Tag Archives: DDD

Domain-Driven Design Europe 2018 in Amsterdam

I’m excited to be attending and speaking at DDD Europe 2018 in Amsterdam on Feb 1-2 2018. It’s an honor to be on the speaker roster with so many DDD gurus and other people with amazing DDD experiences stories to share.

The lowest early-bird ticket prices can still be bought through Nov 30 at €599 (+21% VAT =  €724 (app. ~$860US +). The ticket will go to €699 (+VAT) from Dec 1-Dec31 and then to €749 (+VAT) until the conference.

Prior to the conference,  there are also 10 amazing workshops ranging from 1/2 day to 2 days across January 30 – 31st.

I’ll be doing a 2  hour workshop during the conference proper on using EF Core 2 to map DDD patterns in your domain. It will be a hands-on workshop and my intention is to build some koans for attendees to work with. Although the flavor of hands-on may shift as I continue to percolate ideas.


Another Use Case for DbContext.Add in EFCore (and a DDD win)

If you are like me and design your classes following Domain-Driven Design principals , you may find yourself with code like this for controlling how objects get added to collections in the root entity.

public class Samurai {

  public Samurai (string name) : this()
     Name = name;

  private Samurai ()
    _quotes=new List<Quote>();

  public int Id { get; private set; }
  public string Name { get; private set; }
  private readonly List _quotes = new List ()
  private IEnumerable Quotes => _quotes.ToList ();
  public void AddQuote (string quoteText) {
      var newQuote=new Quote(quoteText,Id);
      _quotes.Add (newQuote);

I have a fully encapsulated collection of Quotes. The only way to add a new quote is through the AddQuote method. You can’t just call Samura.Quotes.Add(myquote).

Additionally, because I want to control how developers interact with my API, there is no DbSet for Quotes. You have to do all of your queries and updates via context.Samurais.

A big downside to this is that if I have a new quote and I know the ID of the samurai, I have to first query for the samurai and then use the AddQuote. That really bugs me. I just want to create a new quote, push in the Samurai’s ID value and save it. And that requires either raw SQL or a DbSet<Quote>. I don’t like either option. Raw SQL is a hack in this case and DbSet<Quote> will open my API up to potential misuse.

I was thinking about this problem while laying in bed this morning (admit it, that’s the first thing you do when you wake up, too, right?) and had an idea.

In EF Core, we can now add objects directly to the context without going through the DbSet. The context can figure out what DbSet the entity belongs to and apply the right info to the change tracker. I thought this was handy for being able to call

myContext.AddRange(personobjectA, accountobjectB, productObjectC);

Although I haven’t run into a good use case for leveraging that yet.

What occurred to me is that if DbContext.Add is  using  reflection, maybe EF Core can find a private DbSet.

So I added a private DbSet to my DbContext class:

private DbSet<Quote> Quotes { get; set; }
 And tried out this code (notice I’m using context.Add, not context.Quotes.Add):
static void AddQuoteToSamurai () 
  using (var context =newSamuraiContext ()) 
    var quote=newQuote("Voila",1);
And it worked! But this isn’t complete yet. I’m breaking my rule of ensuring that only my aggregate root can manage quotes. So this is “dangerous” code from my DDD perspective. However, I was happy to know that EF Core would support this capability.
Currently, Samurai.AddQuote does not have any additional logic to be performed on the quote. What if I were to add in a “RemoveBadWords” rule before a quote can get added?
public void AddQuote (string quoteText) 
 var newQuote=new Quote(quoteText,Id);
  _quotes.Add (newQuote);
 Now I have an important reason to use Samurai to do the deed. I can add a second, static AddQuote method that also takes an int. Because it’s static, it’s a pass through method.
public static Quote AddQuote(string quoteText,int samuraiId)
  var newQuote=newQuote(quoteText,samuraiId);
  return newQuote;

This works and now I don’t have to have an instance of Samurai to use it:

staticvoid AddQuoteToSamurai () 
  using (var context =newSamuraiContext ()) {
    context.Add(Samurai.AddQuote("static voila",1));

One thing I was worried about was if I had an instance of Samurai and tried to use this to add a quote to a different samurai. That would break the aggregate root…it’s job is to manage its own quotes only. It shouldn’t know about other Samurais.

