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DDD East Anglia, 29th June 2013 - Async Patterns presentation and source code

- by Liam Westley

Originally posted on: http://geekswithblogs.net/twickers/archive/2013/07/01/ddd-east-anglia-29th-june-2013---async-patterns-presentation.aspxMany thanks to the team in Cambridge for an awesome first conference DDD East Anglia. I definitely appreciate how each of the different areas have their own distinctive atmosphere and feel. Thanks to some great sponsors we enjoyed a great venue and some excellent nibbles. For those who attended my Async my source code and presentation are available on GitHub, https://github.com/westleyl/DDDEastAnglia2013-Async.git If you are new to Git then the easiest client to install is GitHub for Windows, a graphical UI for accessing GitHub. Personally, I also have Git Extensions and Tortoise Git installed. Tortoise Git is the file explorer add-in that works in a familiar manner to TortoiseSVN. As I mentioned during the presentation I have not included the sample data, the music files, in the source code placed on GitHub but I have included instructions on how to download them from http://silents.bandcamp.comand place them in the correct folders. Also, Windows Media Player, by default, does not play Ogg Vorbis and Flac music files, however you can download the codec installer for these, for free, from http://xiph.org/dshow. I have included the .Net 4.0 version of the source code that uses the Microsoft.Bcl.Async NuGet package - once you have got the project from GitHub you will need to install this NuGet package for the code to compile. Load Project into Visual Studio 2012 Access the NuGet package manager (Tools -> Library Package Manager -> Manage NuGet Packages For Solution) Highlight Online and then Search Online for microsoft.bcl.async Click on Install button Resources : You can download the Task-based Asynchronous Pattern white paper by Stephen Toub, which was the inspiration for this presentation from here - http://www.microsoft.com/en-us/download/details.aspx?id=19957 Presentation : If you just want the presentation and don’t want to bother with a GitHub login you can download the PowerPoint presentation from here.

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What is the best way to use EF 4 and DDD

- by William

I would like to use EFf 4 as my ORM in my DDD project. I am going to generate my model based on my classes. Should I create classes that are basically dto objects for my business objects to consumer or should I implement the actuall BO classes in my EF model?

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DDD: Service or Repository

- by tikhop

I am developing an app in DDD manner. And I have a little problem with it. I have a Fare (airline fare) and FareRepository objects. And at some point I should load additional fare information and set this information to existing Fare. I guess that I need to create an Application Service (FareAdditionalInformationService) that will deal with obtaining data from the server and than update existing Fare. However, some people said me that it is necessary to use FareRepository for this problem. I don't know wich place is better for my problem Service or Repository.

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How to manage long running background threads and report progress with DDD

- by Mr Happy

Title says most of it. I have found surprising little information about this. I have a long running operation of which the user wants to see the progress (as in, item x of y processed). I also need to be able to pause and stop the operation. (Stopping doesn't rollback the items already processed.) The thing is, it's not that each item takes a long time to get processed, it's that that there are usually a lot of items. And what I've read about so far is that it's somewhat of an anti-pattern to put something like a queue in the DB. I currently don't have any messaging system in place, and I've never worked with one either. Another thing I read somewhere is that progress reporting is something that belongs in the application layer, but it didn't go into the details. So having said all this, what I have in mind is the following. User request with list of items enters the application layer. Application layer gets some information from the domain needed to process the items. Application layer passes the items and the information off to some domain service (should the implementation of this service belong in the infrastructure layer?) This service spins up a worker thread with callbacks for both progress reporting and pausing/stopping it. This worker thread will process each item in it's own UoW. This means the domain information from earlier needs to be stored in some DTO. Since nothing is really persisted, the service should be singleton and thread safe Whenever a user requests a progress report or wants to pause/stop the operation, the application layer will ask the service. Would this be a correct solution? Or am I at least on the right track with this? Especially the singleton and thread safe part makes the whole thing feel icky.

