Related
Well from the book POEAA, Martin Fowler introduced this idea of Unit of Work. It works very well if you want to have auto-commit system, in which your domain model uses Unit of work to label itself as new, dirty, removed or clean. Then you only need to call UnitofWork.commit() and all changes of models will be saved. Below is a domain model class with such methods:
public abstract class DomainModel{
protected void markNew(){
UnitOfWork.getCurrent().registerNew(this);
}
protected void markDirty(){
UnitOfWork.getCurrent().registerDirty(this);
}
protected void markRemoved(){
UnitOfWork.getCurrent().registerRemoved(this);
}
protected void markClean(){
UnitOfWork.getCurrent().registerClean(this);
}
}
With this implementation, you can mark a domain model as any save state through business logic method:
public class Message extends DomainModel{
public void updateContent(User user, string content){
// This method update message content if the the message posted time is not longer than 24 hrs, and the user has permission to update messate content.
if(!canUpdateContent(user) && timeExpired()) throw new IllegalOperationException("An error occurred, cannot update content.");
this.content = content;
markDirty();
}
}
At first glance, it looks marvelous, since you dont have to manually call insert, save and delete method on your repository/data mapper. However, I see two problems with this approach:
Tight coupling of domain model with Unit of work: This implementation of Unit of Work will make domain models dependent on UnitOfWork class. UnitOfWork has to come from somewhere, the implementation of static class/method is bad. To improve this, we need to switch to dependency injection, and pass an instance of UnitOfWork to the constructor of Domain Model. But this still couples domain model with Unit of work. Also ideally a domain model should only accept parameters for its data fields(ie. Message domain model's constructor should only accept whats relevant to message, such as title, content, dateposted, etc). If it will need to accept a parameter of UnitOfWork, it will pollute the constructor.
The domain model now becomes persistent-aware: In modern application design, especially DDD, we strive for persistent-ignorant model. The domain model shouldnt care about whether it is being persisted or not, it should not even care about whether there's persistence layer at all. By having those markNew(), markDirty(), etc methods on domain model, our domain models now have the responsibility of informing the rest of our application that it needs to be persisted. Although it does not handle the persistence logic, the model still is aware of the existence of persistence layer. I am not sure if this is a good idea, to me it seems to have violate the single responsibility principle. There's also an article talking about this:
http://blog.sapiensworks.com/post/2014/06/04/Unit-Of-Work-is-the-new-Singleton.aspx/
So what do you think? Does the original Unit of Work pattern described in Martin Fowler violate good OO design principles? If so, do you consider it an antipattern?
To be entirely accurate, there is no one "Martin Fowler's implementation of Unit of Work". In the book he distinguishes between two types of registration of a modified object into a UoW.
Caller registration where only the calling object knows about the UoW and has to mark the (callee) domain object as dirty with it. No anti pattern or bad practice here as far as I can tell.
Object registration where the domain object registers itself with the UoW. Here again there are two options :
For this scheme to work the Unit of Work needs either to be passed to
the object or to be in a well-known place. Passing the Unit of Work
around is tedious but usually no problem to have it present in some
kind of session object.
The code sample is using UnitOfWork.GetCurrent() which is closer to the latter option and admittedly widely considered an anti-pattern today because of the tightly coupled, implicit dependency (Service Locator style).
However, if the first option was chosen, i.e. passing the UoW over to the domain object, and let's assume a Unit of Work abstraction, would it be bad practice ? From a dependency management perspective, clearly not.
Now remains the persistence ignorance aspect. Can we say about an object which can signal another object it's just been edited/created/removed that it is persistence-aware ? Highly debatable.
In comparison, if we look at more recent domain object implementations out there, for instance ones in Event Sourcing, we can see that aggregates can be responsible for keeping a list of their own uncommitted changes which is more or less the same idea. Does this violate persistence ignorance ? I don't think so.
Bottom line : the specific code Fowler chose to illustrate one of many UoW possibilities would clearly be considered bad practice now, but much more so with regard to problem #1 you pointed out and not really problem #2. And this doesn't disqualify other implementations he writes about, nor the whole UoW pattern whose change-tracking mechanics are anyway most of the time hidden away in third party library magic (read: ORM) nowadays and not hardcoded as in the book's example.
From a DDD perspective, this is something you shouldn't do.
DDD contains the following rule:
An application service should only modify one aggregate per transaction.
If you follow this rule, it's clear which aggregate changed during an app service operation. This aggregate then in turn needs to be passed to a repository for saving to the DB:
repository.update(theAggregate);
No other call is required. This defeats the gain from the pattern in the form you describe it.
On the other hand, the pattern you describe introduces a dependency from the domain to the persistence mechanism (depending on the design either a real dependency or just a conceptual dependency). Now this is something you should avoid, because it increases the complexity of your model a lot (not only internally, also for clients).
