Emitting Events

Events can be emitted by Hyperledger Composer and subscribed to by external applications. Events are defined in the model file of a business network definition, and are emitted by transaction JavaScript in the transaction processor functions file.

Before you begin

Before you begin adding events to your business network, you should have a good understanding of the modeling language for business networks, and what makes up a full business network definition.

Procedure

1. Events are defined in the model file (.cto) of your business network definition, in the same way as assets and participants. Events use the following format:

   event BasicEvent {
   }
   

2. In order for the event to be published the transaction which creates the event must call three functions, the first is the getFactory function. The getFactory allows events to be created as part of a transaction. Next, an event must be created by using factory.newEvent('org.namespace', 'BasicEvent'). This creates a BasicEvent defined in a specified namespace. Then the required properties on the event must be set. Lastly, the event must be emitted by using emit(BasicEvent). A simple transaction which calls this event would look like this:

   /**
    * @param {org.namespace.BasicEventTransaction} basicEventTransaction
    * @transaction
    */
   function basicEventTransaction(basicEventTransaction) {
       var factory = getFactory();
   
       var basicEvent = factory.newEvent('org.namespace', 'BasicEvent');
       emit(basicEvent);
   }
   

This transaction creates and emits an event of the BasicEvent type as defined in the business network's model file. For more information on the getFactory function, see the Composer API documentation.

What next?

• Subscribing to events
• Developing applications