But .NET protects me from that. I can’t call the static method from an instance of Samurai.

I still think that there’s a little bit of code smell from a DDD perspective about having this static, pass-through method in an aggregate root so will have to investigate that (or wait for any unhappy DDDers in my comments). But for now I am happy that I can avoid having to query for an instance of Samurai just to do this one task.

Completely New EF in the Enterprise Course on Pluralsight

My baby is here! A brand new Entity Framework in the Enterprise.

[See also:  New EF Core Course on Pluralsight!]

In 2012, I published a course on Pluralsight called Entity Framework in the Enterprise. Since then I have learned so much, most importantly, I’ve become very active with Domain-Driven Design, even publishing the DDD Fundamentals course that I co-created with Steve Smith. This has had a big impact on how I think about designing and architecting software and in turn,how I approach incorporating EF in to large, complex applications.

I’ve been wanting to re-do that old course to share my new views. I finally began in January of this year, but had a 3 month conference travel hiatus. So while it feels like a baby that I spent 9 months on, it was really only 6 months. Still quite a long time!

The course is now live! Entity Framework in the Enterprise

In the course, I use VS2015 and EF6. Why EF6? Because EF Core is too new. Most of the patterns I discuss and demonstrate are totally applicable to EF Core. There is one thing that is not yet in EF Core: Value Objects, but that is coming. Also the module on testing focuses on mocking and does not take EF Core’s new In Memory provider into account. Other than that, you can use what you learn here with EF Core as well.

I put a lot of thought into this course and I think this comment on the discussion forum for the course expresses it so well:

I just watched Julie Lerman’s prior Entity Framework in the Enterprise three weeks ago, before this new course was released, and boy am I glad she’s updated the course. I had thought the previous version was a bit dated (2016 vs 2012 & EF6 vs EF4) and a bit basic with what Julie refers to as Demo Ware. This updated course goes into more details about architecting projects, improved Moq testing with EF6, and a better explanation of DDD with Bounded Contexts using Schema to segregate areas. I already had a good understanding of EF, DDD, Repositories, UoW, and CQRS before watching and while I wouldn’t set up things 100% this way in my own applications they did jump start some refactorings and rethinking on how I maintain my solution, which is the purpose of these courses, to give fresh ideas as technology evolves, just as Entity Framework has. Thanks for updating the course and for those who have watched the previous version, definitely give this new one a watch.


Upcoming EF Course on Pluralsight

9/14: I’ve been told “a few more business days”. Believe me I’m as eager as anyone can be for this to get released! 🙂

9/23: It’s here!!! 


At the beginning of the year, I started dong a completely new version of my EF in the Enterprise course. That one is years old and was done with EF4 before DbContext and Code First even existed. Also before I started learning about Domain Driven-Design.

I had a lot of side-tracks in my schedule along the way including 3 months of conference travel and  a lot of hard thinking about how to explain and demonstrate some of these concepts. But I’ve finally finished the last module yesterday. I have some work to do in response to tech reviews of some of the module and I have to do the dreaded task of coming up with the questions for a few of the modules. But then it will be ready for Pluralsight to push through and get published. I don’t think it will be long now.

While I used EF6 for this course, most of the ideas also apply to EFCore as well.

In the meantime, I can tell you the titles of the 8 modules of this course which seems to have come out a little under 5 hours total:

  1. Architecting a Data Layer
  2. Understanding EF Encapsulation and the Great Repository Debates
  3. Implementing Encapsulation Patterns with EF6
  4. Managing Complex Domains and Data Models: Lessons from DDD Bounded Context
  5. Refactoring to Domain-Driven Design Bounded Contexts:A Walkthrough
  6. Handling the State of Disconnected Graphs
  7. Mapping DDD Domain Models with Entity Framework
  8. Testing Your Apps When Entity Framework is Involved

Watch this space for the new course: bit.ly/PS-Julie

A quote about isolated data stores from Eric Evans

I need to keep this quote around so where better than my blog?

