Does the following fit the definition of a bi-directional business rule?
An employee of the car rental company handles the client but each client can only be handled by one employee.
Directionality is often important to consider in UML and database modeling contexts. These models are then used as the base models for business rule systems. In the context of business rules, directionality allows you to navigate from some some object to another object that it nows about. So in your example you could start with the employee and write some rule about the client:
if the client of the employee is a relative of the employee
then
raise an audit exception.
Here you started with the employee and navigated to the client with the "of the" phrase.
The reverse navigation might be:
if the employee serving the client is a relative of the client
then
raise an audit exception
So here you have bi-directional navigation while authoring the business rules. This might be important and provide some value to the rule authors so that they can start their authoring with either the employee or the client, or it might not. That is a decision you would have to make while designing the rule authoring language.
Another possibility is that you are actually expressing the cardinality of the relationships that are possible in the base model by using rules. But this is not a frequent use case. If you think about it, this kind of cardinality expression is not likely to change much and probably shouldn't be handled at the business rule level. For example, do you need to give the business rule authors the ability to express and change the fact that an employee can only handle one client versus some point in the future where they can handle 1 .. N clients? That might make sense in a non-operational decision model, but will not likely come into play with operational business rule systems.
Related
In brushing up on the SRP I read this document which I located via Uncle Bob's page on principles of OOD. I find the following passage puzzling and somewhat at odds with the rest of the document:
"If, on the other hand, the application is not changing in ways that cause the the two responsibilities to change at different times, then there is no need to separate them. Indeed, separating them would smell of Needless Complexity. There is a corollary here. An axis of change is only an axis of change if the changes actually occur. It is not wise to apply the SRP, or any other principle for that matter, if there is no symptom."
While I understand the answer to many software development questions is "it depends" principles like the SRP appear to be almost universally beneficial and to be implemented as a matter of course. The SRP itself affords code a high adaptability to future changes in requirements. Isn't the point to separate out responsibilities from the get-go to avoid struggling with highly coupled code and cascading changes later on?
I would really appreciate some clarification on this to make sure my understanding of this core principle is correct. Thanks in advance!
From my humble understanding, in the Modem example that is presented here, it might be possible that responsibilities of the modem (Connection and Data Exchange) will change as one.
You have two possibilities here :
When the protocol changes, it is possible that only the connection part change, or only the data exchange part change. It this case you should have two interfaces, because a change of protocol in the data does not imply a change of protocol in the connection.
When the protocol changes, it will always change the connection part and the data exchange part. In that case, you don't need two interfaces, because everytime you will have to rewrite the connection part, you are sure that the data exchange will change as well. In that case, you have two responsibilities put on the same change Axis (which is the protocol handled by the modem), so you can leave them inside a single interface.
The key to this statement is "not changing in ways that cause the two responsibilities to change at different times". Let's say for the sake of argument you have a PaymentLogger and a Payment class. Every time you create a new PaymentType (CreditCard, Cash, Paypal, etc) you need to update the PaymentLogger to log actions specific to those Payments. Instead of splitting out a PaymentLogger class you could have Payment class have a method called Log which does whatever is specific for itself.
In this case it could be that the act of recording actions should be build into the class itself since creating a new Payment requires also creating a new PaymentLogger. It's a responsibility that should have been part of Payment all along.
I need to express
A passing grade in a pre-requite class, in order to access the next one.
An exemption policy to the above, where the professor can override the requirement.
A student can only have one such exemption in a given span of time (year)
Students must be older than 21
Students can only be enrolled in 3 eligible majors
Or similarly, the NY DMV has a complex set of documents and prerequisites in order to obtain a driver's license.
Question
What abstract syntax will allow me to describe the rules listed above, in a platform neutral way?
Use case
I'm developing an app where people define eligibility rules to access certain data. Those rules may have prerequisites, or key/value ranges to access it.
I am thinking XML, but that might become cumbersome. Just as swagger has become the "easier way" to handle web services, I'm looking for a similar "easy" syntax to handle these rules.
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Where I work, we've gone back and forth on this subject a number of times and are looking for a sanity check. Here's the question: Should Business Objects be data containers (more like DTOs) or should they also contain logic that can perform some functionality on that object.