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DDD and validation of aggregate root

- by Mik378

Suppose an aggregate root : MailConfiguration (wrapping an AddressPart object). The AddressPart object is a simple immutable value object with some fields like senderAdress, recipentAddress (to make example simple). As being an invariant object, AddressPart should logically wrap its own Validator (by the way of external a kind of AddressValidator for respecting Single Responsibility Principle) I read some articles that claimed an aggregateRoot must validate its 'children'. However, if we follow this principle, one could create an AddressPart with an uncohesive/invalid state. What are your opinion? Should I move the collaborator AddressValidator(used in constructor so in order to validate immediately the cohesion of an AddressPart) from AddressPart and assign it to aggregateRoot (MailConfiguration) ?

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How to keep your unit test Arrange step simple and still guarantee DDD invariants ?

- by ian31

DDD recommends that the domain objects should be in a valid state at any time. Aggregate roots are responsible for guaranteeing the invariants and Factories for assembling objects with all the required parts so that they are initialized in a valid state. However this seems to complicate the task of creating simple, isolated unit tests a lot. Let's assume we have a BookRepository that contains Books. A Book has : an Author a Category a list of Bookstores you can find the book in These are required attributes : a book has to have an author, a category and at least a book store you can buy the book from. There's likely to be a BookFactory since it is quite a complex object, and the Factory will initialize the Book with at least all the mentioned attributes. Now we want to unit test a method of the BookRepository that returns all the Books. To test if the method returns the books, we have to set up a test context (the Arrange step in AAA terms) where some Books are already in the Repository. If the only tool at our disposal to create Book objects is the Factory, the unit test now also uses and is dependent on the Factory and inderectly on Category, Author and Store since we need those objects to build up a Book and then place it in the test context. Would you consider this is a dependency in the same way that in a Service unit test we would be dependent on, say, a Repository that the Service would call ? How would you solve the problem of having to re-create a whole cluster of objects in order to be able to test a simple thing ? How would you break that dependency and get rid of all these attributes we don't need in our test ? By using mocks or stubs ? If you mock up things a Repository contains, what kind of mock/stubs would you use as opposed to when you mock up something the object under test talks to or consumes ?

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How to keep your unit tests simple and isolated and still guarantee DDD invariants ?

- by ian31

DDD recommends that the domain objects should be in a valid state at any time. Aggregate roots are responsible for guaranteeing the invariants and Factories for assembling objects with all the required parts so that they are initialized in a valid state. However this seems to complicate the task of creating simple, isolated unit tests a lot. Let's assume we have a BookRepository that contains Books. A Book has : an Author a Category a list of Bookstores you can find the book in These are required attributes : a book has to have an author, a category and at least a book store you can buy the book from. There's likely to be a BookFactory since it is quite a complex object, and the Factory will initialize the Book with at least all the mentioned attributes. Now we want to unit test a method of the BookRepository that returns all the Books. To test if the method returns the books, we have to set up a test context (the Arrange step in AAA terms) where some Books are already in the Repository. If the only tool at our disposal to create Book objects is the Factory, the unit test now also uses and is dependent on the Factory and inderectly on Category, Author and Store since we need those objects to build up a Book and then place it in the test context. Would you consider this is a dependency in the same way that in a Service unit test we would be dependent on, say, a Repository that the Service would call ? How would you solve the problem of having to re-create a whole cluster of objects in order to be able to test a simple thing ? How would you break that dependency and get rid of all these attributes we don't need in our test ? By using mocks or stubs ? If you mock up things a Repository contains, what kind of mock/stubs would you use as opposed to when you mock up something the object under test talks to or consumes ?

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DDD and MVC Models hold ID of separate entity or the entity itself?

