What is the difference between Facade and Gateway design patterns?

Question

or Facade==Gateway?

Answer 1

Reviewing Facade in the GoF book and the link in another answer to Martin Fowler's Gateway, it appears that their focus is in opposite directions.

Facade provides a simple uniform view of complex internals to (one or more) external clients;

Gateway provides a simple uniform view of external resources to the internals of an application.

This distinction lets us focus on which is more important in a design:

With the Facade, the external system is our customer; it is better to add complexity facing inwards if it makes the external interface simpler.

With the Gateway, the internal system is our customer; give it all the help we can, even if the externals are more complex.

Answer 2

These two patterns are very similar in the way that both they serve as wrappers over something. The difference is in the context: facade stands over a group of subsystems, while gateway can be standing over any functionality. From this point of view, to me Facade is the concrete case of Gateway (not opposite).

Facade is applied when we think that working with subsystems is complex or if we want to group several subsystem calls into one [method] execution. However, this does not necessarily mean that subsystems are not accessible, or that they are complex enough. It simply means that we have subsystems.

Gateway is applied when we want to wrap some things and expose them into different way. Gateway may be wrapping not a subsystem, but just a relatively complex functionality. Gateway is a general pattern which can be thought as a basis for Facade, Proxy, and other patterns.

If example is still needed for clarification:

Facade can calculate credit worthiness of a customer by querying checking accounts subsystem, credit accounts subsystem, savings subsystem, and back office subsystem (I guess I have seen similar example in GOF books).

class MortgateFacade {     bool IsCreditWorth(string customerName) {         return !_checkingAccSystem.HasNegativeBalance(customerName) && !_creditAccSystem.HasNegativeCredit(customerName) && !_backOfficeSystem.IsInBlackList(customerName);     } } 

Gateway can query the database table and return customer by ID. (Yes, that simple!)

class CustomersGateway {     Customer GetCustomer(int id) {         return _db.ExecuteOne("SELECT TOP 1 FROM CUSTOMERS WHERE CUSTOMER_ID="+id).AsCustomer();     } } 

[Obviously this is a pseudo code]