Example - Take a customer object, it probably contains some common properties (Name, Id, etc), should that customer object also include functions (Save, Calc, etc.)?
One line of reasoning says separate the object from the functionality (single responsibility principal) and put the functionality in a Business Logic layer or object.
The other line of reasoning says, no, if I have a customer object I just want to call Customer.Save and be done with it. Why do I need to know about how to save a customer if I'm consuming the object?
Our last two projects have had the objects separated from the functionality, but the debate has been raised again on a new project. Which makes more sense?
EDIT
These results are very similar to our debates. One vote to one side or another completely changes the direction. Does anyone else want to add their 2 cents?
EDIT
Eventhough the answer sampling is small, it appears that the majority believe that functionality in a business object is acceptable as long as it is simple but persistence is best placed in a separate class/layer. We'll give this a try. Thanks for everyone's input...
Objects are state and behavior together. If an object has sensible behavior (e.g., calculating age for a Person from their birth date, or a total tax for an Invoice), by all means add it. Business objects that are nothing more than DTOs are termed an "anemic domain model." I don't think it's a design requirement.
Persistence is a special kind of behavior. What I'm calling "sensible" is business behavior. A business object need not know that it's persistent. I'd say that a DAO can keep persistence separate from business behavior. I don't put "save" in the "sensible" category.
Business objects CAN have business functionality.
Persistence is not a business functionality , but is technical implementation.
Long story short:
Save/Update/Delete/Find etc - keep away from the business objects in a persistence layer.
CalculateSalary, ApplyDiscount etc are business related methods and can be:
methods of the business objects (so BO is self contained representation of entity) or;
separate services implementing particular functionality (so BOs are acting more like DTOs).
As for the point 2.
I should mention that the approach 2.1 tends to make the BOs too bloated and violate SRP. While 2.2 introduces more maintenance complexity.
I usually balance in between 2.1 and 2.2 so that I put trivial things related to the data into Business Objects and create services for a bit more complex scenarious (if there are more than 4 lines of code - make it a service).
This shifts the paradigm of Business Objects to be more Data Transfer Objects instead.
But this all makes project easier to develop, test and maintain.
The answer is the same regardless of platform or language. The key to this question is whether an object should be able to be autonomous or whether it is better for any given behavior to be spread out among objects with more focused responsibility.
For each class the answer might be different. We end up with a spectrum along which we can place classes based upon the Density of Responsibility.
(Level of responsibility for behavior)
Autonomy - - - - - - - - - - - - - - - - - - - Dependence
High
C - <<GOD object>> <<Spaghetti code>>
l -
a -
s -
s -
-
s -
i -
z -
e - <<Template>> <<Framework>>
low
Let's say you favor letting the class perform all the behaviours itself, or as many as you can. Starting on the left side of this graph, when you make your class more autonomous, the size of the class will grow unless you continuously refactor it to make it more generic. This leads to a template. If no refactoring is done, the temdency is for the class to become more "god-like" because if there is some behavior it needs, it has a method for that. The number of fields and methods grow and soon become both unmanageable and unavoidable. Since the class already does so much, coders would rather add to the monstrosity than try to piece it apart and cut the Gordian knot.
The right side of the graph has classes that depend on other classes to a large degree. If the dependency level is high but the individual class is small, that is a sign of a framework; each class doesn't do much and requires lots of dependent classes to accomplish some function. On the other hand, a highly-dependent class that also has a large amount of code is a sign that the class is full of Spaghetti.
The key to this question is to determine where you feel more comfortable on the graph. In any event, individual classes will end up spread out on the graph unless some organizational principle is applied, which is how you can achieve the results of Template or Framework.
Having just written that, I would say that there is a correlation between class size and degree of organization. Robert C. Martin (or "Uncle Bob") covers similar ground with package dependencies in his very thorough paper on Design Principles and Design Patterns. JDepend is an implementation of the ideas behind the graph on page 26 and complements static analysis tools such as Checkstyle and PMD.
I think it makes more sense for business objects to know how to "handle" themselves, then to have to put that burden elsewhere in the system. In your example, the most logical place to deal with how to "save" customer data would be, to me, in the Customer object.
