I'm wondering if there's a metric similar to the Package Stability Metric defined by Robert Martin that can be used to know when a package should or shouldn't depend on another, by its Instability (I) metric:
Ca = Afferent Couplings
Ce = Efferent Couplings
I = Ce / (Ce+Ca)
But for classes, where instead of being Afferent and Efferent couplings between classes inside a package to classes in other packages; they where afferent and efferent couplings between classes within the same package (maybe and/or also other packages, I don't really know), letting one know if a class should or shouldn't depend on another class, by its 'instability'.
Edit: Supposedly the Instability metric measures the reasons to change : reasons not to change ratio, but now that I think about it, a class should have only 1 reason to change, meaning that if such similar Instability metric existed, a class' I would be 0, but still, some classes do 'use' object instances of other classes, making them dependent of those classes; but I'm uncertain of this, any insights on this?
Afferent and Efferent coupling are valid metrics on a class, and it is possible to calculate Instability for a class. You could use Instability on a class to determine where to focus on creating stable or instable classes - but in practice this could lead to some poor design choices.
For example, instable components should have as few dependers (Afferent coupling) as possible, while Stable classes should have as few dependencies (Efferent coupling) as possible. In a rich domain model , it is quite possible to have bi-directional associations - meaning you are beginning to violate the "rules" associated with the metric no matter whether your classes are intended to be stable or instable. Notice that at a package/component level, any cyclical dependencies are discouraged or even prohibited.
Your efforts are better spent focusing on larger components, i.e. packages or layers. Generally, you want your Domain model should be stable (when you change the domain, you do so because this represents an actual change to the domain, or at least your understanding of the domain). Things that are more likely to change, such as visual elements and data access components are instable, and have dependencies on the domain.
Related
I'm looking the way to understand full influence of changing some class. For example I added new field to DTO class. It is needed to find all web-services and methods that are affected by this change. For a now I need to find all classes that extend changed class, all classes that aggregate this class or any of its children, all classes that use type of changed class or of its children, etc. Risk to lose affected web-services is very high. And if I lose even one web-services it means bug during integration process.
Is there tool that makes this process automatic?
It sounds like you want to use the Dependency Matrix, which is available under the Analyze menu from the action Analyze Dependency Matrix. This shows you efferent and afferent dependencies, along with metrics that can be used to infer the strength of the dependency.
Although an experimented programmer and architect, the same old basic problem comes back recurrently. I have my own religion about it, but I need some authoritative source.
Are anemic data models ( (c) Martin Fowler?) inherently bad? Should a cake be able to bake itself? Should an invoice know how (and when it should allow) to add lines to itself, or should another layer do that? rabbit.addToHole(hole) or hole.addRabbit(rabbit)? Has it been proved that an ADM is more bug-prone, or easier to maintain, or anything?
You can find a lot of claims on the web, but I'd really want some authoritative quotes, references or facts, if possible from both sides.
See this stackoverflow answer for enlightment.
And this is my opinion:
ADM (Anemic Domain Model) cannot be represented with class diagram UML
Anemic domain model is bad, only in terms of full oop. It is considered as bad design, mainly because you cannot create UML classes and relations with embedded behavior inside it. For example, in your Invoice class with Rich Domain Model (RDM):
Class Name: Order
Implemented: ICommittable, IDraftable, ...
Attributes: No, UserId, TotalAmount, ...
Behavior: Commit(), SaveDraft(), ...
The class is self-documented and self explaining about what it can do and what can't.
If it is anemic domain model, it does not has the behavior, and we need to search which class is responsible for Committing and Saving Draft. And since the UML class diagram only shows the relation between each classes (one to many / many to many / aggregate / composite), the relation with service class cannot be documented, and Martin Fowler has his point right.
In general, the more behavior you find in the services, the more
likely you are to be robbing yourself of the benefits of a domain
model. If all your logic is in services, you've robbed yourself blind.
This is based on class diagram UML in OOAD book by Lars Mathiassen. I don't know if newer class diagram UML can represent service class.
SRP
In ADM's point of view and compisition over inheritance, RDM (rich domain model) violates SRP. It may be true, but you can refer to this question for discussion.
Shortly, in ADM's point of view, SRP equals one class doing one thing and one thing only. Any change into the class has one and only one reason.
In RDM's point of view, SRP equals all responsibility related to and only to the interface itself. As soon as the operation involve other class, then the operation need to be put into other interface. The implementation itself may vary, as such if a class can implement 2 or more interfaces. It is simply said as if an operation in interface need to be changed, it is for and only for one reason.
