I'm starting with DDD philosophy and I'd like to implement an integration with a legacy system that we have here. In my researches in the internet, I found some articles and samples but I must to admit: is pretty hard to understand how to do that integration correctly.
Before to ask this question, I did a search here but the results were not useful for me, so I'd like to know if is possible to send or show me a implementation sample of an ACL.
Here I have this items:
The Legacy system
The legacy database (I need to access)
The new application that will be created using the DDD approach
The initial idea is to access that database throughout NHibernate, creating just some the needed mapping classes, the domain entities and implement the business rules. According to Eric Evans, this strategy is called [Bubble Context][1]. I think this strategy will solve my problem, but I need some sample to do that in a right way.
Can someone help me?
The ACL is a pattern and not just the piece of code. In what you described you didn't say do you have strong dependencies upon legacy system or you just want to have some independent piece of code built into current system? With this you could decide will be your ACL just a service to database or will it incorporate some wrapping upon legacy system logic?
The actual pieces that you'd put into the ALC are highly depend on your implementation.
There is a generic schema of what you're asking for:
You could find more info in Eric Evan's talk.
I am quite confident with developing DDD applications, but one area which is continuing to cause me problems is when two applications integrate with each other. I am struggling to find any useful books or resources on the subject. Books such as Patterns of EAI go into depth about messaging patterns and message construction, but don't really explain how to architect systems that make use of these patterns.
I've searched high and low and I'm quite sure there are no sample applications that demostrate how to integrate two systems. I understand the concept of asynchronous messaging, but again can't find good examples of how to apply it.
Resources on SOA seem to keep repeating the same concepts without demonstarting how to implement them, and more often than not seem more concerned with selling me products.
Here are the sort of questions I am struggling to answer:
Should each application have it's own copy of the data? For example, should every application within an organisation have it's own list of clients, which it updates upon the receipt of a message?
At what point in the DDD stack are messages passed? Are they the result of domain events?
Can I combine asynchrounous messaging and WCF or do I have to chose? Do I use WCF for request/response and messaging for publish/subscribe?
How does one DDD application consume the services of another? Should one DDD application query another system for its data via its application services, or should it already have its own local copy of the data, as mentioned in point 1?
Apparently I can't have a transaction across two systems. How do I avoid this?
If I sound confused it's because I am. I'm not looking for answers to the above questions, just pointing in the direction of resources that will answer this and simmilar questions.
I've been making a similar transition. My advice:
Start at http://cqrsinfo.com/.
Listen the the Distributed Podcast.
Catch any of Greg Young's talks. For example, here is Eric Evans interviewing Greg. He's got some all-day sessions that are recorded as well.
Read/listen to anything from Udi Dahan (poscasts, lectures, articles, etc.). He's got some good stuff on InfoQ.
Wait for Greg's book.
Read whatever you can find on EDA (Event Driven Architecture).
In addition to what Eric Farr had said, it may be worth looking closely at Part 4 of the DDD book (Strategic Design). It does not approach the problem from 'Distributed' angle but has a lot of information on how to integrate applications (Bounded Contexts). Patterns like Anticorruption Layer can be very helpful when designing at the boundary of the application.
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 currently been looking into Domain programming solutions and trying to predicate the future of the technology for my current customer.
Where is Microsoft going with this considering that there seems to be many different solutions depending on which technology your looking at. For example there is Entity Framework in .net and also there is the whole SQL Server Modelling solution that will be making its appearance in SQL Server. Part of the latter contains a language called M that seems to do the modelling and describing of entities in a decoupled way.
Where are they going with Visio?
Is there a reason that Microsoft seems to be avoiding the whole UML thing and reinventing the wheel, after looking at UML for a while now it seems to be turin complete and does the whole shooting match that could be decoupled from code and storage.
Any help greatly appreciated.
I get the feeling Microsoft is betting on many horses in this race.
The support for POCOs in the latest Entity Framework release, to me, is a (good) sign that DDD is one of them.
With regards to UML. I actually downloaded Visio 2010 recently, and nothing has changed UML-wise the last ten years or so.
Visual Studio 2010, however, has gotten increased UML support (five diagram types supported).
After some reading around, Microsoft are tying things together in the future M and Entity Framework are linked in some mysterious way. From what I gather the idea is that M will create the SQL Server Model (other databases are also valid) and feed into the Entity Framework. To take this even further it looks like you can use your favourite XSI UML tool and feed it into SQL Server Model.
So the disjointed stuff at present will come together.
Having recently discovered design patterns, and having acquired the excellent Head First Design Patterns book (can really recommend it!), I am now wondering about design patterns for security and controlling access to records in data stores.
My use case is a bespoke CRM style application, with contacts, businesses, and users who have different levels of access, including being limited to read only access, or even a subset of records. I will only be doing distinct entity level access control, not field level.
Can anyone recommend any security orientated design patterns that would fit the above?
If it makes a difference, I am using ASP.Net MVC, Entity Framework 4 and SQL Server 2008.
Security is what we call Cross-cutting concern and it's never easy deal with.
If you need to deal with the security from ASP.NET MVC level you would consider to look at MVC tutorial :
http://www.asp.net/learn/mvc/
If you want to know more about the security from the domain model level, an interesting question was already asked :
DDD User Security Policies
Hope this helps
There does exists a group of patterns realted to security, though most of them fucuses on securing integrated systems. I have found no book that is as well written and usable as GOF/Head-first, though I did enjoy the one online at www.securitypatterns.org.
Security is as much about architecture (sever setup, network topology...) as its about programing, so I would recommend that you start out with a general security book. Also pick up a book specifically on .NET/Windows security, since robust security programming is very technology specific (I, as UNIX/Java programmer, will have a completly different toolbox than
a .NET programmer and can unfortunatly not help you with a book on this last subject).
A good place to start on security (although not necessarily a "security design patterns" book) is Ross Anderson's Security Engineering.