As a result, you shouldn't use the pattern in this form together with DDD.
Outside of DDD
Having that said, I think the pattern is one of many solutions to a certain problem. That solution has pros and cons, some of which you describe in the question. In some situations, the pattern may be the best trade-off, so
No, this is not an anti-pattern.
I don't think the model should not have a dependency on the UoW. It would be more like a repository that would depend on the UoW and, in turn, the repository would depend on the model.
If your repositories only depend on an abstract UoW, then the only piece of the puzzle that knows about the persistence technology is the concrete UoW.
The only classes I tend to allow the model to depend on are other pieces of the model: domain services, factories, etc.
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.
Yes, another question on separation of responsibilities in an MVC architecture for a web application - I think this one has a subtle difference however...
My existing implementation looks like this:
Controllers: Very 'thin'; ASide from calls to Models & Views, Routing & Presentation Logic Only
Models: Very 'thick'; All Business Logic
Views: Very 'thin'; Aside from Content & Markup, Code is limited to Loops & Data Formatting
Additionally, the project utilizes an ORM as an abstraction layer above the database and 'Connectors' as wrapper classes to external services.
My question concerns the separation of responsibilities between models. For the most part, our Models mimic the 'things' within our system - 'Users', 'Products', 'Orders', etc.
I'm finding that this works quite well for serving simple data retrieval requests - the Controller(s) instantiate(s) the proper Model(s) & calls the relevant 'getter(s)'.
The issue arises when more complex processes are initiated such as 'PlaceOrder' or 'RegisterUser'. Sometimes these processes can be implemented within a single model, other times they require communication or coordination between models to implement.
As it stands, the Models communicate with each other directly in these cases rather than the process being managed by the Controller. Keeping the process within the Models seems proper (the Controller needn't be aware that a business rule of 'RegisterUser' requires a confirmation email to be sent, for instance).
What I'm finding with this implementation are two issues which concern me somewhat:
Models often seem to know too much about other Models - Models seem too tightly coupled in this implementation.
Methods within the Models are of two general types: 'getters/setters' and what I've taken to calling 'Process Methods', methods which manage a process, calling other methods within the Model or other Models as appropriate - these methods seem 'un-model-like', for lack of a better description.
Would it be appropriate to implement two sorts of Models - 'Data/Object Models' (populated primarily with 'getters/setters' and perhaps simple 'Process Methods' which are exclusively internal and 'Process Models' (populated with 'Process Methods' which require the collaboration of multiple ('Data/Object') Models)?
In this implementation, we'd have Models representing 'Users, 'Products', 'Orders' as well as 'Registration', 'Ordering', etc.
Thoughts?
The solution to this problem is to have a separate layer, a thin layer on top of Model. This layer is sometimes called the Service Layer or Application Layer. This layer does not too much of state, it rather calls various model methods and Data Access Methods.
For example, you may have one service class for managing orders.
class OrderService {
placeOrder(Order order) {
order.doModelStuff();
orderDao.save(order);
}
removeOrder(order){
order.cancel();
orderDao.delete(order);
...
}
or
class UserService {
registerUser(User user) {
if(userDao.userExists(user)) {
throw exception: user exists;
}
user.doRegistrationStuff();
userDao.save(user);
}
The methods in service layer are not confined to manipulate a single entity. In general, they can access and manipulate multiple models. For example,
placeOrder(Customer customer, Order order) {
customer.placeOrder(order);
save customer, if necessary.
save order, if necessary
customer.sendEmail();
Shipper shipper = new shipper;
shipper.ship(order, customer.getAddress());
...
}
The idea of this layer is that, its methods do a unit of work (typically corresponding to a single use case). This is in fact more of a procedural nature. You can read more about this layer from Martin Fowler, and others.
Note: my point is to show what a service/application layer is, not to show implementation of order, customer etc.
Martin Fowler, in his "Refactoring" book, seems to have the opinion that a "Data" model consisting of data, accessors, and nothing else, is a good candidate for refactoring into another class. He calls it "Data Class" in his library of "bad smells" in code.
That suggests it may be better to look at simplifying interactions between different processes, but allowing a process to be closely coupled to its own data
e.g. PlaceOrder and OrderData can be tightly coupled, but PlaceOrder involves a minimum of interactions such as AddOrderToCustomerRecord with the Customer process.
In design pattern terms, separating your model objects into simple objects (with getters and setters) and process objects (with process logic) would be turning your Domain Model into an Anemic Domain Model with Transaction Scripts.
You don't want to do that. Model objects telling each other to do things (your process methods) is good. That kind of coupling is preferable to the kind of coupling you get from using getters and setters.
Objects have to interact with each other, so there has to be some level of coupling. If you limit that coupling to methods that are meant to be exposed to the outside world (the object's API if you will), you can change the implementation of the object without side effects.
