The iBATIS philosophy - iBATIS in Action - page 17

reducing network traffic and memory requirements for our application. i BATIS
can help achieve these goals using features that allow querying for specific ranges
of data.

1.2.3 The business logic layer

The business logic layer of the application describes the coarse-grained services
that the application provides. For this reason they are sometimes called service
classes. At a high level, anyone should be able to look at the classes and methods
in the business logic layer and understand what the system does. For example, in a
banking application, the business logic layer might have a class called TellerSer-
vice , with methods like openAccount() , deposit() , withdrawal() , and getBal-
ance() . These are very large functions that involve complex interactions with
databases and possibly other systems. They are much too heavy to place into a
domain class, as the code would quickly become incohesive, coupled, and gener-
ally unmanageable. The solution is to separate the coarse-grained business func-
tions from their related business object model. This separation of object model
classes from logic classes is sometimes called noun-verb separation .
Object-oriented purists might claim that this design is less object oriented than
having such methods directly on the related domain class. Regardless of what is
more or less object oriented , it is a better design choice to separate these concerns.
The primary reason is that business functions are often very complex. They usu-
ally involve more than one class and deal with a number of infrastructural compo-
nents, including databases, message queues, and other systems. Furthermore,
there are often a number of domain classes involved in a business function, which
would make it hard to decide which class the method should belong to. It is for
these reasons that coarse-grained business functions are best implemented as sep-
arate methods on a class that is part of the business logic layer.
Don't be afraid to put finer-grained business logic directly on related domain
classes. The coarse-grained service methods in the business logic layer are free to
call the finer-grained pure logic methods built into domain classes.
In our layered architecture, the business logic layer is the consumer of the per-
sistence layer services. It makes calls to the persistence layer to fetch and change
data. The business logic layer also makes an excellent place to demarcate transac-
tions, because it defines the coarse-grained business functions that can be con-
sumed by a number of different user interfaces or possibly other interfaces, such
as a web service. There are other schools of thought regarding transaction demar-
cation, but we'll discuss the topic more in chapter 8.