trigger, and the action.
$ wsk rule create myRule locationUpdate hello

At any time, you can choose to disable a rule.

$ wsk rule disable myRule
  1. Fire the locationUpdate trigger. Each time that you fire an event, the hello action is called with the event parameters.
$ wsk trigger fire locationUpdate --param name Donald --param place "Washington, D.C."

ok: triggered locationUpdate with id 878998285cad448b8998285cad948b30
  1. Verify that the action was invoked by checking the most recent activation.
$ wsk activation list --limit 1 hello

Datetime            Activation ID                    Kind     Start Duration   Status  Entity
2019-02-18 11:51:41 0efe54d8fb96486bbe54d8fb96d86bbe nodejs:6 cold  54ms       success guest/hello:0.0.1

$ wsk activation result 0efe54d8fb96486bbe54d8fb96d86bbe

{
   "payload": "Hello, Donald from Washington, D.C."
}

You see that the hello action received the event payload and returned the expected string.

You can create multiple rules that associate the same trigger with different actions. Triggers and rules cannot belong to a package. The rule may be associated with an action that belongs to a package however, for example:

$ wsk rule create recordLocation locationUpdate /whisk.system/utils/echo

You can also use rules with sequences. For example, one can create an action sequence recordLocationAndHello that is activated by the rule anotherRule.

$ wsk action create recordLocationAndHello --sequence /whisk.system/utils/echo,hello
$ wsk rule create anotherRule locationUpdate recordLocationAndHello