ADM tend to be abused with static methods and dirty hacks may apply
ADM is very easy to be abused with static methods - service class. It can be done with RDM too, but it need another layer of abstraction and not worth it. Static methods are usually a sign of bad design, it reduced testability and may introduce race conditions, as well as hiding the dependency.
ADM can has many dirty hacks because the operations are not being constrained by the object definition (hey, I can create another class for this!). In hand of bad designer, this can become catastrophic. In RDM it is harder, please read next point for information.
RDM's implementation usually cannot be reused and cannot be mocked. RDM require to know the system's behavior beforehand
Usually RDM's implementation cannot be reused and mocked. In TDD manner, it reduced testability (please correct me if there is RDM which can be mocked and reused). Imagine a situation with this inheritance tree:
A
/ \
B C
If B need logic implemented in C, it cannot be done. Using composition over inheritance, it can be achieved. In RDM, it can be done with a design like this:
A
|
D
/ \
B C
In which introduce more inheritance. However, in order to achieve neat design early, you will need to know the system flow firsthand. That said, RDM require you to know the system's behavior before doing any design, or you won't know any of the interfaces named ISubmitable, IUpdateable, ICrushable, IRenderable, ISoluble, etc, suitable for your system.
Conclusion
That's all my opinion about this kind of holy war. Both has pros and cons. I usually go for ADM because it seems like higher flexibility even has less reliability. Regardless of ADM or RDM, if you design your system bad, the maintenance is hard. Any type of chainsaw will only shines when held by skillful carpenter.
I think the accepted answer to this question is one best answering your question too.
Things that I think are esential to remember:
ADM is adequate for CRUD applications, and since most apps start out this way, it's OK as a starting architecture; you can evolve from there via refactoring, if needed, but there's no point of over-designing an application right from the start
once complexity starts to grow - once business rules start to pile up - it's less convenient to keep the model anemic - separating the rules from the objects they act upon makes it hard to remember all rules that apply when you look at the object
if the rules are in the domain objects, they are also conducive to writing tests, if they're elsewhere (say in stateless services), you don't know what a domain object can do and what all constraints that apply to it are, to write proper tests for it (think orthogonal rules modelled in distinct services)
there's a distinction to be made between really simple applications and anemic domain models: in a really simple application, there is not much business logic, in an anemic domain model the logic exists, but is kept separately from the domain model
I was trying to find tutorials and good examples which would explain difference between those two, but not able to find any information.
Pure fabrication and indirection acts to create and assign responsibilities to intermediate object, so could anyone explain what is difference between those design patterns?
Thanks!
You use Indirection if you want to create a lower coupling between components. The example Larman suggests in Applying UML and Patterns is a class TaxCalculatorAdapter. In order to shield clients from having to know inner workings of a possible adapter, he hides them with an indirection, only exposing the required API. This Indirection will be highly coupled to the adaptees, but only loosely coupled to the clients.
The PersistentStorage from Pure Fabrication is indeed an Indirecton (Larman states so in the book) in that it provides lower coupling. Pure Fabrication goes beyond that though in that it creates objects that are not part of your Domain Model.
The example Larman gives is a domain class Sale. Since Sale has all the data to save, it would be a candidate to hold the logic for saving a Sale as well (Information Expert). However, persistence logic is not related to the concept of a Sale, hence the class would become incohesive. Also, by coupling the Sale to a particular DB API, you limit reuse (Indirection to the rescue). And because saving is a general activity, you would likely also duplicate code in objects which also need to be saved. To avoid this, you make something up (the pure fabrication), meaning you create something that is not part of the Domain model (here: a PersistentStorage), but still captures an essential activity in your application.
As such, Pure Fabrication it is a specialization or rather a variant of Indirection.
Pure fabrication and indirection both are principles from GRASP.
Following examples in this dzone article might clear your concept about pure fabrication and indirection.
Pure Fabrication:
We know the domain model for a banking system contains classes like Account, Branch, Cash, Check, Transaction, etc. The domain classes need to store information about the customers. In order to do that one option is to delegate data storage responsibility to domain classes. This option will reduce the cohesiveness of the domain classes (more than one responsibility). Ultimately, this option violates the SRP principle.
Another option is to introduce another class which does not represent any domain concept. In the banking example, we can introduce a class called, PersistenceProvider. This class does not represent any domain entity. The purpose of this class is to handle data storage functions. Therefore PersistenceProvider is a pure fabrication.