“For as long as I can remember, I’ve been advocating that a team that’s developing some complex piece of logic should have their own isolated data store and not have to share some huge database that has some kind of mishmash of different people’s ideas of what the data should be and so forth.”

(Eric Evans, “DDD & Microservices: At Last, Some Boundaries!” GOTO Berlin Dec 2015 (gotober.com) https://www.youtube.com/watch?v=yPvef9R3k-M)

This is such a difficult thing for so many developers to buy into. I frequently share a story about Eric trying to ease me out of a near mental meltdown, explaining to me when I was struggling with this (& I am paraphrasing and possibly adding some of my own comprehension at this point) that you have to pay the price somewhere and often its just less painful to solve the problem of having data in different places that needs to connect once in a while than it is to deal with the complexity of a system that tries to provide all answers to all of the problems. We have patterns to solve the “connect the data once in a while” problem (e.g. message queues).

I admit that I have to advise people to aim for this but it’s often not practical (or cost-effective) so if we’re talking about relational databases then we can at least isolate with schemas. But I do try to start with “separate database” and work our way backwards from there if it’s just not doable (or if the person or team I’m talking to looks like they are about to murder me). However each step backwards comes with a price. You just have to choose – pick your poison. As long as you are doing so with the right information at hand to make those decisions & choices, I think that’s the most important part.

My TechEd Europe Schedule


TEEU_2014_I’m speaking_1I’m so very excited to be heading to Barcelona shortly for TechEd Europe. You may or may not be aware of my dashed hopes last year when I was scheduled to speak at TechEd Europe in Madrid. But when I found myself stuck in the Chicago O’Hare airport on my 2nd day of travel, only to be told I wouldn’t get to Madrid until Thursday afternoon (after 4 days of travel) it was time to turn around and go home. I also had scheduled a post event side trip to Barcelona – this too, I canceled.

I was a little surprised that TechEd was willing to try again considering my bad history but I will be speaking at the 2014 version of the event.

Not only am I looking forward to my first ever trip to Spain and the amazing city of Barcelona, but it will be my first opportunity to meet so many developers that I’ve never met before.

My Session

My talk is on Friday, Oct 31 at 12-1:15pm. Yes, on Halloween!

Entity Framework Model Partitioning in Domain-Driven Design Bounded Contexts
Bounded Context is one of the most critical patterns in Domain-Driven Design (DDD). While you shouldn’t have to worry about your data persistence while you are modeling your domain, it is still worrisome to follow DDD patterns not knowing how you’ll get your ORM to fall in line when it’s time to implement the data layer. Entity Framework sample solutions have a long history of creating one huge model to map to one huge database to satisfy all of the needs of your application. In this session we look at patterns using Entity Framework that will support not only multiple bounded contexts in your software but even those areas that are satisfied with pure CRUD logic. You also see patterns to bridge the divides between those bounded contexts when implementing the data persistence with Entity Framework.

Getting the Most Out of My Session

Dino Esposito will be doing a relevant talk on Tuesday that will be useful if you are new to Domain-Driven Design.

DEV-B211 Architecting and Implementing Domain-Driven Design Patterns with Microsoft .NET  Tuesday, October 28 1:30 PM – 2:45 PM Room: Hall 8.1 Room G

I’d also recommend watching the Domain-Driven Design Fundamentals course that Steve Smith and I created for Pluralsight. It’s about 4 hours long and is a great first look as well as a refresher on some of the key concepts of DDD. You’ll definitely get a lot more out of my talk if you’ve seen this in advance. If you don’t have a subscription to Pluralsight, send me a note and I can get you a 30 day trial code.

Where to Find Me

Here is my current schedule. This does not list talks I plan to attend, but the opportunities I will have to meet people and geek out! Smile


So much bling, I just can’t choose!

