Domain Logic Patterns

Domain Logic Patterns
by Pavlo Livchak,
Software Engineer

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Contents
1. What is domain logic?
2. Domain logic patterns:
1. Transaction script
2. Domain model
3. Table module
4. Service layer
3. Conclusion

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What is domain logic?
▪ Domain logic describe the functional algorithms or business logic that handle the information
exchange between a database and a user interface.
▪ The Domain logic patterns examine different ways to implement the logic for your application.
▪ Main concern is the complexity of the application’s logic.

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Domain logic patterns
● Transaction Script
● Domain Model
● Table Module
● Service Layer
Taken from “Patterns of Enterprise Application Architecture” by Martin
Fowler

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Transaction script
● Organizes business logic by procedures where each procedure handles a single request
from the presentation
● A Transaction Script organizes all this logic primarily as a single procedure, making calls
directly to the database or through a thin database wrapper.
Transaction script

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Transaction script
Advantages:
● It is independent from other transactions
● Very simple to implement and understand
Disadvantages:
● Code duplication
For more complex domain logic, Domain model pattern should be used instead.

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Domain Model
● An object model of the domain that incorporates both behavior and data.

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Types of Domain model
● Simple Domain model
○ Looks very much like the database design with mostly one domain object for each
database table
○ Uses Active record
● Rich Domain model
○ Looks different from the database design, with inheritance, strategies, and other
patterns, and complex webs of small interconnected objects
○ Uses Data mapper

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When to use it?
● In cases, when you have complicated and changing business rules involving additional
operations like validation, calculation.

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Table module
▪ The Table Module pattern takes the middle road between a Domain Model and
Transaction Script.
▪ Using this approach the logic for the application is implemented as a set of components
called Table Modules.
▪ Your solution would contain a Table Module for each Table in your database. These
Table Modules implement the functionality related to the entity described in the table.

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When to use it?
▪ Table module is very much based on table-oriented data, so we can use it when we
access tabular data using Record Set.
▪ Table module allows you to fit business logic into the application in a well-organized
manner, without losing the way the elements work on the tabular data.

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Service layer
▪ Service Layer defines an application’s boundary with a layer of services that establishes
a set of available operations and coordinates the application’s response in each
operation.
▪ It encapsulates the application’s business logic, controlling transactions and
coordinating responses in the implementation of its operations.

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Service layer

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Service layer
2 Basic Implementation variations are:
● Domain facade approach
○ Service Layer is implemented as a set of thin facades over a Domain model
○ The classes implementing the facades don’t implement any business logic. All logic
is implemented in Domain Model
● Operation script approach
○ Service Layer is implemented as a set of thicker classes that directly implement
application logic but delegate to encapsulated domain object classes for domain
logic.
○ The operations available to clients of a Service Layer are implemented as scripts,
organized several to a class defining a subject area of related logic.

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When to use it?
▪ The benefit of Service Layer is that it defines a common set of application operations
available to many kinds of clients and it coordinates an application’s response in each
operation.
▪ In an application with more than one kind of client of its business logic, and complex
responses in its use cases involving multiple transactional resources, it makes a lot of
sense to include a Service Layer with container-managed transactions, even in an
undistributed architecture.

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Conclusion
● Transaction Script: This pattern abstracts each business transaction into a Script object. The
transactions are then performed by running the script.
● Domain Model: This pattern manages the application's complexity within an object oriented
model. Using this approach the business functionality is implemented within the model.
● Table Module: Using this pattern, the data related to the business entities is contained within
DataSet, while the logic is maintained with Table Modules. Each Table Module contains
methods that act upon the DataSet.
● Service Layer: This pattern provides a layer through which client applications can access the
services offered by an application (or service).

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