Use of Facade Pattern

How can I know that I need a facade Pattern at a point in my application development?

How can开发者_运维百科 I draw the line between Facade Pattern and Template Pattern?

For example: In [this] article, we see that, int placeOrder(int CustomerID, List<BasketItem> Products) has a number of predefined steps in the algorithm. So why don't the author use Template Pattern here?

Facade deals with interface, not implementation. Its purpose is to hide internal complexity behind a single interface that appears simple on the outside. In the example from your question, the facade hides four classes (Order, OrderLine, Address, BasketItem) behind a single method.

Template method deals with implementation. Its purpose is to extract the common algorithm from several ones that differ only in a 'fill in the blanks' way. The template method in the superclass implements the common algorithm and each subclass 'fills in the blanks' in its own specific way.

So why don't the author use Template Pattern here?

It would make sense to make placeOrder a template method if there were several similar versions of the operation. Maybe a few methods like placePhoneOrder, placeInternetOrder, placeManuallyEnteredOrder could be refactored into a single template placeOrder with some subclasses implementing only the {phone,internet,manual}-specific differences.

The facade pattern is appropriate when you have a complex system that you want to expose to clients in a simplified way, or you want to make an external communication layer over an existing system which is incompatible with your system. It is a structural pattern. See here: http://en.wikipedia.org/wiki/Facade_pattern

The template pattern, on the other hand, is a behavioral pattern that will help you when dealing with the inner implementation of a component. See here: http://en.wikipedia.org/wiki/Template_method_pattern

Suppose you have a few services, libraries or whatever. These libraries need interoperation in order to perform some higher level services. Then you may wish to wrap those calls and intialization code that usually go together and offer a bunch of functions to hide those details and make it simple to use those services for specific scenarios. Then it is a good use for facade pattern.

UPDATE: In the article mentioned the PlaceOrder method has one single implementation that works for all orders. Template pattern is meant to prescribe a series of steps that have to be followed but allow subclasses to offer their custom implementation of those fixed steps. For example, if you needed orders for televisions to be processed differently from orders for microwaves you could use the template pattern to redefine some imaginary DispatchParcel method (to send microwave as a simple package but television with extra service to help lift the heavy device to the upper floor). In our case there is no need for reimplementation of ProcessOrder steps so there is no need for template pattern as one single implementation suits all types of orders.