Subscription

Description

Subscriptions represent "standing queries" in the Catalog.  Like a Query, Subscriptions are based on the OGC Filter specification.

Subscription Lifecycle

1. Creation
   1. Subscriptions are created directly with the Event Processor or declaratively through use of the Whiteboard Design Pattern.
   2. The Event Processor will invoke each Pre-Subscription Plugin and, if the Subscription is not rejected, the Subscription will be activated.
2. Evaluation
   1. When a Metacard matching the Subscription is created, updated, or deleted in any Source, each Pre-Delivery Plugin will be invoked.
   2. If the delivery is not rejected, the associated Delivery Method callback will be invoked.

Update evaluation

Notably, the Catalog allows event evaluation on both the previous value (if available) and new value of a Metacard when an update occurs.

Durability

Subscription durability is not provided by the Event Processor.  Thus, all Subscriptions are transient and will not be recreated in the event of a system restart.  It is the responsibility of Endpoints using Subscriptions to persist and re-establish the Subscription on startup.  This decision was made for the sake of simplicity, flexibility, and the inability of the Event Processor to recreate a fully-configured Delivery Method without being overly restrictive.

Additional Information

• See Working with Subscriptions in the Developer's Guide section.
• See Use of the Whiteboard Design Pattern in the Developer's Guide section.