Answer Posted / sabir hussain

Singleton ensure that class has only one instance and
provide a global point of access to it.

It can be applied in a situation where your implementation
is independent and utilize resource that should be
initialized once within your application environment.

1) So Configuration can be singleton as we configure
resources that are globally used with your application. (ex:
database connection info.)

2) Authentication and Authorization component can also be
singleton as there implementation are independent and will
not solve any purpose if instantiated twice, if you look in
terms of spring framework all such components are singleton.
Both the component can independently authenticate or
authorize user or user info (validating credit card info).

Cart, SelectionItem and Billing are user specific generally
session scoped, and there can be more then one session per
application, so they cant be Singleton for obvious reason.

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