This may be because I consider the database to be the "data container", so I'm in favor of "business objects" being the higher level that protects the data container from direct access AND enforces standard "business rules" about how that data is accessed/manipulated.
We've used Rocky Lhotka's CSLA framework for years and love the way it is designed. In that framework all of the functionality is contained in the objects. While I can see the value of separting the logic out, I don't think we'll switch away from this philosophy anytime soon.
Business objects should be about encapsulating data and associated behaviors of the business entity modeled by that object. Think of it like this: one of the major tenets of object-oriented programming is encapsulating data and associated behaviors on that data.
Persistence is not a behavior of the modeled object. I find development progresses more smoothly if business objects are persistence ignornant. Developing new code and unit testing new code happen more quickly and more smoother if the business objects are not specifically tied to the underlying plumbing. This is because I can mock those aspects and forget about having to go through hoops to get to the database, etc. My unit tests will execute more quickly (a huge plus if you have thousands of automated tests that run with each build) and I will have less stress because I won't have tests failing because of database connection issues (great if you often work offline or remotely and can't always access your database and oh, by the way, those aspects (database connectivity etc.) should be tested elsewhere!).
The other line of reasoning says, no, if I have a customer object I just want to call Customer.Save and be done with it. Why do I need to know about how to save a customer if I'm consuming the object?
Knowing that Customer has a Save method is already knowing how to save a customer object. You haven't avoided the problem by embedding that logic in your business object. Instead, you've made your code base more tightly coupled and therefore harder to maintain and test. Push off the responsibility of persisting the object to someone else.
The Business objects, as they are named, should obviously coutain their own business logic, the dynamic of the business logic among the domain being in the service layer.
On the other side, could the BO be a data container (DTO?) composition and methods; meaning BO are pure functionnal? That could avoid all the conversions between BO and DTO.
In an MVC architecture,
Can we say that Model contains business objects.
I know the rule of thumb is that a noun used by the user is potentially a class. Similarly, a verb may be made into an action class e.g. predicate
Given a description from the user, how do you -
identify what is not not to be made into a class
The only real answer is experience. However, some things fairly obviously (to me, anyway) cannot be modelled in your design. For example if the use case says:
"and then the parcel is put on the UPS van"
There is no need to model the van. You can make decisions of this kind by considering the system boundaries - you don't and can't control the van. However,
"we make a request to UPS for pickup"
might well result in a UPSPickup object.
The rules are simple.
Everything is an object.
All objects belong to classes.
In rare (very rare circumstances) you have some specialized class/object confusion.
A "library" of all static methods. This is an implementation choice, and no user can see this.
A Singleton where there can only be one object of the given class. This does happen sometimes.
In an OO language it is not a question of what to be made into a class, but rather 'what class does this data/functionality go into?'
Like other software architecture aspects there are rules, but ultimately it is an art that requires experience. There are lots of books on software design, but a simple reference is Coupling and Cohesion.
Cohesion of a single module/component is the degree to which its responsibilities form a meaningful unit; higher cohesion is better.
Coupling between modules/components is their degree of mutual interdependence; lower coupling is better.
It seems that the decision to make your objects fully cognizant of their roles within the system, and still avoid having too many dependencies within the domain model on the database, and service layers?
For example: Say that I've got an entity with a revision history, and several "lookup tables" that the data references, your entity object should have methods to get the details from some of the lookup tables, whether by providing access to the lookup table rows, or by delegating methods down to them, but in order to do so it depends on the database layer to read the data from those rows. Also, when the entity is saved, It needs to know not only how to save itself, but also to save entries into the revision history. Is it necessary to pass references to dozens of different data layer objects and service objects to the model object? This seems like it makes the logic far more complex to understand than just passing back and forth thin models to service layer objects, but I've heard many "wise men" recommending this sort of structure.
Really really good question. I have spent quite a bit of time thinking about such topics.
You demonstrate great insight by noting the tension between an expressive domain model and separation of concerns. This is much like the tension in the question I asked about Tell Don't Ask and Single Responsibility Principle.
Here is my view on the topic.