Once you expose implementation details (getters and setters expose object internals, which are implementation specific), you can't change the implementation without side effects. That's bad coupling. See Getters and Setters Are Evil for a more thorough explanation.
Back to your process methods and excessive coupling, there are ways to reduce coupling between model objects. Check the Law of Demeter for some guidelines on what is reasonable and what should be a red flag.
Also take a look at Domain Driven Design for patterns for reducing coupling. Something like an Aggregate Root can reduce coupling and complexity.
The tl;dr version: don't separate your data and methods, hide your data and only expose your API.
I'm in a project that takes the Single Responsibility Principle pretty seriously. We have a lot of small classes and things are quite simple. However, we have an anemic domain model - there is no behaviour in any of our model classes, they are just property bags. This isn't a complaint about our design - it actually seems to work quite well
During design reviews, SRP is brought out whenever new behaviour is added to the system, and so new behaviour typically ends up in a new class. This keeps things very easily unit testable, but I am perplexed sometimes because it feels like pulling behaviour out of the place where it's relevant.
I'm trying to improve my understanding of how to apply SRP properly. It seems to me that SRP is in opposition to adding business modelling behaviour that shares the same context to one object, because the object inevitably ends up either doing more than one related thing, or doing one thing but knowing multiple business rules that change the shape of its outputs.
If that is so, then it feels like the end result is an Anemic Domain Model, which is certainly the case in our project. Yet the Anemic Domain Model is an anti-pattern.
Can these two ideas coexist?
EDIT: A couple of context related links:
SRP - http://www.objectmentor.com/resources/articles/srp.pdf
Anemic Domain Model - http://martinfowler.com/bliki/AnemicDomainModel.html
I'm not the kind of developer who just likes to find a prophet and follow what they say as gospel. So I don't provide links to these as a way of stating "these are the rules", just as a source of definition of the two concepts.
Rich Domain Model (RDM) and Single Responsibility Principle (SRP) are not necessarily at odds. RDM is more at odds with a very specialised subclassof SRP - the model advocating "data beans + all business logic in controller classes" (DBABLICC).
If you read Martin's SRP chapter, you'll see his modem example is entirely in the domain layer, but abstracting the DataChannel and Connection concepts as separate classes. He keeps the Modem itself as a wrapper, since that is useful abstraction for client code. It's much more about proper (re)factoring than mere layering. Cohesion and coupling are still the base principles of design.
Finally, three issues:
As Martin notes himself, it's not always easy to see the different 'reasons for change'. The very concepts of YAGNI, Agile, etc. discourage the anticipation of future reasons for change, so we shouldn't invent ones where they aren't immediately obvious. I see 'premature, anticipated reasons for change' as a real risk in applying SRP and should be managed by the developer.
Further to the previous, even correct (but unnecessary anal) application of SRP may result in unwanted complexity. Always think about the next poor sod who has to maintain your class: will the diligent abstraction of trivial behaviour into its own interfaces, base classes and one-line implementations really aid his understanding of what should simply have been a single class?
Software design is often about getting the best compromise between competing forces. For example, a layered architecture is mostly a good application of SRP, but what about the fact that, for example, the change of a property of a business class from, say, a boolean to an enum has a ripple effect across all the layers - from db through domain, facades, web service, to GUI? Does this point to bad design? Not necessarily: it points to the fact that your design favours one aspect of change to another.
I'd have to say "yes", but you have to do your SRP properly. If the same operation applies to only one class, it belongs in that class, wouldn't you say? How about if the same operation applies to multiple classes? In that case, if you want to follow the OO model of combining data and behavior, you'd put the operation into a base class, no?
I suspect that from your description, you're ending up with classes which are basically bags of operations, so you've essentially recreated the C-style of coding: structs and modules.
From the linked SRP paper:
"The SRP is one of the simplest of the principle, and one of the hardest to get right."
The quote from the SRP paper is very correct; SRP is hard to get right. This one and OCP are the two elements of SOLID that simply must be relaxed to at least some degree in order to actually get a project done. Overzealous application of either will very quickly produce ravioli code.
SRP can indeed be taken to ridiculous lengths, if the "reasons for change" are too specific. Even a POCO/POJO "data bag" can be thought of as violating SRP, if you consider the type of a field changing as a "change". You'd think common sense would tell you that a field's type changing is a necessary allowance for "change", but I've seen domain layers with wrappers for built-in value types; a hell that makes ADM look like Utopia.
It's often good to ground yourself with some realistic goal, based on readability or a desired cohesion level. When you say, "I want this class to do one thing", it should have no more or less than what is necessary to do it. You can maintain at least procedural cohesion with this basic philosophy. "I want this class to maintain all the data for an invoice" will generally allow SOME business logic, even summing subtotals or calculating sales tax, based on the object's responsibility to know how to give you an accurate, internally-consistent value for any field it contains.
