I read that Domain Driven Design is about concentrating on the problem domain instead of concentrating on the software. They say that it is easier to solve the complexities of the problem domain than the complexities of the software, because after you have solved the domain, you know better how to build the software, too. Also they say that actually the domain is more complex than the structures of the software or that if you don't see the forest from the woods, you are in trouble.
But how is Domain Driven Design different from just using a specification for the software? I mean, of course we should get to know the problem domain before we start coding. Is DDD reinventing the wheel?
Domain-driven design is more about establishing a common model of the world (and an associated common language) that both you and the domain experts can use.
In theory this means that developers can write code that reads like a description of the problem domain, and domain experts can look over developers' shoulders and see what's going on.
A specification makes no such promises about a common language or model of the world, it just says "we're going to build something specific". The highly specified class model you come up with might work, but it may not reflect 'reality' particularly well.
There's a good free book on Domain-Driven design here (login required unfortunately).
Related
a) Do Core Domain and Generic Subdomain ( GS ) in most cases contain different parts of the same domain model or does each GS define its own domain model, which is usually different from the model used in Core Domain?
b) If the former, then I assume the reason for both using the same model is because the primary purpose of GS is to "serve" a Core Domain, and GS can "serve" best if there's no need for a translation layer between the Core Domain and GS ( if each used its own model, then we'd also need a translation layer between the GS and Core Domain )?
thanks
Core Domains, Supporting Subdomains and Generic Subdomains evolve around the concept of the Bounded Context in DDD.
To answer your question, the Core Domain is the domain which makes your business unique and gives you an advantage over your competitors - you will put most efforts (developers/monry) into improving it. A Generic Subdomain handles a topic that is still important but there is a chance you'll find an existing solution (either as a concept or reusable code) that handles the tasks good enough.
The Generic Subdomain would have a different model because it tackles a different domain.
A Generic Subdomain may describe anything from date/time(zone) handling see (2, Ch. 15), persistence, the user-interface toolkit up to a mail server or a complete inventory management system (1, Ch. 2). On the other hand, the inventory management logic is the Core Domain of the inventory management system's vendor.
You can find in-depth information in the book Implementing Domain-Driven Design and of course in the original book introducing Domain-Driven Design by Eric Evans.
Update: (see comment)
In my opinion, the most important aspect in Core Domains using any kind of Subdomain is not to overthink this topic on an abstract level. You'll probably agree that the biggest challenge in Domain-Driven Design is to find good examples that, by plan or accidentally, match the patterns/strategies from the Strategic Design section of the Domain-Driven Design book.
Now, from my understanding, the need for a translation layer between Core Domain and Generic Subdomains arises simply from necessity. In Chapter 15, Distillation of Domain-Driven Design four options on how to develop Generic Subdomains are discussed with their corresponding pros and cons:
An Off-the-Shelf Solution
A Published Design or Model
An Outsourced Implementation
An In-House Implementation
I won't repeat the discussion because this would just include quoting from the excellent book. You'll probably agree that a custom tailored and well-designed in-house solution, that is only used for this project, does not need a translation layer. On the other hand, a commercial or open-source Off-the-Shelf Solution is more likely to require an abstraction because you have little control on how the product evolves, if it has an appropriate Intention-Revealing Interface and so on.
There are two other aspects that are related but should not be mixed up with Subdomains:
Communication between Bounded Contexts
Cohesive Mechanisms
Bounded Contexts need some kind of translation by sheer definition. For each Bounded Context, there is a Model in Context. A Context Map documents the relationships and interactions of Bounded Contexts with Translation Map. The different ways of relating models of BoundedContexts are discussed in Chapter 14, Mainting Model Integrity (Anti-Corruption Layer, Open-Host Service and others).
Cohesive Mechanisms (see Chapter 15 of Domain-Driven Design), on the other hand, are similar to Generic Subdomains as both are introduced to relieve the Core Domain from unnecessary clutter. Eric Evans describes Cohesive Mechanisms as a lightweight framework but admits that in practice the distinction between Cohesive Mechanisms and Generic Subdomains is mostly not pure.
I'd like to say that I had to read those sections again as I do not deal with this on a daily basis so please be forgiving. Additionally, I am not in the inner circle of the DDD community and thus I am not aware if these issues are evaluated differently today. They still seem very useful to me and I have not encountered a better set of tools in this area so I assume they are still valid.
