function onMessage( message ) {
	var child;
	if ( message === 'close' ) {
		debug( 'Received a message to close. Closing...' );
		return closeProcess();
	}
	debug( 'Received a message to run a script: %s. pid: %s.', message, proc.pid );
	if ( proc.env.WORKER_ORDERED ) {
		debug( 'Environment configuration specifies to preserve script output.' );
		child = exec( proc.env.WORKER_CMD, message, done );
	} else {
		debug( 'Environment configuration allows interleaved script output.' );
		child = spawn( proc.env.WORKER_CMD, message, done );
	}
	db[ child.pid ] = child;
}
Callback invoked upon receiving a message from a parent process.

@private
@param {string} message - message from parent
@returns {void}