I personally do not have a big problem with a "lightweight" domain. Just having the one role of being the "data expert" makes the domain object the keeper of every field/property pertinent to the class, as well as all calculated field logic, any explicit/implicit data type conversions, and possibly the simpler validation rules (i.e. required fields, value limits, things that would break the instance internally if allowed). If a calculation algorithm, perhaps for a weighted or rolling average, is likely to change, encapsulate the algorithm and refer to it in the calculated field (that's just good OCP/PV).
I don't consider such a domain object to be "anemic". My perception of that term is a "data bag", a collection of fields that has no concept whatsoever of the outside world or even the relation between its fields other than that it contains them. I've seen that too, and it's not fun tracking down inconsistencies in object state that the object never knew was a problem. Overzealous SRP will lead to this by stating that a data object is not responsible for any business logic, but common sense would generally intervene first and say that the object, as the data expert, must be responsible for maintaining a consistent internal state.
Again, personal opinion, I prefer the Repository pattern to Active Record. One object, with one responsibility, and very little if anything else in the system above that layer has to know anything about how it works. Active Record requires the domain layer to know at least some specific details about the persistence method or framework (whether that be the names of stored procedures used to read/write each class, framework-specific object references, or attributes decorating the fields with ORM information), and thus injects a second reason to change into every domain class by default.
My $0.02.
I've found following the solid principles did in fact lead me away from DDD's rich domain model, in the end, I found I didn't care. More to the point, I found that the logical concept of a domain model, and a class in whatever language weren't mapped 1:1, unless we were talking about a facade of some sort.
I wouldn't say this is exactly a c-style of programming where you have structs and modules, but rather you'll probably end up with something more functional, I realise the styles are similar, but the details make a big difference. I found my class instances end up behaving like higher order functions, partial functions application, lazily evaluated functions, or some combination of the above. It's somewhat ineffable for me, but that's the feeling I get from writing code following TDD + SOLID, it ended up behaving like a hybrid OO/Functional style.
As for inheritance being a bad word, i think that's more due to the fact that the inheritance isn't sufficiently fine grained enough in languages like Java/C#. In other languages, it's less of an issue, and more useful.
I like the definition of SRP as:
"A class has only one business reason to change"
So, as long as behaviours can be grouped into single "business reasons" then there is no reason for them not to co-exist in the same class. Of course, what defines a "business reason" is open to debate (and should be debated by all stakeholders).
Before I get into my rant, here's my opinion in a nutshell: somewhere everything has got to come together... and then a river runs through it.
I am haunted by coding.
=======
Anemic data model and me... well, we pal around a lot. Maybe it's just the nature of small to medium sized applications with very little business logic built into them. Maybe I am just a bit 'tarded.
However, here's my 2 cents:
Couldn't you just factor out the code in the entities and tie it up to an interface?
public class Object1
{
public string Property1 { get; set; }
public string Property2 { get; set; }
private IAction1 action1;
public Object1(IAction1 action1)
{
this.action1 = action1;
}
public void DoAction1()
{
action1.Do(Property1);
}
}
public interface IAction1
{
void Do(string input1);
}
Does this somehow violate the principles of SRP?
Furthermore, isn't having a bunch of classes sitting around not tied to each other by anything but the consuming code actually a larger violation of SRP, but pushed up a layer?
Imagine the guy writing the client code sitting there trying to figure out how to do something related to Object1. If he has to work with your model he will be working with Object1, the data bag, and a bunch of 'services' each with a single responsibility. It'll be his job to make sure all those things interact properly. So now his code becomes a transaction script, and that script will itself contain every responsibility necessary to properly complete that particular transaction (or unit of work).
Furthermore, you could say, "no brah, all he needs to do is access the service layer. It's like Object1Service.DoActionX(Object1). Piece of cake." Well then, where's the logic now? All in that one method? Your still just pushing code around, and no matter what, you'll end up with data and the logic being separated.
So in this scenario, why not expose to the client code that particular Object1Service and have it's DoActionX() basically just be another hook for your domain model? By this I mean:
public class Object1Service
{
private Object1Repository repository;
public Object1Service(Object1Repository repository)
{
this.repository = repository;
}
// Tie in your Unit of Work Aspect'ing stuff or whatever if need be
public void DoAction1(Object1DTO object1DTO)
{
Object1 object1 = repository.GetById(object1DTO.Id);
object1.DoAction1();
repository.Save(object1);
}
}
You still have factored out the actual code for Action1 from Object1 but for all intensive purposes, have a non-anemic Object1.
Say you need Action1 to represent 2 (or more) different operations that you would like to make atomic and separated into their own classes. Just create an interface for each atomic operation and hook it up inside of DoAction1.
That's how I might approach this situation. But then again, I don't really know what SRP is all about.