- by Dr. Zim

If you have an Order that references a customer, does the model include the ID of the customer or a copy of the customer object like a value object (thinking DDD)? I would like to do ths: public class Order { public int ID {get;set;} public Customer customer {get;set;} ... } right now I do this: public class Order { public int ID {get;set;} public int customerID {get;set;} ... } It would be more convenient to include the complete customer object rather than an ID to the View Model passed to the form. Otherwise, I need to figure out how to get the vendor information to the view that the order references by ID. This also implies that the repository understand how to deal with the customer object that it finds within the order object when they call save (if we select the first option). If we select the second option, we will need to know where in the view model to put it. It is certain they will select an existing customer. However, it is also certain they may want to change the information in-place on the display form. One could argue to have the controller extract the customer object, submit customer changes separately to the repository, then submit changes to the order, keeping the customerID in the order.

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Strange behavior of DDD debugger in Ubuntu

- by Alex Farber

I installed DDD debugger in Ubuntu and trying to work with it. It looks like DDD UI doesn't work properly with Ubuntu desktop environment. Edit boxes are almost unusable: sometimes they accept keyboard input, most of times input is ignored. Internal resizing panes are not working. Is there some way to get DDD UI working properly? The same behavior is in Ubuntu 9.10 32 bit, and 10.4 64 bit, so this is not Ubuntu version issue.

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DDD and Client/Server apps

- by Christophe Herreman

I was wondering if any of you had successfully implemented DDD in a Client/Server app and would like to share some experiences. We are currently working on a smart client in Flex and a backend in Java. On the server we have a service layer exposed to the client that offers CRUD operations amongst some other service methods. I understand that in DDD these services should be repositories and services should be used to handle use cases that do not fit inside a repository. Right now, we mimic these services on the client behind an interface and inject implementations (Webservices, RMI, etc) via an IoC container. So some questions arise: should the server expose repositories to the client or do we need to have some sort of a facade (that is able to handle security for instance) should the client implement repositories (and DDD in general?) knowing that in the client, most of the logic is view related and real business logic lives on the server. All communication with the server happens asynchronously and we have a single threaded programming model on the client. how about mapping client to server objects and vice versa? We tried DTO's but reverted back to exposing the state of our objects and mapping directly to them. I know this is considered bad practice, but it saves us an incredible amount of time) In general I think a new generation of applications is coming with the growth of Flex, Silverlight, JavaFX and I'm curious how DDD fits into this.

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DDD Infrastructure services

- by Zygimantas

Hello, I am learning DDD and I am a little bit lost in Infrastructure layer: As I understand, "all good DDD applications" should have 4 layers: Presentation, Application, Domain and Infrastructure. Database should be accessed using Repositories. Repository interfaces should be in Domain layer and repository implementation - in Infrastructure (reference http://stackoverflow.com/questions/693221/ddd-where-to-keep-domain-interfaces-the-infrastructure). Application, Domain and Infrastructure layer should/may have services (reference www.lostechies.com/blogs/jimmy_bogard/archive/2008/08/21/services-in-domain-driven-design.aspx), in example EmailService in Infrastructure layer which sends Email messages. BUT, inside Infrastructure layer we have repository implementations, which are used to access database. So, in this case, repositories are database services? What is the difference between Infrastructure service and repository? Thanks in advance!

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How to model a many to many from a DDD perspective in UML?

- by JD01

I have a two entity objects Site and Customer where there is a many to many relationship. I have read you try not to model this in DDD as it is in the data model and go for a unidirectional flow. If I wanted to show this in UML, would I show it as it is in the data model: Site * ----->*Customer but the direction arrow gives the flow? or as following Site ----->*Customer But then this would imply that Customer can only go in one site.

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DDD with Grails

- by Paul

I cannot find any info about doing Domain Driven Design (DDD) with Grails. I'm looking for any best practices, experience notes or even open source projects that are good examples of DDD with Grails.

