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principles:model_principle [2018-12-08 10:43] – [Description Status] christianprinciples:model_principle [2021-09-02 20:01] – old revision restored (2021-05-11 22:08) 65.21.179.175
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 Real world actions are then //mirrored// in the software system. This means that each action in the real world triggers a corresponding action in the model world which ensures that the model stays consistent with the real world. So the software is a kind of a simulation of what actually happens. If customer orders some product, the software reacts by creating an order object, which is connected to the customer and the product objects corresponding to the customer and the product involved in the real world action. Real world actions are then //mirrored// in the software system. This means that each action in the real world triggers a corresponding action in the model world which ensures that the model stays consistent with the real world. So the software is a kind of a simulation of what actually happens. If customer orders some product, the software reacts by creating an order object, which is connected to the customer and the product objects corresponding to the customer and the product involved in the real world action.
  
-Be precise with semantics. An operation ''cancelOrder()'' should get an ''Order'' or ''OrderId'' as a parameter. In some cases you might be inclined to supply a ''Product'' instead, maybe because you already have a variable with that object at hand. This might work totally fine in the concrete situation but in fact it only works "by accident". Similarly you might be inclined to invoke ''cancelOrder()'' in a situation when you want to delete the order from the repository. This might work as expected but actually ''cancelOrder'' is semantically slightly different from ''deleteOrder''. So even if ''cancelOrder'' currently does nothing but to call ''deleteOrder'' you should call the method on the correct abstraction level that has precisely the right semantics.+FIXME more detailed description
  
-Keep sure that your software models the reality by invoking the method that has the correct semantics and supplying it with the parameters that are needed from a requirements perspective. 
  
 ===== Rationale ===== ===== Rationale =====
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 When the structures in the software roughly correspond to the structures of the problem domain, a developer doesn't have to learn both of them. Knowing the problem domain is inevitably necessary. Any further structure of the software has to be learned and understood in addition. So creating a direct mapping between them, makes understanding the software easier, which improves maintainability. In such a system for most functionality there is a "natural", i.e. an intuitively clear place to implement it. This makes structuring the software easier and helps finding the implementation for a given functionality. When the structures in the software roughly correspond to the structures of the problem domain, a developer doesn't have to learn both of them. Knowing the problem domain is inevitably necessary. Any further structure of the software has to be learned and understood in addition. So creating a direct mapping between them, makes understanding the software easier, which improves maintainability. In such a system for most functionality there is a "natural", i.e. an intuitively clear place to implement it. This makes structuring the software easier and helps finding the implementation for a given functionality.
  
-Moreover if something works accidentally, it breaks accidentally. Many many bugs are created because you are not precise with semantics. 
  
-In the example above supplying a ''Product'' to the ''cancelOrder'' method only works because by some circumstance it is made sure that  when ''cancelOrder'' is called, there is only one order for that particular product. It's a hidden precondition. As the software is changed, this hidden precondition may not be guaranteed anymore. This results in a non-obvious bug in a part of the system you haven't directly touched. Similarly in the ''deleteOrder'' example: The ''cancelOrder'' operation might get enhanced by creating a reverse invoice, a credit note, or a message to the customer. But an order might have to be deleted for purely technical reasons (migration to a new order system, etc.). So if you call ''cancelOrder'' instead of ''deleteOrder'' this will produce nasty bugs if ''cancelOrder'' gets enhanced as described.  
 ===== Strategies ===== ===== Strategies =====
  
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   * Create methods corresponding to real-world actions   * Create methods corresponding to real-world actions
   * Map additionally necessary behavior to natural classes instead of creating artificial classes   * Map additionally necessary behavior to natural classes instead of creating artificial classes
-  * For artificial behavior that cannot be mapped to a natural class at least create a metaphor or an artificial model (like a state machine) +  * For artificial behavior that cannot be mapped to a natural class at least create a metaphor or an artificial model (like for example a state machine) 
-  * Be precise with semantics. If you have an operation that currently does what you need but for slightly different reasons because it's an operation on the wrong abstraction level, create a new operation with the correct semantics. Have that new operation call the existing one as an implementation detail (e.g. have a ''cancelOrder'' method call the ''deleteOrder'' method).+
  
 ===== Caveats ===== ===== Caveats =====
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 > "**Object-oriented programming.** A program execution is regarded as a physical model, simulating the behavior of either a real or imaginary part of the world."((see for example Ole Lehrmann Madsen, Birger Møller-Pedersen and Kristen Nygaard: //[[ftp://ftp.daimi.au.dk/pub/beta/betabook/betabook.pdf|Object-Oriented Programming in the BETA Programming Language]]//)). > "**Object-oriented programming.** A program execution is regarded as a physical model, simulating the behavior of either a real or imaginary part of the world."((see for example Ole Lehrmann Madsen, Birger Møller-Pedersen and Kristen Nygaard: //[[ftp://ftp.daimi.au.dk/pub/beta/betabook/betabook.pdf|Object-Oriented Programming in the BETA Programming Language]]//)).
  
-Although this view is disputed as a definition for object-oriented programming, it became the key idea of object-oriented analysis. In ((Grady Booch: //[[resources:Object-Oriented Design with Applications]]//, p. 191)) Grady Booch clearly states that objects "directly reflect our model of reality".+Although this view is disputed as a definition for object-oriented programming, it became the key idea of object-oriented analysis. In ((Grady Booch: //Object Oriented Design with Applications//, p. 191)) Grady Booch clearly states that objects "directly reflect our model of reality".
  
 ===== Evidence ===== ===== Evidence =====
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   * **[[Tell, don't Ask/Information Expert]] (TdA/IE)**: TdA/IE tells how to distribute functionality among the natural classes which are created according to the Model Principle.   * **[[Tell, don't Ask/Information Expert]] (TdA/IE)**: TdA/IE tells how to distribute functionality among the natural classes which are created according to the Model Principle.
   * [[Low Coupling]] (LC): When designing a model for a software, it has to be borne in mind that structures with low coupling are desirable.   * [[Low Coupling]] (LC): When designing a model for a software, it has to be borne in mind that structures with low coupling are desirable.
-  * [[Law of Leaky Abstractions]] (LLA): When building abstractions according to MP, keep in mind that there will most likely be abstraction leaks. A good abstraction minimizes those leaks.+
  
 ==== Principle Collections ==== ==== Principle Collections ====
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 /* Choose one of the following and comment out the rest: */ /* Choose one of the following and comment out the rest: */
 /*[[wiki:Stub]]*/ /*[[wiki:Stub]]*/
-/*[[wiki:Incomplete]]*/ +[[wiki:Incomplete]] 
-[[wiki:Complete]]+/*[[wiki:Complete]]*/
  
  
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   * [[http://martinfowler.com/articles/dblogic.html|Martin Fowler: Domain Logic and SQL]] (other possible to implement a system and their difference to domain models)   * [[http://martinfowler.com/articles/dblogic.html|Martin Fowler: Domain Logic and SQL]] (other possible to implement a system and their difference to domain models)
  
-===== Discussion ===== 
- 
-Discuss this wiki article and the principle on the corresponding [[talk:principles:Model Principle|talk page]]. 
principles/model_principle.txt · Last modified: 2021-10-18 21:47 by christian