Excerpt
hidden true
Description of how to create and use subscriptions with Insert excerpt ApplicationName ApplicationName nopanel true ApplicationName Eventing.

Overview

The Eventing capability of

Insert excerpt
ApplicationName ApplicationName nopanel true ApplicationName

Insert excerpt
Eventing Eventing nopanel trueEventing

.

To better understand why this would be useful, suppose that there has been increased pirating activity off the coast of Somalia.  Because of these events, a group of intelligence analysts is interested in determining the reason for the heightened hostility and discovering its cause.  To do this, analysts need to monitor interesting events occurring in that area.  Without

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

Eventing, the analysts would need to repeatedly query for any records of events or intelligence gathered in that area.  Analysts would have to keep an eye out for changes or anything of interest.  However, with

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

Eventing, the analysts can create a subscription indicating criteria for the types of intelligence of interest.  In this scenario, analysts could specify interest in metacards added, updated, or deleted that describe data obtained around the coast of Somalia.  Through this subscription,

Insert excerpt
ApplicationName ApplicationName nopanel true ApplicationName

will send event notifications back to the team of analysts containing metadata of interest.  Furthermore, they could filter the records not only spatially, but by any other criteria that would zero in on the most interesting records.  For example, a fishing company that has operated ships peacefully in the same region for a long time may not be interesting.  To exclude metadata about that company, analysts may add contextual criteria indicating to only return records containing the keyword "pirate."  How these event notifications are handled and processed is up to the client's event consumer which receives all callbacks from

Insert excerpt
ApplicationName ApplicationName nopanel true ApplicationName

.  With the subscription in place, the analysts will be notified only of metadata related to the pirating activity, so analysts can obtain better situational awareness.

The key components of

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

Eventing include:

• Subscription -

  Insert excerpt
  Subscription Subscription nopanel trueSubscription

  .
• Delivery Method -

  Insert excerpt
  Delivery Method Delivery Method nopanel trueDelivery Method

  .
• Event Processor -

  Insert excerpt
  Event Processor Event Processor nopanel true Event Processor

  .
• Event Consumer - service that receives and processes event notifications on the client side.
• Callback URL - location of Event Consumer.

...

This section discusses how a developer can create subscriptions and event consumers to work with 

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

Eventing.  More information about

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

Eventing in general can be found in the Eventing section of the Integrator's Guide.

...

To create a subscription in

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

the developer needs to implement the ddf.catalog.event.Subscription interface.  This interface extends org.opengis.filter.Filter in order to represent the subscription's filter criteria.  Furthermore, the Subscription interface contains a DeliveryMethod implementation.  

When implementing Subscription, the developer will need to override the methods accept and evaluate from the Filter.  The accept method allows the visitor pattern to be applied to the Subscription.  A FilterVisitor can be passed into this method in order to process the Subscription's Filter.  In 

Insert excerpt
ApplicationName ApplicationName nopanel true ApplicationName

this method is used to convert the Subscription's Filter into a predicate format that is understood by the Event Processor.  The second method inherited from Filter is evaluate.  This method is used to evaluate an object against the Filter's criteria in order to determine if it matches the criteria.  See the Creating Filters section of the Developer's Guide for more information on OGC Filters. 

...

The Event Processor obtains the subscription's DeliveryMethod and invokes one of its four methods when an event occurs.  The DeliveryMethod then handles that invocation and communicates an event to a specified consumer service outside of 

Insert excerpt
ApplicationName ApplicationName nopanel true ApplicationName

.  The DeliveryMethod implementation typically contains a callback URL.  It uses this URL to know where the event consumer is located.

...

An event consumer is a service designed to receive and process event notifications.  This event consumer can be any type of service running at a location accessible to

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

.  If the service is a SOAP service, the DeliveryMethod will need to communicate with it according to its WSDL.  On the other hand, if it is a REST service, the DeliveryMethod will invoke HTTP operations on the callback URL.  Typically, whatever type of service the event consumer is, it will have operations corresponding to each of the created, updated, and deleted events.  This way, the consumer will know what type of event occurred on

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

. 

Deleting a Subscription

To remove a subscription from

Insert excerpt
ApplicationName ApplicationName nopanel trueApplicationName

, the subscription ID is required.  Once this is provided, the ServiceRegistration for the indicated Subscription should be obtained from the Subscriptions Map.  Then the Subscription can be removed by unregistering the service.  The following code demonstrates how this is done:

...