public Expression getPartitionSelectorExpression() {
		String partionSelectorExpression = getProperty(PARTITION_SELECTOR_EXPRESSION);
		Expression expression = null;
		if (partionSelectorExpression != null) {
			expression = spelExpressionParser.parseExpression(partionSelectorExpression);
		}
		return expression;
	}
The expression evaluated against the partition key to determine
the partition to which the message will be sent. The result should
be an integer that will subsequently be mod'd with the module's
partition count.
@return The expression.
spring-xd-messagebus-spi/src/main/java/org/springframework/xd/dirt/integration/bus/AbstractBusPropertiesAccessor.java