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I'm a 24yo Web Developer trying to improve my knowledge in this field.
I've been working on web since I was 12 and feel like I lack some fundamentals.
Many times I'm being rejected in interviews not because lack of talent, programming knowledge or a small portfolio (In fact, my portfolio is pretty big for a 24 yo dev), but because I can't answer many fundamental questions such as difference between/terminology about CRUD, REST, SOAP, OOP-related questions and such..
Going to university right now is impossible for many reasons so I was trying to get my hands on some books about dev fundamentals (mainly oriented to web dev). What are the best ones, and why? Which resources (Shouldn't necessary be books) should I look deeply into? And in the end.. What suggestions could you give to become a better developer?
I can only share from my own experience. During technical interviews, i had a cheat sheet printed and ready. That helped a lot on the telephone interview but also as a study guide. I can recommend the "PHP Zend Certification Study Guide" and php-fig.org to freshen up on Design Patterns and other things.
When the interviewer thinks you are qualified, you need to write code anyway. During the code writing you will probably write in OOP PHP with no framework. Prepare a simple mvc with some simple crud functionality, sessions and user login.
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 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.
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I'm learning about Domain Driven Design, however there are some practical issues that are confusing to me that I think seeing some good samples might clear up.
Does anyone know of some good working code samples that do a good job of modelling basic DDD concepts?
Particularly interested in
An illustrative Domain Model
Repositories
Use of Domain/Application Services
Value Objects
Aggregate Roots
The difficulty with DDD samples is that they're often very domain specific and the technical implementation of the resulting system doesn't always show the design decisions and transitions that were made in modelling the domain, which is really at the core of DDD. DDD is much more about the process than it is the code. (as some say, the best DDD sample is the book itself!)
That said, a well commented sample app should at least reveal some of these decisions and give you some direction in terms of matching up your domain model with the technical patterns used to implement it.
You haven't specified which language you're using, but I'll give you a few in a few different languages:
DDDSample - a Java sample that reflects the examples Eric Evans talks about in his book. This is well commented and shows a number of different methods of solving various problems with separate bounded contexts (ie, the presentation layer). It's being actively worked on, so check it regularly for updates.
dddps - Tim McCarthy's sample C# app for his book, .NET Domain-Driven Design with C#
S#arp Architecture - a pragmatic C# example, not as "pure" a DDD approach perhaps due to its lack of a real domain problem, but still a nice clean approach.
With all of these sample apps, it's probably best to check out the latest trunk versions from SVN/whatever to really get an idea of the thinking and technology patterns as they should be updated regularly.
Not source projects per say but I stumbled upon Parleys.com which has a few good videos that cover DDD quite well (requires flash):
Improving Application Design with a Rich Domain Model
Get Value Objects Right for Domain Driven Design (unavailable)
I found these much more helpful than the almost non-existent DDD examples that are currently available.
.NET DDD Sample from Domain-Driven Design Book by Eric Evans can be found here: http://dddsamplenet.codeplex.com
Cheers,
Jakub G
Check out Project Silk. Not only does it demonstrate DDD but other cutting edge patterns. This is an excellent resource for any Web Developer. A full overview of the project can be found on MSDN.
This is a good example based on domain driven design and explains why it is important to have separate domain layer.
Microsoft spain - DDD N Layer Architecture
ddd-cqrs-sample is also a good resource. Written with Java, Spring and JPA.
Updated link:
https://github.com/BottegaIT/ddd-leaven-v2
Code Camp Server, Jeffrey Palermo's sample code for the book ASP.NET MVC in Action. While the book is focused on the presentation layer, the application is modeled using DDD.