UML Modeling Techniques > Concurrency modeling > Messaging through events and operations
Messaging through events and operations
Instances on the Communication Diagram must be stimulated by either Events or Operations carried through the message flows and is the means by which one object communicates with another. The mechanism for stimulating an Instance is dependent upon the items at either end of the flow, as follows:
• START:Actor -> END:Instance
You can add Operations to the end only, and Events in both directions
• START:Instance -> END:Instance
You can add Operations and Events in both directions
• START:Instance -> END:Actor
You can add Operations to the start only, and Events in both directions.
For a particular message flow, you can:
• Call a new Operation.
• Send a new Event.
• Annotate the message flow with an Event or Operation name, message parameters and a directional arrow.
• Add or change properties.
• Structure message descriptions by adding sequence numbers in Dewey decimal format, where letters indicate concurrency of messages; return name arguments using the assignment sign (:=); indicate a potentially iterative message by prefixing an asterisk (*).
To call a new operation:
1. On the Communication Diagram tab, click the Operation Message button.
2. Click the required link.
3. Select the destination object from the context menu.
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You can also call an existing operation by dragging it from an appropriate pane to the diagram.
To send a new event:
1. On the Communication Diagram tab, click the Event Message button.
2. Click the required link.
3. Select the destination object from the context menu.
* 
You can also send an existing Event by dragging it from an appropriate pane to the diagram.
To change the instance properties of an event or operation instance:
1. Right-click the Event or Operation message, and then click Instance Properties.
2. Change the required properties in the Property pages.
To change the properties of an event or operation:
1. Click the Event or Operation message.
2. Change the required properties in the Property pages.
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