Interface Segregation

Interface Segregation

It states that interfaces should be small and At their simplest, interfaces contain single methods that serve a single purpose.

The reasons for segregating (splitting ) large interface into smaller interfaces  :

1. Self-documentation for other developers to follow
2.  Decorating interfaces : we may have to decorate implementations using decorate pattern/adapter pattern  in the cases  where cross-cutting concerns such as logging transaction management ,caching are required
3. Architectural Need :The interfaces are segregated based on the architectural need  e.g. the Data access technology  is different for the same data
4. Client Need :When a set of certain set of operation is only available to a certain clients, the large interfaces are segregated into smaller interface

Examples of client needs  :

1. Client A is having requirement to get read only a particular data from service and Client B is to get write only to the same data
2. A set of certain set of operation is only available to a certain clients when the application is in specific state For example, the operations that a user can perform are typically different depending on whether that user is logged in or not.                       

Client construction :

          Client can construct segregated interfaces in 2 ways  as below.

1. Single implementation, single instance : all of the segregated interfaces are implemented in a     single class, a single instance is sufficient for all of the related  dependencies on the client.
2. Multiple implementations, multiple instances : If each segregated interface is given its own       implementation, each of those implementations needs to be constructed and supplied to the       client.

The Interface Soup anti-pattern    :

A common mistake is to take the segregated interfaces and reunify them in an aggregate as below.

In the above fig, the segregated interfaces are reunified in a single interface forming “interface soup”, resulting in providing implementations of all operations and so there is no scope for targeted decoration