A graph function must implement an undeliverableMessageHandler method that handles with any undeliverable vertex messages. The following example shows the GraphSearch implementation of the undeliverableMessageHandler method. The method simply issues a global halt if any undeliverable messages are received. Another option would have been for the method to log each message and return, thus allowing graph processing to proceed.
/**
* If there are any undeliverable messages, then stop graph processing.
* Alternatively, one can log messages and continued.
* @param globals - provides the iteration number and other global state
* @param undeliverableMessages - the messages that could not be delivered
*/
public void undeliverableMessagesHandler(GraphGlobals globals,
VertexMessageIterator undeliverableMessages) {
if (undeliverableMessages.advanceToNextMessage()){
globals.globalHalt();
}
return;
}