Organizing Domain Logic in Java

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2 Organizing Domain Logic
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In organizing domain logic I've separated it into three primary patterns: Transaction Script (110), Domain Model (116), and Table Module (125)
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The simplest approach to storing domain logic is the Transaction Script (110) A Transaction Script (110) is essentially a procedure that takes the input from the presentation, processes it with validations and calculations, stores data in the database, and invokes any operations from other systems It then replies with more data to the presentation, perhaps doing more calculation to help organize and format the reply The fundamental organization is of a single procedure for each action that a user might want to do Hence, we can think of this pattern as being a script for an action, or business transaction It doesn't have to be a single inline procedure of code Pieces get separated into subroutines, and these subroutines can be shared between different Transaction Scripts (110) However, the driving force is still that of a procedure for each action, so a retailing system might have Transaction Scripts (110) for checkout, for adding something to the shopping cart, for displaying delivery status, and so on
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A Transaction Script (110) offers several advantages:
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It's a simple procedural model that most developers understand It works well with a simple data source layer using Row Data Gateway (152) or Table Data Gateway (144) It's obvious how to set the transaction boundaries: Start with opening a transaction and end with closing it It's easy for tools to do this behind the scenes
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Sadly, there are also plenty of disadvantages, which tend to appear as the complexity of the domain logic increases Often there will be duplicated code as several transactions need to do similar things Some of this can be dealt with by factoring out common subroutines, but even so much of the duplication is tricky to remove and harder to spot The resulting application can end up being quite a tangled web of routines without a clear structure
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Of course, complex logic is where objects come in, and the object-oriented way to handle this problem is with a Domain Model (116) With a Domain Model (116) we build a model of our domain which, at least on a first approximation, is organized primarily around the nouns in the domain Thus, a leasing system would have classes for lease, asset, and so forth The logic for handling validations and calculations would be placed into this domain model, so shipment object might contain the logic to calculate the shipping charge for a delivery There might still be routines for calculating a bill, but such a procedure would quickly delegate to a Domain Model (116) method
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Using a Domain Model (116) as opposed to a Transaction Script (110) is the essence of the paradigm shift that object-oriented people talk about so much Rather than one routine having all the logic for a user action, each object takes a part of the logic that's relevant to it If you're not used to a Domain Model (116), learning to work with one can be very frustrating as you rush from object to object trying to find where the behavior is
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It's hard to capture the essence of the difference between the two patterns with a simple example, but in the
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discussions of the patterns I've tried to do that by building a simple piece of domain logic both ways The easiest way to see the difference is to look at sequence diagrams for the two approaches (Figures 21 and 22) The essential problem is that different kinds of product have different algorithms for recognizing revenue on a given contract (see 9, page 109, for more background) The calculation method has to determine what kind of product a given contract is for, apply the correct algorithm, and then create revenue recognition objects to capture the results of the calculation (For simplicity I'm ignoring the database interaction issues)
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