Convert your plain domain objects to ActiveRecord pattern with a common base class to all domain objects. Put common behaviour in the base class and override the behaviour in derived classes wherever necessary or define the new behaviour wherever required.
What is the dependency inversion principle and why is it important?
What Is It?
The books Agile Software Development, Principles, Patterns, and Practices and Agile Principles, Patterns, and Practices in C# are the best resources for fully understanding the original goals and motivations behind the Dependency Inversion Principle. The article "The Dependency Inversion Principle" is also a good resource, but due to the fact that it is a condensed version of a draft which eventually made its way into the previously mentioned books, it leaves out some important discussion on the concept of a package and interface ownership which are key to distinguishing this principle from the more general advise to "program to an interface, not an implementation" found within the book Design Patterns (Gamma, et. al).
To provide a summary, the Dependency Inversion Principle is primarily about reversing the conventional direction of dependencies from "higher level" components to "lower level" components such that "lower level" components are dependent upon the interfaces owned by the "higher level" components. (Note: "higher level" component here refers to the component requiring external dependencies/services, not necessarily its conceptual position within a layered architecture.) In doing so, coupling isn't reduced so much as it is shifted from components that are theoretically less valuable to components which are theoretically more valuable.
This is achieved by designing components whose external dependencies are expressed in terms of an interface for which an implementation must be provided by the consumer of the component. In other words, the defined interfaces express what is needed by the component, not how you use the component (e.g. "INeedSomething", not "IDoSomething").
What the Dependency Inversion Principle does not refer to is the simple practice of abstracting dependencies through the use of interfaces (e.g. MyService → [ILogger ⇐ Logger]). While this decouples a component from the specific implementation detail of the dependency, it does not invert the relationship between the consumer and dependency (e.g. [MyService → IMyServiceLogger] ⇐ Logger.
Why Is It Important?
The importance of the Dependency Inversion Principle can be distilled down to a singular goal of being able to reuse software components which rely upon external dependencies for a portion of their functionality (logging, validation, etc.)
Within this general goal of reuse, we can delineate two sub-types of reuse:
Using a software component within multiple applications with sub-dependency implementations (e.g. You've developed a DI container and want to provide logging, but don't want to couple your container to a specific logger such that everyone that uses your container has to also use your chosen logging library).
Using software components within an evolving context (e.g. You've developed business-logic components which remain the same across multiple versions of an application where the implementation details are evolving).
With the first case of reusing components across multiple applications, such as with an infrastructure library, the goal is to provide a core infrastructure need to your consumers without coupling your consumers to sub-dependencies of your own library since coupling to such dependencies requires your consumers to require the same dependencies as well. This can be problematic when consumers of your library choose to use a different library for the same infrastructure needs (e.g. NLog vs. log4net), or if they choose to use a later version of the required library which isn't backward compatible with the version required by your library.
With the second case of reusing business-logic components (i.e. "higher-level components"), the goal is to isolate the core domain implementation of your application from the changing needs of your implementation details (i.e. changing/upgrading persistence libraries, messaging libraries, encryption strategies, etc.). Ideally, changing the implementation details of an application shouldn't break the components encapsulating the application's business logic.
Note: Some may object to describing this second case as actual reuse, reasoning that components such as business-logic components used within a single evolving application represents only a single use. The idea here, however, is that each change to the application's implementation details renders a new context and therefore a different use case, though the ultimate goals could be distinguished as isolation vs. portability.
While following the Dependency Inversion Principle in this second case can offer some benefit, it should be noted that its value as applied to modern languages such as Java and C# is much reduced, perhaps to the point of being irrelevant. As discussed earlier, the DIP involves separating implementation details into separate packages completely. In the case of an evolving application, however, simply utilizing interfaces defined in terms of the business domain will guard against needing to modify higher-level components due to changing needs of implementation detail components, even if the implementation details ultimately reside within the same package. This portion of the principle reflects aspects that were pertinent to the language in view when the principle was codified (i.e. C++) which aren't relevant to newer languages. That said, the importance of the Dependency Inversion Principle primarily lies with the development of reusable software components/libraries.
A longer discussion of this principle as it relates to the simple use of interfaces, Dependency Injection, and the Separated Interface pattern can be found here. Additionally, a discussion of how the principle relates to dynamically-typed languages such as JavaScript can be found here.
Check this document out: The Dependency Inversion Principle.
It basically says:
High level modules should not depend upon low-level modules. Both should depend upon abstractions.
Abstractions should never depend upon details. Details should depend upon abstractions.
As to why it is important, in short: changes are risky, and by depending on a concept instead of on an implementation, you reduce the need for change at call sites.
Effectively, the DIP reduces coupling between different pieces of code. The idea is that although there are many ways of implementing, say, a logging facility, the way you would use it should be relatively stable in time. If you can extract an interface that represents the concept of logging, this interface should be much more stable in time than its implementation, and call sites should be much less affected by changes you could make while maintaining or extending that logging mechanism.