A domain model is anemic because it contains no domain logic. Other objects get and set data using an anemic domain object. What you describe doesn't sound like domain logic to me. It might be, but generally, look-up tables and other technical language is most likely terms that mean something to us but not necessarily anything to the customers. If this is incorrect, please clarify.
Anyway, the construction and persistence of domain objects shouldn't be contained in the domain objects themselves because that isn't domain logic.
So to answer the question, no, you shouldn't inject a whole bunch of non-domain objects/concepts like lookup tables and other infrastructure details. This is a leak of one concern into another. The Factory and Repository patterns from Domain-Driven Design are best suited to keep these concerns apart from the domain model itself.
But note that if you don't have any domain logic, then you will end up with anemic domain objects, i.e. bags of brainless getters and setters, which is how some shops claim to do SOA / service layers.
So how do you get the best of both worlds? How do you focus your domain objects only domain logic, while keeping UI, construction, persistence, etc. out of the way? I recommend you use a technique like Double Dispatch, or some form of restricted method access.
Here's an example of Double Dispatch. Say you have this line of code:
entity.saveIn(repository);
In your question, saveIn() would have all sorts of knowledge about the data layer. Using Double Dispatch, saveIn() does this:
repository.saveEntity(this.foo, this.bar, this.baz);
And the saveEntity() method of the repository has all of the knowledge of how to save in the data layer, as it should.
In addition to this setup, you could have:
repository.save(entity);
which just calls
entity.saveIn(this);
I re-read this and I notice that the entity is still thin because it is simply dispatching its persistence to the repository. But in this case, the entity is supposed to be thin because you didn't describe any other domain logic. In this situation, you could say "screw Double Dispatch, give me accessors."
And yeah, you could, but IMO it exposes too much of how your entity is implemented, and those accessors are distractions from domain logic. I think the only class that should have gets and sets is a class whose name ends in "Accessor".
I'll wrap this up soon. Personally, I don't write my entities with saveIn() methods, because I think even just having a saveIn() method tends to litter the domain object with distractions. I use either the friend class pattern, package-private access, or possibly the Builder pattern.
OK, I'm done. As I said, I've obsessed on this topic quite a bit.
"thin models to service layer objects" is what you do when you really want to write the service layer.
ORM is what you do when you don't want to write the service layer.
When you work with an ORM, you are still aware of the fact that navigation may involve a query, but you don't dwell on it.
Lookup tables can be a relational crutch that gets used when there isn't a very complete object model. Instead of things referencing things, you have codes, which must be looked up. In many cases, the codes devolve to little more than a static pool of strings with database keys. And the relevant methods wind up in odd places in the software.
However, if there is a more complete object model, we have first-class things instead of these degenerate lookup values.
For example, I've got some business transactions which have one of n different "rate plans" -- a kind of pricing model. Right now, the legacy relational database has the rate plan as a lookup table with a code, some pricing numbers, and (sometimes) a description.
[Everyone knows the codes -- the codes are sacred. No one is sure what the proper descriptions should be. But they know the codes.]
But really, a "rate plan" is an object that is associated with a contract; the rate plan has the method that computes the final price. When an app asks the contract for a price, the contract delegates some of the pricing work to the associated rate plan object.
There may have been some database query going on to lookup the rate plan when producing a contract price, but that's incidental to the delegation of responsibility between the two classes.
I aggree with DeadBeef - therein lies the tension. I don't really see though how a domain model is 'anemic' simply because it doesn't save itself.
There has to be much more to it. ie. It's anemic because the service is doing all the business rules and not the domain entity.
Service(IRepository) injected
Save(){
DomainEntity.DoSomething();
Repository.Save(DomainEntity);
}
'Do Something' is the business logic of the domain entity.
**This would be anemic**:
Service(IRepository) injected
Save(){
if(DomainEntity.IsSomething)
DomainEntity.SetItProperty();
Repository.Save(DomainEntity);
}
See the inherit difference ? I do :)
Try the "repository pattern" and "Domain driven design". DDD suggests to define certain entities as Aggregate-roots of other objects. Each Aggregate is encapsulated. The entities are "persistence ignorant". All the persistence-related code is put in a repository object which manages Data-access for the entity. This way you don't have to mix persistence-related code with your business logic. If you are interested in DDD, check out eric evans book.