Indirection:
This principle answers one question: How do you cause objects to interact in a manner that makes bond among them remain weak?
The solution is: Give the responsibility of interaction to an intermediate object so that the coupling among different components remains low.
For example, a software application works with different configurations and options. To decouple the domain code from the configuration a specific class is added - which shown in the following listing:
Public Configuration{
public int GetFrameLength(){
// implementation
}
public string GetNextFileName(){
}
// Remaining configuration methods
}
In this way, if any domain object wants to read a certain configuration setting it will ask the Configuration class object. Therefore, the main code is decoupled from the configuration code.
If you have read the Pure Fabrication Principle, this Configuration class is an example of pure fabrication. But the purpose of indirection is to create de-coupling. On the other hand, the purpose of pure fabrication is to keep the domain model clean and represent only domain concepts and responsibilities.
Many software design patterns like Adapter, Facade, and Observer are specializations of the Indirection Principle.
Pure fabrication class is a type of class ,which does not concept in a problem domain designed ,This class is assigned with high cohesion ^,low coupling & reuse.
Indirection
It solves the problem of assigning the responsibility of avoiding direct coupling between things.it also ensures low coupling between the objects & maintains higher reside capabilities.
I have problems understanding the statement low in coupling and high in cohesion. I have googled and read a lot about this, but still finding it hard to understand.
To what I understand is High cohesion means, that we should have classes that are specialized to perform a particular function. Hope this is correct? Like a credit card validation class, which is specialized to validate credit cards only.
And still don't understand what low Coupling means?
What I believe is this:
Cohesion refers to the degree to which the elements of a module/class belong together, it is suggested that the related code should be close to each other, so we should strive for high cohesion and bind all related code together as close as possible. It has to do with the elements within the module/class.
Coupling refers to the degree to which the different modules/classes depend on each other, it is suggested that all modules should be independent as far as possible, that's why low coupling. It has to do with the elements among different modules/classes.
To visualize the whole picture will be helpful:
The screenshot was taken from Coursera.
Cohesion in software engineering, as in real life, is how much the elements consisting a whole(in our case let's say a class) can be said that they actually belong together. Thus, it is a measure of how strongly related each piece of functionality expressed by the source code of a software module is.
One way of looking at cohesion in terms of OO is if the methods in the class are using any of the private attributes.
Now the discussion is bigger than this but High Cohesion (or the cohesion's best type - the functional cohesion) is when parts of a module are grouped because they all contribute to a single well-defined task of the module.
Coupling in simple words, is how much one component (again, imagine a class, although not necessarily) knows about the inner workings or inner elements of another one, i.e. how much knowledge it has of the other component.
Loose coupling is a method of interconnecting the components in a system or network so that those components, depend on each other to the least extent practically possible…
I wrote a blog post about this. It discusses all this in much detail, with examples etc. It also explains the benefits of why you should follow these principles.
In software design high cohesion means that class should do one thing and one thing very well. High cohesion is closely related to Single responsibility principle.
Low coupling suggest that class should have least possible dependencies. Also, dependencies that must exist should be weak dependencies - prefer dependency on interface rather than dependency on concrete class, or prefer composition over inheritance .
High Cohesion and low coupling give us better designed code that is easier to maintain.
Short and clear answer
High cohesion: Elements within one class/module should functionally belong together and do one particular thing.
Loose coupling: Among different classes/modules should be minimal dependency.
Low coupling is in the context of two or many modules. If a change in one module results in many changes in other module then they are said to be highly coupled. This is where interface based programming helps. Any change within the module will not impact the other module as the interface (the mean of interaction ) between them has not changed.
High cohesion- Put the similar things together. So a class should have method or behaviors to do related job. Just to give an exaggerated bad example: An implementation of List interface should not have operation related to String. String class should have methods, fields which is relevant for String and similarly, the implementation of List should have corresponding things.
Hope that helps.
Cohesion - how closely related everything is with one another.
Coupling - how everything is connected to one another.
Let's take an example - We want to design a self-driving car.
(1) We need the motor to run properly.
(2) We need the car to drive on its own.
All of the classes and functions in (1) starting the motor and making it run work great together, but do not help the car steer. So we place those classes behind an Engine Controller.
All of the classes and functions in (2) work great to make the car steer, accelerate and brake. They do not help the car start or send gasoline to the pistons. So we place these classes behind its own Driving Controller.