By also making the implementation depend on an interface, you get the possibility to choose at run-time which implementation is better suited for your particular environment. Depending on the cases, this may be interesting too.
When we design software applications we can consider the low level classes the classes which implement basic and primary operations (disk access, network protocols,...) and high level classes the classes which encapsulate complex logic (business flows, ...).
The last ones rely on the low level classes. A natural way of implementing such structures would be to write low level classes and once we have them to write the complex high level classes. Since high level classes are defined in terms of others this seems the logical way to do it. But this is not a flexible design. What happens if we need to replace a low level class?
The Dependency Inversion Principle states that:
High level modules should not depend upon low level modules. Both should depend upon abstractions.
Abstractions should not depend upon details. Details should depend upon abstractions.
This principle seeks to "invert" the conventional notion that high level modules in software should depend upon the lower level modules. Here high level modules own the abstraction (for example, deciding the methods of the interface) which are implemented by lower level modules. Thus making lower level modules dependent on higher level modules.
Dependency inversion well applied gives flexibility and stability at the level of the entire architecture of your application. It will allow your application to evolve more securely and stable.
Traditional layered architecture
Traditionally a layered architecture UI depended on the business layer and this in turn depended on the data access layer.
You have to understand layer, package, or library. Let's see how the code would be.
We would have a library or package for the data access layer.
// DataAccessLayer.dll
public class ProductDAO {
}
And another library or package layer business logic that depends on the data access layer.
// BusinessLogicLayer.dll
using DataAccessLayer;
public class ProductBO {
private ProductDAO productDAO;
}
Layered architecture with dependency inversion
The dependency inversion indicates the following:
High-level modules should not depend on low-level modules. Both should depend on abstractions.
Abstractions should not depend on details. Details should depend on abstractions.
What are the high-level modules and low level? Thinking modules such as libraries or packages, high-level module would be those that traditionally have dependencies and low level on which they depend.
In other words, module high level would be where the action is invoked and low level where the action is performed.
A reasonable conclusion to draw from this principle is that there should be no dependence between concretions, but there must be a dependence on an abstraction. But according to the approach we take we can be misapplying investment depend dependency, but an abstraction.
Imagine that we adapt our code as follows:
We would have a library or package for the data access layer which define the abstraction.
// DataAccessLayer.dll
public interface IProductDAO
public class ProductDAO : IProductDAO{
}
And another library or package layer business logic that depends on the data access layer.
// BusinessLogicLayer.dll
using DataAccessLayer;
public class ProductBO {
private IProductDAO productDAO;
}
Although we are depending on an abstraction dependency between business and data access remains the same.
To get dependency inversion, the persistence interface must be defined in the module or package where this high level logic or domain is and not in the low-level module.
First define what the domain layer is and the abstraction of its communication is defined persistence.
// Domain.dll
public interface IProductRepository;
using DataAccessLayer;
public class ProductBO {
private IProductRepository productRepository;
}
After the persistence layer depends on the domain, getting to invert now if a dependency is defined.
// Persistence.dll
public class ProductDAO : IProductRepository{
}
(source: xurxodev.com)
Deepening the principle
It is important to assimilate the concept well, deepening the purpose and benefits. If we stay in mechanically and learn the typical case repository, we will not be able to identify where we can apply the principle of dependence.
But why do we invert a dependency? What is the main objective beyond specific examples?
Such commonly allows the most stable things, that are not dependent on less stable things, to change more frequently.
It is easier for the persistence type to be changed, either the database or technology to access the same database than the domain logic or actions designed to communicate with persistence. Because of this, the dependence is reversed because as it is easier to change the persistence if this change occurs. In this way we will not have to change the domain. The domain layer is the most stable of all, which is why it should not depend on anything.
But there is not just this repository example. There are many scenarios where this principle applies and there are architectures based on this principle.
Architectures
There are architectures where dependency inversion is key to its definition. In all the domains it is the most important and it is abstractions that will indicate the communication protocol between the domain and the rest of the packages or libraries are defined.
Clean Architecture
In Clean architecture the domain is located in the center and if you look in the direction of the arrows indicating dependency, it is clear what are the most important and stable layers. The outer layers are considered unstable tools so avoid depending on them.
(source: 8thlight.com)
Hexagonal Architecture
It happens the same way with the hexagonal architecture, where the domain is also located in the central part and ports are abstractions of communication from the domino outward. Here again it is evident that the domain is the most stable and traditional dependence is inverted.
(source: pragprog.com)
Basically it says:
Class should depend on abstractions (e.g interface, abstract classes), not specific details (implementations).