I understand the urge to understand these concepts but a real understanding can only be achieved by looking at concrete examples. Some are mentioned in the books. None of them are claimed perfect. The understanding and assessment of these complex problems, large or small, changes over time and this the soul of DDD in my opinion.
Can somebody please explain (in succinct terms) what exactly is domain driven design? I see the term quite a lot but really don't understand what it is or what it looks like. How does it differ from non-domain driven design?
Also, can somebody explain what a Domain Object is? How does domain differ from normal objects?
EDIT:
As this seem to be a top result on Google and my answer below is not, please refer to this much better answer:
https://stackoverflow.com/a/1222488/1240557
OLD ANSWER (not so complete :))
In order to create good software, you have to know what that software
is all about. You cannot create a banking software system unless you
have a good understanding of what banking is all about, one must
understand the domain of banking.
From: Domain Driven Design by Eric Evans.
This book does a pretty good job of describing DDD.
Register to download a summary of the book.
Domain Driven Design is a methodology and process prescription for the development of complex systems whose focus is mapping activities, tasks, events, and data within a problem domain into the technology artifacts of a solution domain.
The emphasis of Domain Driven Design is to understand the problem domain in order to create an abstract model of the problem domain which can then be implemented in a particular set of technologies. Domain Driven Design as a methodology provides guidelines for how this model development and technology development can result in a system that meets the needs of the people using it while also being robust in the face of change in the problem domain.
The process side of Domain Driven Design involves the collaboration between domain experts, people who know the problem domain, and the design/architecture experts, people who know the solution domain. The idea is to have a shared model with shared language so that as people from these two different domains with their two different perspectives discuss the solution they are actually discussing a shared knowledge base with shared concepts.
The lack of a shared problem domain understanding between the people who need a particular system and the people who are designing and implementing the system seems to be a core impediment to successful projects. Domain Driven Design is a methodology to address this impediment.
It is more than having an object model. The focus is really about the shared communication and improving collaboration so that the actual needs within the problem domain can be discovered and an appropriate solution created to meet those needs.
Domain-Driven Design: The Good and The Challenging provides a brief overview with this comment:
DDD helps discover the top-level architecture and inform about the
mechanics and dynamics of the domain that the software needs to
replicate. Concretely, it means that a well done DDD analysis
minimizes misunderstandings between domain experts and software
architects, and it reduces the subsequent number of expensive requests
for change. By splitting the domain complexity in smaller contexts,
DDD avoids forcing project architects to design a bloated object
model, which is where a lot of time is lost in working out
implementation details — in part because the number of entities to
deal with often grows beyond the size of conference-room white boards.
Also see this article Domain Driven Design for Services Architecture which provides a short example. The article provides the following thumbnail description of Domain Driven Design.
Domain Driven Design advocates modeling based on the reality of
business as relevant to our use cases. As it is now getting older and
hype level decreasing, many of us forget that the DDD approach really
helps in understanding the problem at hand and design software towards
the common understanding of the solution. When building applications,
DDD talks about problems as domains and subdomains. It describes
independent steps/areas of problems as bounded contexts, emphasizes a
common language to talk about these problems, and adds many technical
concepts, like entities, value objects and aggregate root rules to
support the implementation.
Martin Fowler has written a number of articles in which Domain Driven Design as a methodology is mentioned. For instance this article, BoundedContext, provides an overview of the bounded context concept from Domain Driven Development.
In those younger days we were advised to build a unified model of the
entire business, but DDD recognizes that we've learned that "total
unification of the domain model for a large system will not be
feasible or cost-effective" 1. So instead DDD divides up a large
system into Bounded Contexts, each of which can have a unified model -
essentially a way of structuring MultipleCanonicalModels.
You CAN ONLY understand Domain driven design by first comprehending what the following are:
What is a domain?
The field for which a system is built. Airport management, insurance sales, coffee shops, orbital flight, you name it.
It's not unusual for an application to span several different domains. For example, an online retail system might be working in the domains of shipping (picking appropriate ways to deliver, depending on items and destination), pricing (including promotions and user-specific pricing by, say, location), and recommendations (calculating related products by purchase history).
What is a model?
"A useful approximation to the problem at hand." -- Gerry Sussman
An Employee class is not a real employee. It models a real employee. We know that the model does not capture everything about real employees, and that's not the point of it. It's only meant to capture what we are interested in for the current context.
Different domains may be interested in different ways to model the same thing. For example, the salary department and the human resources department may model employees in different ways.
