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The event library provides functions for understanding an event and the environment in which it was raised as well as actions for sending events.

Event Functions

Event functions in the following categories are available:

Event Environment

For historical reasons, some of these are Web-centric. The following functions are available:

event:env()- returns information about the event's environment. The function takes an argument that determines what will be returned:

  • caller -return the URL of the Web page on which the event is firing (assumes an event in the web domain)
  • ip - return the IP number of the endpoint (client).
  • referer - return the URL of the referring page to the caller (assumes a web event).
  • title - return the page title of the calling page.
  • txn_id - return the transaction ID of this ruleset evaluation.

Event Type and Domain

  • event:domain() - return the event domain that caused this rule to execute.
  • event:type() - return the event type (name) that caused this rule to execute.
  • event:name() - alias for event:type()

Event Attributes

  • event:attr() - returns a specific event attribute. The function takes the name of the attribute to be returned as its sole argument.
  • event:attrs() - returns all the event attributes as a map.

Event Channels

event:channel() - returns information about the event channel. The function takes an argument that determines what is returned:

  • id - returns the event channel identifier. 

Event Actions

The event library supports sending events to event channels. 

event:send(<subscription_map>, <domain>, <type>)

The <domain> and <type> are expressions that evaluate to strings. These arguments are optional. If they are not present, the values with keys _domain and _type from the subscription map will be used.

A subscription map is any map that contains an event channel identifier (identified by the key cid) or an event signal URL (identified by the key esl). The other key-value pairs in the map are optional. The following code sample shows a subscription map:

Given the preceding subscription map declaration, we could send a notification:status event to the event channel identified by the CID in the map as follows:

If the subscription map contains both a CID and an ESL, the ESL is used to send the event. 

Event attributes can be specified in the optional action parameter attrs. To send a notification:status event with the attributes priority and subject, we would modify the preceding example as follows:

Of course, the arguments and parameters of an action can be calculated. 

If the ESL or CID have non-standard keys in the subscription map, you can use the parameters cid_key or esl_key to give the key name:

Asynchronicity

The default behavior of send() is to use the Sky event protocol in asynchronous mode. To force synchronous event processing, add the attribute _async with value 0 to the attributes. 

Results

Results are only available for synchronous mode events. In general, you shouldn't be relying on results from sending an event. Rather, the recommended pattern is to have the receiving event network communicate by sending an event to the original sender. However, it is often useful to know whether or not the send() succeeded.  To support this, the system will raise a system:send_complete event when all threads have completed. The event has a single attribute called send_results that contains an array of the results of the individual sends. Each member of the array has the following structure:

You can use a rule like the following listen for the system event and loop over the results:

Obviously, you probably don't want to merely print the status out, but rather check it and do something if it is not 200.

Patterns

Raising an Event On Final

A common pattern when sending events is to loop over the subscribers and then raise an explicit event when done. The on final guard condition is used to ensure the explicit event is only raised once. 

The difference between raising an explicit event on final and using the system generated system:send_complete event shown above might not be obvious. 

The primary difference is that the explicit event does have access to result information, including status. But, both the explicit event shown in the preceding code and the system:send_complete event will cause any rules that are selected based on the respective events to be scheduled. Similarly neither will cause rules to be run before send() is complete since parallel send() actions must all complete before the rule they are in can complete. 

Encoding Structured Attributes

event:send() uses URL Form Encoding, which means that structured, nested attributes won't be passed correctly unless they are encoded. Here is a rule that sends and event and encodes the structured data:

In the rule listening for the event, you have to decode the attribute:

 

Scheduled Event Functions

A scheduled event is created in the Postlude. As such, it is not yet an actual event–event protoplasm if you will; an event yet to be realized. There are a few functions available to list, delete and get the history of scheduled events.

get_list

Returns an array of scheduled event arrays. A scheduled event array looks like [<SchedEv Id>,<domain>/<event name>,<event type>,<next scheduled fire>];

SchedEv Id

string

domainstring
event namestring
event type'once' | 'repeat'
next scheduled fireepoch time

event:get_list returns the scheduled events for the current account/entity

 [
  [
    '51cb5a2461bc204e53090000',
    'notification/_test_Islamization',
    'repeat',
    'cs_test',
    1372281480
  ]
]

*note: the format of the result is flexible.  I chose an array of arrays because KRL has some powerful array operators.  This could easily be a map of schedEv ids and values if that is a better usecase.

delete

event:delete accepts an id for a scheduled event.  This id could come from event:get_list() or from the postlude. It removes the scheduled event from the queue.

get_history

event:get_history accepts an id for a scheduled event.  This id could come from event:get_list() or from the postlude. This will return the status from the last time the schedEv raised the event as a map.

 

History status for a single/once event:

{
  'fired' : '2013-06-26T21:16:21+00:00',
  'status' : '200 OK',
  'keep_until' : '2013-06-26T21:16:21Z',
  'code' : '200'
}

If the event had not fired the call would have returned null.  In this case, the scheduled event was a single/once so the field 'keep_until' indicates when the already completed event and history will be completely flushed from the system. The default is to keep the history for 4 hours.

History status for a repeating event:

{
  'fired' : '2013-06-26T21:16:21+00:00',
  'status' : '200 OK',
  'next' : '2013-06-26T21:20:00Z',
  'code' : '200'
}

In this case, the history is for a repeating event, so it will never be deleted unless event:delete is called for the id. Status is similar; noting when the last time the event fired, but also provides an estimate for the next time the event is scheduled to fire.  In this case, approximately 4 minutes after the last event was raised

Additional Information

The blog post Federating Personal Clouds shows an example using send()