To me, the Dependency Inversion Principle, as described in the official article, is really a misguided attempt to increase the reusability of modules that are inherently less reusable, as well as a way to workaround an issue in the C++ language.
The issue in C++ is that header files typically contain declarations of private fields and methods. Therefore, if a high-level C++ module includes the header file for a low-level module, it will depend on actual implementation details of that module. And that, obviously, is not a good thing. But this is not an issue in the more modern languages commonly used today.
High-level modules are inherently less reusable than low-level modules because the former are normally more application/context specific than the latter. For example, a component that implements an UI screen is of the highest-level and also very (completely?) specific to the application. Trying to reuse such a component in a different application is counter-productive, and can only lead to over-engineering.
So, the creation of a separate abstraction at the same level of a component A that depends on a component B (which does not depend on A) can be done only if component A will really be useful for reuse in different applications or contexts. If that's not the case, then applying DIP would be bad design.
A much clearer way to state the Dependency Inversion Principle is:
Your modules which encapsulate complex business logic should not depend directly on other modules which encapsulate business logic. Instead, they should depend only on interfaces to simple data.
I.e., instead of implementing your class Logic as people usually do:
class Dependency { ... }
class Logic {
private Dependency dep;
int doSomething() {
// Business logic using dep here
}
}
you should do something like:
class Dependency { ... }
interface Data { ... }
class DataFromDependency implements Data {
private Dependency dep;
...
}
class Logic {
int doSomething(Data data) {
// compute something with data
}
}
Data and DataFromDependency should live in the same module as Logic, not with Dependency.
Why do this?
The two business logic modules are now decoupled. When Dependency changes, you don't need to change Logic.
Understanding what Logic does is a much simpler task: it operates only on what looks like an ADT.
Logic can now be more easily tested. You can now directly instantiate Data with fake data and pass it in. No need for mocks or complex test scaffolding.
Good answers and good examples are already given by others here.
The reason DIP is important is because it ensures the OO-principle "loosely coupled design".
The objects in your software should NOT get into a hierarchy where some objects are the top-level ones, dependent on low-level objects. Changes in low-level objects will then ripple-through to your top-level objects which makes the software very fragile for change.
You want your 'top-level' objects to be very stable and not fragile for change, therefore you need to invert the dependencies.
Inversion of control (IoC) is a design pattern where an object gets handed its dependency by an outside framework, rather than asking a framework for its dependency.
Pseudocode example using traditional lookup:
class Service {
Database database;
init() {
database = FrameworkSingleton.getService("database");
}
}
Similar code using IoC:
class Service {
Database database;
init(database) {
this.database = database;
}
}
The benefits of IoC are:
You have no dependency on a central
framework, so this can be changed if
desired.
Since objects are created
by injection, preferably using
interfaces, it's easy to create unit
tests that replace dependencies with
mock versions.
Decoupling off code.
Dependency Inversion Principle(DIP)
It is a part of SOLID[About] which is a part of OOD and was introduced by Uncle Bob. It is about loose coupling between classes(layers...). Class should not be depended on concrete realization, class should be depended on abstraction/interface
Problem:
//A -> B
class A {
B b
func foo() {
b = B();
}
}
Solution:
//A -> IB <|- B
//client[A -> IB] <|- B is the Inversion
class A {
IB ib // An abstraction between High level module A and low level module B
func foo() {
ib = B()
}
}
Now A is not depended on B(one to one), now A is depended on interface IB which is implemented by B, it means that A depends on multiple realization of IB(one to many)
[DIP vs DI vs IoC]
The point of dependency inversion is to make reusable software.
The idea is that instead of two pieces of code relying on each other, they rely on some abstracted interface. Then you can reuse either piece without the other.
The way this is most commonly achieved is through an inversion of control (IoC) container like Spring in Java. In this model, properties of objects are set up through an XML configuration instead of the objects going out and finding their dependency.
Imagine this pseudocode...
public class MyClass
{
public Service myService = ServiceLocator.service;
}
MyClass directly depends on both the Service class and the ServiceLocator class. It needs both of those if you want to use it in another application. Now imagine this...
public class MyClass
{
public IService myService;
}
Now, MyClass relies on a single interface, the IService interface. We'd let the IoC container actually set the value of that variable.
So now, MyClass can easily be reused in other projects, without bringing the dependency of those other two classes along with it.
Even better, you don't have to drag the dependencies of MyService, and the dependencies of those dependencies, and the... well, you get the idea.
If we can take it as a given that a "high level" employee at a corporation is paid for the execution of their plans, and that these plans are delivered by the aggregate execution of many "low level" employee's plans, then we could say it is generally a terrible plan if the high level employee's plan description in any way is coupled to the specific plan of any lower level employee.