What is a domain model?
A model for a domain.
What is Domain-Driven Design (DDD)?
It is a development approach that deeply values the domain model and connects it to the implementation. DDD was coined and initially developed by Eric Evans.
Culled from here
Here is another good article that you may check out on Domain Driven Design. if your application is anything serious than college assignment. The basic premise is structure everything around your entities and have a strong domain model. Differentiate between services that provide infrastructure related things (like sending email, persisting data) and services that actually do things that are your core business requirments.
Hope that helps.
As in TDD & BDD you/ team focus the most on test and behavior of the system than code implementation.
Similar way when system analyst, product owner, development team and ofcourse the code - entities/ classes, variables, functions, user interfaces processes communicate using the same language, its called Domain Driven Design
DDD is a thought process. When modeling a design of software you need to keep business domain/process in the center of attention rather than data structures, data flows, technology, internal and external dependencies.
There are many approaches to model systerm using DDD
event sourcing (using events as a single source of truth)
relational databases
graph databases
using functional languages
Domain object:
In very naive words, an object which
has name based on business process/flow
has complete control on its internal state i.e exposes methods to manipulate state.
always fulfill all business invariants/business rules in context of its use.
follows single responsibility principle
DDD(domain driven design) is a useful concept for analyse of requirements of a project and handling the complexity of these requirements.Before that people were analysing these requirements with considering the relationships between classes and tables and in fact their design were based on database tables relationships it is not old but it has some problems:
In big projects with complex requirements it is not useful although this is a great way of design for small projects.
when you are dealing with none technical persons that they don,t have technical concept, this conflict may cause some huge problems in our project.
So DDD handle the first problem with considering the main project as a Domain and splitting each part of this project to small pieces which we are famous to Bounded Context and each of them do not have any influence on other pieces.
And the second problem has been solved with a ubiquitous language which is a common language between technical team members and Product owners which are not technical but have enough knowledge about their requirements
Generally the simple definition for Domain is the main project that makes money for the owners and other teams.
I do not want to repeat others' answers, so, in short I explain some common misunderstanding
Practical resource: PATTERNS, PRINCIPLES, AND PRACTICES OF DOMAIN-DRIVEN DESIGN by Scott Millett
It is a methodology for complicated business systems. It takes all the technical matters out when communicating with business experts
It provides an extensive understanding of (simplified and distilled model of) business across the whole dev team.
it keeps business model in sync with code model by using ubiquitous language (the language understood by the whole dev team, business experts, business analysts, ...), which is used for communication within the dev team or dev with other teams
It has nothing to do with Project Management. Although it can be perfectly used in project management methods like Agile.
You should avoid using it all across your project
DDD stresses the need to focus the most effort on the core subdomain. The core subdomain is the
area of your product that will be the difference between it being a success and it being a failure. It’s
the product’s unique selling point, the reason it is being built rather than bought.
Basically, it is because it takes too much time and effort. So, it is suggested to break down the whole domain into subdomain and just apply it in those with high business value. (ex not in generic subdomain like email, ...)
It is not object oriented programming. It is mostly problem solving approach and (sometimes) you do not need to use OO patterns (such as Gang of Four) in your domain models. Simply because it can not be understood by Business Experts (they do not know much about Factory, Decorator, ...). There are even some patterns in DDD (such as The Transaction Script, Table Module) which are not 100% in line with OO concepts.
I believe the following pdf will give you the bigger picture. Domain Driven Design by Eric Evans
NOTE: Think of a project you can work on, apply the little things you understood and see best practices. It will help you to grow your ability to the micro service architecture design approach too.
Get an organization wide understanding of the problem domain by
developing a ubiquitous language (a common mental model) per sub-problem-domain.
Use that language as close as possible in solution domains (code).
Only then choose technologies.
Don't be technology driven but problem domain or business driven.
I have found quite a few material (books and other stuff online) on how to make UML diagrams. So now I understand UML and the diagramming (with a tool).
However, where I am stuck is the approach / methodology. My hunt for approach / methodology always leads to how to use UML and which diagram fits where. Frankly my intent is to know how to start the journey from putting down the domain understanding (and how) to drafting the blueprint of the system that is ready for the use of developers.
I really don't care if it is UML (good if it is so) or not. I should be able to communicate the target application's domain understanding, it's analysis and eventually it's intended design in as clear terms as possible.