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Silverlight 4 with WCF RIA architecture applying DDD

- by doteneter

Hello, In my ASP.NET MVC applications I use DDD and it works very well. I'm new to Silverlight development and would like to know how could I apply DDD to build a new architecture. I had a look on WCF RIA Services and what is exposed by default it's the simple CRUD methods. I would like to use MVVM pattern. I thought about general architecture and don't know if what I'm thinking about make sense in Silverlight development. I thought about creating Domain Model on the top of SVC. I would than expose by WCF RIA some operation that deals with aggreates in my Domain Model instead of simple CRUD. What I would aloso expose is the ViewModel entieties that could be used by the view. I don't know if it's make sense, if I'm going in a good direction or if applying DDD in Silverlight 4 development is a good practice. I didn't find much informations on Internet. I'll appreciate if you could point me to some interesting links or if you can give me some hints. Thanks for your help.

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DDD: Client-side script to enforce invariants

- by Mosh

Hello, One thing that I'm confused about in regards to DDD is that our domain is supposed to handle all business logic and enforce invariants. I have noticed some people (me included) handle certain invariants in the presentation layer (i.e. WebForms, Views, etc) with javascript. This is mainly done to improve performance so the server is not hit for every request which may be invalid. Even though this approach may be beneficial performance-wise, it violates DDD principles. What if the business rules are changed? This way we don't have a rich domain where all the business rules are captured. In case of a change, we should change the domain as well as the presentation layer. Has anyone come across this situation before? I'd like to know your thoughts on this. Cheers, Mosh

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in DDD (gdb) how to skip passed loops

- by Andrei

During many, sometimes inundating, debugging sessions using DDD, I stumble upon loops. And I keep pressing next to get passed it, and if there are many iterations, I just set a break point right after it, and press "continue." Is there any other way to go passed loops? Thanks

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in DDD (gdb) how to skip past loops

- by Andrei

During many, sometimes inundating, debugging sessions using DDD, I stumble upon loops. And I keep pressing next to get past it, and if there are many iterations, I just set a break point right after it, and press "continue." Is there any other way to go past loops? Thanks

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DDD: Getting aggregate roots for other aggregates

- by Ed

I've been studying DDD for the past 2 weeks, and one of the things that really stuck out to me was how aggregate roots can contain other aggregate roots. Aggregate roots are retrieved from the repository, but if a root contains another root, does the repository have a reference to the other repository and asks it to build the subroot?

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TDD, DDD and the No-getters principle

- by Justin

Hi all, After several years of following the bad practice handed down from 'architects' at my place of work and thinking that there must be a better way, I've recently been reading up around TDD and DDD and I think the principles and practices would be a great fit for the complexity of the software we write. However, many of the TDD samples I have seen call a method on the domain object and then test properties of the object to ensure the behaviour executed correctly. On the other hand, several respected people in the industry (Greg Young most noticeably so) advocate the "no-getters" principle on our domain objects. My question therefore is: How does one test the functionality of a domain object if it is forbidden to retrieve its state? I believe I am missing something fundamental so please feel free to call me an idiot and enlighten me - any guidance would be greatly appreciated.

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DDD and IOC Containers

- by MegaByte

Hi Im fairly new with DDD and Im trying to use IOC in order to loosen up my tightly coupled layers :) My C# web application consists of a UI, domain and persistence layer. My persistence layer references my domain layer and contains my concrete repository implementations and nhibernate mappings. Currently my UI references my domain layer. My question : How do I use an IOC container to inject my concrete classes, in my persistence layer, into my domain layer? Does this meen that my UI should also reference my persistence layer?

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DDD - Validation of unique constraint

- by W3Max

In DDD you should never let your entities enter an invalid state. That being said, how do you handle the validation of a unique constraint? The creation of an entity is not a real problem. But let say you have an entity that must have a unique name and there is a thousand instances of this entity type - they are not in memory but stored in a database. Now let say you want to rename an instance. You can't just use a setter... the object could enter an invalid state - you have to validate against the database. How do you handle this scenario in a web environment?