These controllers are used to communicate with all of the classes and functions that are available. The controllers then communicate only with each other. This means I can't call a function in the piston class from the gas pedal class to make the car go faster.
The pedal class has to ask the Driving Controller to talk to the Engine Controller which then tells the piston class to go faster. This allows us programmers to be able to find issues and allows us to combine large programs without worrying. This is because the code was all working behind the controller.
Take the example of an old PC motherboard.
Mouse had its own PS/2 port.
Printer had its own Printer port.
Monitor had its own VGA port.
This meant that a particular port was meant only for a particular device, and for none other.
This is Strong / High Coupling
Since a mouse is used only for operating the cursor and related functionalities, a keyboard for typing keys, etc i.e they perform only the task they are intended for, this is High Cohesion
If a mouse had a few buttons 'a' 'b' 'c' to enter, then it is doing more than what it should, since a keyboard is already performing them, this is Low Cohesion
The outdated usage of exclusive ports was thankfully replaced by a standard (interface) we call USB. This is Loose / Low Coupling
Looking at these physical attributes, it looks obvious that this is how it is supposed to be, but while writing software it is very easy to lose track of what functionality is to be put where, etc. and hence as a reminder, in everything in life, always stick to:
'High Cohesion and Loose Coupling'
Metaphorically, if your cat barks, it has poor cohesion, and if your dog needs a cat by his side to bark, it is highly coupled.
"Dogs bark and cats purr, if they barf your pull request will be rejected"
Long story short, low coupling as I understood it meant components can be swapped out without affecting the proper functioning of a system. Basicaly modulize your system into functioning components that can be updated individually without breaking the system
Do you have a smart phone? Is there one big app or lots of little ones? Does one app reply upon another? Can you use one app while installing, updating, and/or uninstalling another? That each app is self-contained is high cohesion. That each app is independent of the others is low coupling. DevOps favours this architecture because it means you can do discrete continuous deployment without disrupting the system entire.
When I was reading about a microservice. I came across the following things:
Cohesion is a measure of the number of relationships that parts of a component have with each other. High cohesion means that all of the parts that are needed to deliver the component's functionality are included in the component
Coupling is a measure of the number of relationships that one component has with other components in the system. Low coupling means that components do not have many relationships with other components
Inheritance or generalization is an example of high coupling (i.e. high interdependence). What I meant by this is that in inheritance often the parent class defines base functionalities that is used by its child class and change in methods of parent class directly impact its child classes. Hence we can say that there is a greater degree of interdependence between classes.
Realization or using interface is an example of high cohesion (i.e. low interdependence). What this means is that an interface put forward a contract for any class that implements it but each class has the right to implement methods declared in interface in its own way and changes in method declared in one class doesn't affect any other class.
Low Coupling:--
Will keep it very simple.
If you change your module how does it impact other modules.
Example:-
If your service API is exposed as JAR, any change to method signature will break calling API (High/Tight coupling).
If your module and other module communicate via async messages. As long as you get messages, your method change signature will be local to your module (Low coupling).
Off-course if there is change in message format, calling client will need to make some change.
Low Coupling and High Cohesion is a recommended phenomenon.
Coupling means to what extent various modules are interdependent and how the other modules are affected on changing some/considerable functionality of a module. Low coupling is emphasized as the dependency has to be maintained low so that very least/negligible changes are made to other modules.
An example might be helpful. Imagine a system which generates data and puts it into a data store, either a file on disk or a database.
High Cohesion can be achieved by separate the data store code from the data production code. (and in fact separating the disk storage from the database storage).
Low Coupling can be achieved by making sure that the data production doesn't have any unnecessary knowledge of the data store (e.g. doesn't ask the data store about filenames or db connections).
Here is an answer from a bit of an abstract, graph theoretic angle:
Let's simplify the problem by only looking at (directed) dependency graphs between stateful objects.
An extremely simple answer can be illustrated by considering two limiting cases of dependency graphs:
The 1st limiting case: a cluster graphs .
A cluster graph is the most perfect realisation of a high cohesion and low coupling (given a set of cluster sizes) dependency graph.
The dependence between clusters is maximal (fully connected), and inter cluster dependence is minimal (zero).
This is an abstract illustration of the answer in one of the limiting cases.
The 2nd limiting case is a fully connected graph, where everything depends on everything.
Reality is somewhere in between, the closer to the cluster graph the better, in my humble understanding.
From another point of view: when looking at a directed dependency graph, ideally it should be acyclic, if not then cycles form the smallest clusters/components.