I think there is no Cast in Stone way of doing this, however, I am looking for potential approaches / methodologies. Please share pointers to any books / training material that is available for the purpose.
Here are a few resources that may help:
Domain Driven Design Quickly (Free summary of Domain Driven Design)
Domain Driven Design
These resources deal with gathering the knowledge of the Domain from domain experts, coming up with terms that are ubiquitous for all parties involved, and then designing the programming model to suit.
Additionally, since you mention UML, and if you haven't come across the following book yet, I highly recommend it:
UML Distilled 3rd Edition
Lastly, in more general terms, I would look further into Agile Development Methodologies.
I repeatedly find that establishing user requirements is one of the hardest parts of my job. This is for several reasons, for example, lack of shared technical vocabulary, incomplete understanding of domain on my part, inability of user to 'imagine' completed UI / product, etc etc.
Since this appears to be an ongoing challenge for me, has anyone here had a 'eureka' moment that has really helped them with this part of developing? For example, I have heard of the book 'Domain Driven Design', but not read it yet. Has anyone found a book, online resource of piece of advice that has really turned things around for them?
I won't aspire for eureka experience, however, if you are interested in DDD, which is about establishing common language for you and the users (among other things), than if you don't have access to the book, look for the Domain-Driven Design Quickly on DZone.
Generally speaking, any time user cannot imagine the thing and therefore state proper requirements, go for the prototyping (if you can). Recently I was pleased by really super simple tool which is a Firefox extension called Pencil, which enables easy and quite fast prototyping even for non-programmers. It is far from perfect, but it enables you to create own components and it is extensible.
Are you creating the user requirements on your own or are you actually interacting with the user to generate the requirements?
If you are creating a piece of software without a customer then starting with a simple high-level mock-up of what I want to create is where I will usually start and will formulate my User Requirements how I think a user would use the software.
If you have a customer I would suggest breaking the software into smaller modules (manageable chunks) and sitting down with the user to talk to them, step-by-step, how they want the module to function.
This is one thing that has been bugging me for a while about DDD. I can clearly see the benefits of the approach when dealing with non-technical business domains with complex models and lots of interaction required between technical people and non-technical domain experts.
However, what about when the 'domain' in question is technical?
For instance, situation A) take a web start-up. Imagine they are trying to accomplish something quite complicated (say a facebook clone), but almost all of the staff are technical (or at least have strong technical understanding).
What about situation B) A similar situation, but with a slightly less ambitious project, and a lone developer trying to create somthing with an elegant architecture.
I'd be really interested to hear what people have to say. What I'm really trying to get to the meat of, is where the benefits of DDD lie, what the downsides might are, and at what point one outweighs the other...
DDD is really just an elaboration of the design pattern Fowler calls Domain Model in Patterns of Enterprise Application Architecture. In that book, he compares Domain Model to other ways of organizing code, such as Transaction Script, but it is clear that he prefers Domain Model over other alternatives for all but the simplest of applications. I do, too.
DDD simply expands greatly on the original concept of a Domain Model and provides tons of guidance on how we should analyze and model our domain in a way that will be beneficial to ourselves as developers.
If the Domain in question is complex, then a Domain model (and hence DDD) is a good choice. It doesn't really matter whether the Domain is business-oriented or more technical in nature. In his book Domain-Driven Design, Eric Evans starts by describing how the DDD techniques helped him model a Printed Circuit Board application. That is surely a technical Domain, if any!
In my current work, we are using DDD to model an application that deals with Claims-based identity - another very technical Domain.
DDD is really just about dealing with complexity in sofware, and this is also the subtitle of Evans' book: "Tackling Complexity in the Heart of Software."
Evans suggest using Domain Driven Design when :
Domain Complexity > Technical Complexity
If your domain is simple but you have many technical limitations (technology choice, performance restrictions etc), do not use DDD.
If your domain in complex, do DDD, put the domain as the core of your system, and move any other concern outside (persistence, performance, technology concerns).
I don't really have a great answer for you, but I can say from my perspective as an outsider to DDD with an interest in DDD I've seen technical domain concepts / drivers creep into the the DDD conversation as first class concepts. A good example of this would be that some people are advocating for technical / infrastructure bounded contexts. The best example I can think of is Greg Young's architecture where he considers reads a bounded context and transactional writes another bounded context. In general I think things like this are "domain constructs" (my term... forgive me if I must mutilated a real DDD term). It's similiar to objects in the OO world that don't model something in the real world but are fully flushed out objects.