If a high level executive has a plan to "improve delivery time", and indicates that an employee in the shipping line must have coffee and do stretches each morning, then that plan is highly coupled and has low cohesion. But if the plan makes no mention of any specific employee, and in fact simply requires "an entity that can perform work is prepared to work", then the plan is loosely coupled and more cohesive: the plans do not overlap and can easily be substituted. Contractors, or robots, can easily replace the employees and the high level's plan remains unchanged.
"High level" in the dependency inversion principle means "more important".
I can see good explanation has been given in above answers. However i wants to provide some easy explanation with simple example.
Dependency Inversion Principle allows the programmer to remove the hardcoded dependencies so that the application becomes loosely coupled and extendable.
How to achieve this : through abstraction
Without dependency inversion:
class Student {
private Address address;
public Student() {
this.address = new Address();
}
}
class Address{
private String perminentAddress;
private String currentAdrress;
public Address() {
}
}
In above code snippet, address object is hard-coded. Instead if we can use dependency inversion and inject the address object by passing through constructor or setter method. Let's see.
With dependency inversion:
class Student{
private Address address;
public Student(Address address) {
this.address = address;
}
//or
public void setAddress(Address address) {
this.address = address;
}
}
Dependency Inversion Principle (DIP) says that
i) High level modules should not depend upon low-level modules. Both should depend upon abstractions.
ii) Abstractions should never depend upon details. Details should depend upon abstractions.
Example:
public interface ICustomer
{
string GetCustomerNameById(int id);
}
public class Customer : ICustomer
{
//ctor
public Customer(){}
public string GetCustomerNameById(int id)
{
return "Dummy Customer Name";
}
}
public class CustomerFactory
{
public static ICustomer GetCustomerData()
{
return new Customer();
}
}
public class CustomerBLL
{
ICustomer _customer;
public CustomerBLL()
{
_customer = CustomerFactory.GetCustomerData();
}
public string GetCustomerNameById(int id)
{
return _customer.GetCustomerNameById(id);
}
}
public class Program
{
static void Main()
{
CustomerBLL customerBLL = new CustomerBLL();
int customerId = 25;
string customerName = customerBLL.GetCustomerNameById(customerId);
Console.WriteLine(customerName);
Console.ReadKey();
}
}
Note: Class should depend on abstractions like interface or abstract classes, not specific details (implementation of interface).
Dependency inversion: Depend on abstractions, not on concretions.
Inversion of control: Main vs Abstraction, and how the Main is the glue of the systems.
These are some good posts talking about this:
https://coderstower.com/2019/03/26/dependency-inversion-why-you-shouldnt-avoid-it/
https://coderstower.com/2019/04/02/main-and-abstraction-the-decoupled-peers/
https://coderstower.com/2019/04/09/inversion-of-control-putting-all-together/
Adding to the flurry of generally good answers, I'd like to add a tiny sample of my own to demonstrate good vs. bad practice. And yes, I'm not one to throw stones!
Say, you want a little program to convert a string into base64 format via console I/O. Here's the naive approach:
class Program
{
static void Main(string[] args)
{
/*
* BadEncoder: High-level class *contains* low-level I/O functionality.
* Hence, you'll have to fiddle with BadEncoder whenever you want to change
* the I/O mode or details. Not good. A good encoder should be I/O-agnostic --
* problems with I/O shouldn't break the encoder!
*/
BadEncoder.Run();
}
}
public static class BadEncoder
{
public static void Run()
{
Console.WriteLine(Convert.ToBase64String(Encoding.UTF8.GetBytes(Console.ReadLine())));
}
}
The DIP basically says that high-level components shouldn't be dependent on low-level implementation, where "level" is the distance from I/O according to Robert C. Martin ("Clean Architecture"). But how do you get out of this predicament? Simply by making the central Encoder dependent only on interfaces without bothering how those are implemented:
class Program
{
static void Main(string[] args)
{
/* Demo of the Dependency Inversion Principle (= "High-level functionality
* should not depend upon low-level implementations"):
* You can easily implement new I/O methods like
* ConsoleReader, ConsoleWriter without ever touching the high-level
* Encoder class!!!
*/
GoodEncoder.Run(new ConsoleReader(), new ConsoleWriter()); }
}
public static class GoodEncoder
{
public static void Run(IReadable input, IWriteable output)
{
output.WriteOutput(Convert.ToBase64String(Encoding.ASCII.GetBytes(input.ReadInput())));
}
}
public interface IReadable
{
string ReadInput();
}
public interface IWriteable
{
void WriteOutput(string txt);
}
public class ConsoleReader : IReadable
{
public string ReadInput()
{
return Console.ReadLine();
}
}
public class ConsoleWriter : IWriteable
{
public void WriteOutput(string txt)
{
Console.WriteLine(txt);
}
}
Note that you don't need to touch GoodEncoder in order to change the I/O mode — that class is happy with the I/O interfaces it knows; any low-level implementation of IReadable and IWriteable won't ever bother it.