One step up/down the hierarchy corresponds to "one instance" of loose coupling, tight cohesion in a software but it is possible to view this loose coupling/tight cohesion principle as a repeating phenomena at different depths of an acyclic directed graph (or on one of its spanning tree's).
Such decomposition of a system into a hierarchy helps to beat exponential complexity (say each cluster has 10 elements). Then at 6 layers it's already 1 million objects:
10 clusters form 1 supercluster, 10 superclusters form 1 hypercluster and so on ... without the concept of tight cohesion, loose coupling, such a hierarchical architecture would not be possible.
So this might be the real importance of the story and not just the high cohesion low coupling within two layers only. The real importance becomes clear when considering higher level abstractions and their interactions.
I think you have red so many definitions but in the case you still have doubts or In case you are new to programming and want to go deep into this then I will suggest you to watch this video,
https://youtu.be/HpJTGW9AwX0
It's just reference to get more info about polymorphism...
Hope you get better understanding with this
What are Cohesion and Decoupling? I found information about coupling but not about decoupling.
That article from Aaron is very good for understanding, also I'd recommend that you read manning publications Spring in Action book, they give very good examples on how the spring solves that problem it will definitely improve your understanding of this.
EDIT :
I came accross this in this great book called Growing object oriented software guided by tests :
Coupling :
Elements are coupled if a change in
one forces a change in the other. For
example, if two classes inherit from a
common parent, then a change in one
class might require a change in the
other. Think of a combo audio system:
It’s tightly coupled because if we
want to change from analog to digital
radio, we must rebuild the whole
system. If we assemble a system from
separates, it would have low coupling
and we could just swap out the
receiver. “Loosely” coupled features
(i.e., those with low coupling) are
easier to maintain.
Cohesion:
An element’s cohesion is a measure
of whether its responsibilities form a
meaningful unit. For example, a class
that parses both dates and URLs is not
coherent, because they’re unrelated
concepts. Think of a machine that
washes both clothes and dishes—it’s
unlikely to do both well.2 At the
other extreme, a class that parses
only the punctuation in a URL is
unlikely to be coherent, because it
doesn’t represent a whole concept. To
get anything done, the programmer will
have to find other parsers for
protocol, host, resource, and so on.
Features with “high” coherence are
easier to maintain.
Cohesion - related to the principle that a class/method should be responsible for one thing only i.e there are no stray methods that don't belong in the encapsulation; a method only does one thing. High/Low cohesion is the degree to which this holds.
Coupling - how interdependent different parts of the system are. e.g how and where there are dependencies. If two classes make calls to methods of each other then they are tightly coupled, as changing one would mean having to change the other. Decoupling is the process of making something that was tightly coupled less so, or not at all.
Flexible systems have High Cohesion and Loose Coupling.
For coupling, this Wikipedia article should answer all your questions. This article deals with cohesion.
"Decoupling" is just another name for "little/low coupling".
So these terms answer these questions:
How much does each part of your project depend on another part?
If you wanted to use just a part of your project (like to solve a specific problem) how much do you need to know about all the rest of the project?
Is every part of your project focused on a single solution to a specific problem or do solutions "leak" to other parts?
Here are my thoughts on cohesion. Imagine there is a module. Inside that module, we have some tasks. When those tasks are highly related to each other, we say it has high cohesion. When those tasks are not related, we say it has low cohesion. My best attempt to explain decoupling is that decoupling is the act of removing coupling.
Low Coupling helps us get to high cohesion! Remember that we want our module to have related tasks and one single responsibility. But what is coupling? Coupling is the degree of dependency on other modules to achieve our single responsibility for that module. So by low coupling, we are saying that we are not very dependent on external modules hence we have high cohesion.
However, if we have many dependencies to external modules, we would have high coupling and low cohesion. Get it?
Other more decorated thinkers and groups say:
Cohesion is the degree to which the tasks performed by a single module are functionally related." IEEE, 1983 "Cohesion is the "glue" that holds a module together. It can be thought of as the type of association among the component elements of a module. Generally, one wants the highest level of cohesion possible." Bergland, 1981
A software component is said to exhibit a high degree of cohesion if the elements in that unit exhibit a high degree of functional relatedness. This means that each element in the program unit should be essential for that unit to achieve its purpose. Sommerville, 1989
decoupling allows the separation of object interaction from classes and inheritance into distinct layers of abstraction used to polymorphic-ally decouple the encapsulation which is the practice of using re-usable code to prevent discrete code modules from interacting with each other.