TEEU_2014_I’m speaking_4TEEU_2014_I’m speaking_1TEEU_2014_I’m speaking_3

DDD Fundamentals Course is Making a Difference

Steve Smith and I are so happy to be getting this kind of feedback about our Domain-Driven Fundamentals course on Pluralsight. We wanted to give developers help understanding the basics so that they can dig in to more indepth resources like Eric Evans’ or Vaughn Vernon’s books.

Here are some recent tweets we’ve seen.

Ryan Moseley @bahamaboy85

Completed the DDD Fundamentals course from @pluralsight by @ardalis @julielerman http://pluralsight.com/courses/domain-driven-design-fundamentals …. This course makes DDD accessible.

It is tough to find DDD examples that don’t assume some level of knowledge. So this course is welcomed.

Tom Davis @photomoose · Sep 9

@julielerman Just watched your DDD course on @pluralsight. Probably the best intro to DDD I’ve seen; things make sense now!

Paul Hale @paulhale · Aug 29

@pluralsight @ardalis @julielerman cracking #DDD course + @ericevans0 book set me on right track. Ironically Im building a domain reg system

Félix-A. Bourbonnais @fbourbonnais · Aug 11

Good #DDDDesign course by @julielerman @ardalis. Key concepts of DDD are well explained. I recommend http://owl.li/AcX4q

Geoff Mazeroff @geoffmazeroff
· Jul 30

My work project uses DDD, so I enjoyed hearing @ardalis and @julielerman present the fundamentals on their @Pluralsight course. Nice work!

Mike Sussman @mikesussman
· Jul 25

Finished @pluralsight DDD Fundamentals course by @julielerman and @ardalis, makes a lot more sense now.

Domain-Driven Design Fundamentals Course on Pluralsight

Steve Smith and I are excited to finally have our Domain-Driven Design Fundamentals course available on Pluralsight (http://juliel.me/PS-DDD ) . It was a long haul creating the course but we wanted it to be just right. And coordinating our schedules became tricky since we really wanted to collaborate completely on this, not just divide up the work and sew it together.

We were also grateful to have Eric Evan’s, the “father” of DDD, participate in the course by letting us interview him and use the resulting video clips where he provides thoughtful and insightful advice throughout the course.

The course has now been out for a few weeks and the reception has been great. Good ratings, great feedback in the discussions and on twitter are making us quite proud of our hard work. And it’s has been in Pluralsight’s Top 10 list (based on past 10 days viewing) in the entire library of some [I believe] 1500 courses since a few days after it was released.

If you don’t have a subscription (but really, they are only $29/month), there is a 10 day free trial on the website and I also have 30 day free trial cards I can share. Let us know what you think!


Some DDD+EF Resources

Could not fit this into a tweet in response to a request.

Here are some resources for how EF can be a partner to a Domain Driven Design world:

Update June 2015:

Microsoft Ignite, Chicago May 2015
Domain-Driven Design for the Database Driven Mind (90 minute presentation video on Channel 9)

Update: October 2014:

MSDN Mag Oct 2014 Issue:
Data Points: A Pattern for Sharing Data Across Domain-Driven Design Bounded Contexts
Julie Lerman explores a way to mirror data as one scenario for sharing data across DDD bounded contexts.

MSDN Mag Dec 2014 Issue:
Data Points: A Pattern for Sharing Data Across Domain-Driven Design Bounded Contexts : Part 2
(will provide link when available)

Video: Oredev, Fall 2013 (by me)

Video: TechEd North America, May 2014 (Vaughn Vernon)
How You Can Implement Aggregates and Domain Entities Effectively in Domain Models, with .NET

Article Series:
MSDN Magazine Data Points Column (by me) Aug, Sept & Oct 2013
Coding for Domain-Driven Design: Tips for Data-Focused Devs, Part 1
Coding for Domain-Driven Design: Tips for Data-Focused Devs, Part 2
Coding for Domain-Driven Design: Tips for Data-Focused Devs, Part 3

Domain modeling with Entity Framework Scorecard (Jimmy Bogard)

Bonus for NHibernate Converts:
And for those who are looking for this by way of moving from DDD + NH, Jimmy Bogard did a great post (with pointers to related posts) on Migrating from NHibernate to EF.