# Programming with the ALE and ALEPC APIs

# Event Triggers

The following sections describe the use of event triggers with the RFID Edge Server:

- Overview of Event Triggers
- OLE for Process Control (OPC) Trigger Driver

## Overview of Event Triggers

You can define event cycle specifications (ECSpecs) and programming cycle specifications (PCSpecs) where the beginning and/or end of each cycle is triggered by external events. BEA provides an extensible mechanism for connecting sources of external events to the ALE engine.

## OLE for Process Control (OPC) Trigger Driver

Object Linking and Embedding (OLE) for OPC is a series of standards specifications that define a standard set of objects, interfaces, and methods for use in process control and manufacturing automation applications to facilitate interoperability. (For more information on OPC, see http://www.opcfoundation.org.)

WebLogic RFID Edge Server includes a driver for OPC triggers. An event or programming cycle in the Edge Server can be triggered by polling for a change in an OPC item. The Edge Server communicates with the OPC service using the OPC XML-DA protocol, which is a SOAP interface to an OPC Data Access provider. The OPC XML-DA implementation is provided by a third party.

The general form of the trigger URI is:

    opcpoll:itemName=item;http://hostname/location

where:

- `item` is the name of the OPC item that the driver polls for changes.
- `http://hostname/location` is the URL that is used to create the connection to the OPC XML-DA server.

When an event cycle or programming cycle that uses an OPC trigger is first requested (that is, when the event cycle or programming cycle is invoked using the `poll` or `immediate` method or subscribed using `subscribe`), the Edge Server polls the OPC XML-DA server for the current value of the specified OPC item. Afterwards, each time the value of the OPC item changes, a trigger will be delivered to the event cycle or programming cycle.