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How to organize a ASP.NET MVC solution (DDD)

- by vandalo

Hello everyone. I am trying to start a new project (asp.net MVC) and I would like to apply some DDD rules I've learned in these last few months. I don't know how to organize my solution though. I would like to use Nhibernate but I don't want to use Fluent Nhibernate cause it must be something like an experiment. I've seen some examples where people keep everything in the same project. Some others tend to create a different project for everything. Do you think I should differentiate the model and the repository or put it in the same project? If someone has some links to articles etc it would be appreciated. thanks Alberto

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Implement DDD and drawing the line between the an Entity and value object

- by William

I am implementing an EMR project. I would like to apply a DDD based approach to the problem. I have identified the "Patient" as being the core object of the system. I understand Patient would be an entity object as well as an aggregrate. I have also identified that every patient must have a "Doctor" and "Medical Records". The medical records would encompass Labs, XRays, Encounter.... I believe those would be entity objects as well. Let us take a Encounter for example. My implementation currently has a few fields as "String" properties, which are the complaint, assessment and plan. The other items necessary for an Encounter are vitals. I have implemented vitals as a value object. Given that it will be necessary to retrieve vitals without haveing to retrieve each Encounter then do vitals become part of the Encounter aggregate and patient aggregrate. I am assuming I could view the Encounter as an aggregrate, because other items are spwaned from the Encounter like prescriptions, lab orders, xrays. Is approach right that I am taking in identifying my entities and aggregates. In the case of vitals, they are specific to a patient, but outside of that there is not any other identity associated with them.

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DDD principlers and ASP.NET MVC project design

- by kaivalya

Two part questions I have a product aggregate that has; Prices PackagingOptions ProductDescriptions ProductImages etc I have modeled one product repository and did not create individual repositories for any of the child classes. All db operations are handled through product repository. Am I understanding the DDD concept correctly so far? Sometimes the question comes to my mind that having a repository for lets say packaging options could make my life easier by directly fetching a the packaging option from the DB by using its ID instead of asking the product repository to find it in its PackagingOptions collection and give it to me.. Second part is managing the edit create operations using ASP.MVC frame work I am currently trying to manage all add edit remove of these child collections of product through product controller(sound right?). One challenge I am now facing is; If I edit a specific packaging option of product through mydomain/product/editpackagingoption/10 I have access to the id of the packaging option But I don't have the ID of the product it self and this forces me to write a query to first find the product that has this specific packaging option then edit that product and the revelant packaging option. I can do this as all packaging option have their unique ID but this would fail if I have collections that don't have unique ID. That feels very wrong.. The next option I thought of is sending both the product and packaging option IDs on the url like; mydomain/product/editpackagingoption/3/10 But I am not sure if that is a good design either. So I am at a point that I am a bit confused. might be having fundamental misunderstandings around all of this... I would appreciate if you bear with the long question and help me put this together. thanks!

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DDD: Persisting aggregates

- by Mosh

Hello, Let's consider the typical Order and OrderItem example. Assuming that OrderItem is part of the Order Aggregate, it an only be added via Order. So, to add a new OrderItem to an Order, we have to load the entire Aggregate via Repository, add a new item to the Order object and persist the entire Aggregate again. This seems to have a lot of overhead. What if our Order has 10 OrderItems? This way, just to add a new OrderItem, not only do we have to read 10 OrderItems, but we should also re-insert all these 10 OrderItems again. (This is the approach that Jimmy Nillson has taken in his DDD book. Everytime he wants to persists an Aggregate, he clears all the child objects, and then re-inserts them again. This can cause other issues as the ID of the children are changed everytime because of the IDENTITY column in database.) I know some people may suggest to apply Unit of Work pattern at the Aggregate Root so it keeps track of what has been changed and only commit those changes. But this violates Persistence Ignorance (PI) principle because persistence logic is leaking into the Domain Model. Has anyone thought about this before? Mosh

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