Udi Dahan   Udi Dahan – The Software Simplist
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Fetching Strategy Design

Monday, April 23rd, 2007.

Following up on my previous post on Better Domain-Driven Design Implementation, I wanted to show some more detail on how this actually works. There are two main concepts here.

The first is that of keeping Service Layer classes independent of Domain Model classes. The reason this is desirable is that the two families change on independent axis. Service Layer classes are affected by changes to the service’s external interface, as well as the interfaces of other services it depends on. Domain Model classes change to support changing business rules internal to the service. The solution is quite simply to introduce a set of interfaces between the two.

The second is tied to performance. When retrieving data from the database, we’d like to cross the wire only once bringing with us all the data we need in order to perform the work required. Lazy loading helps us in one way while hurting us in another. For connected objects that we don’t need when retrieving our target object, lazy loading prevents them from being loaded. However, for connected objects that we do need, lazy loading will cause us to return to the database to retrieve each of those objects in turn (sets of the same kind of object are still one DB call). If those objects need to be traversed as well in order to retrieve other objects, we can see that one simple request can cause many (MANY!) calls to the DB. These problems of latency and throughput are solved by eagerly (in one DB call) fetching and loading all the objects we need.

Here’s the package diagram for the solution so that we’ll have a reference for the following discussion.

fetching strategy package diagram - opens in a new window

Now, the class that is best suited to issue this call to eagerly load all objects is the service layer class, since it is aware of what specific use case we’re in. However, the service layer class does not necessarily know exactly what classes will be used in the domain to handle that request. Obviously, this set of classes may change as the domain model and the database schema change. We can therefore say that is not the service layer class’ responsibility to handle the definition of which classes need to be loaded. Only the Domain Model has that knowledge. So we need some way to pass the knowledge of the request type into the Domain Model.

In Object/Role Modeling terms, we represent that request type as a role. And roles are represented by interfaces. So we create an interface in the Domain Model which represents the request type. That interface will most likely contain only one method – the method which the service layer class will call.

After we have the first role, we can build other things around it, like fetching strategies. We can then define other classes who fullfil the role of “I’m his fetching strategy”. Those classes will expose a property defining the exact set of classes to load, when using NHibernate we use HQL.

Here’s the sequence diagram showing how this works.

fetching strategy sequence diagram - opens in a new window

One question that you might ask is what if there is more than one class that implements the same interface? How could the infrastructure know which class it should be loading? The answer is simple. You shouldn’t do that. Having two classes fulfilling the same role will get you in trouble even if you don’t use this design. The Single Responsibility Principle should be our guiding light.

In summary, achieving high performance is possible when using Domain Models and Object/Relational Mapping, but requires minimizing calls to the database. This design decreases coupling between all the cooperating parts of the solution without giving up the ability to optimize over a specific technology.

Download the full detailed design (in HTML format). I’ll be offering the edittable Enterprise Architect format shortly.

  
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20 Comments

  1. Evan Says:

    very nice..a question on your packaging however..

    we know that the Order type is an aggregate root, but we also know that the Customer type could also be an aggregate root (in another context)..

    Do you duplicate your package diagram for each aggregate in your application/service? How does this packaging work with your Customer Service (where the context of the Customer aggregate includes other non-Order items)

    This packaging scheme would seem ideal per aggregate (so that changing a table name is a single package change, adding a database column and new method would only affect 2 packages, etc)..but would end up with many dlls (which just means better change/configuration mgmt i suppose)..

    I get the sneaky feeling you are going to tell me the Customer aggregate goes into a seperate service but figured i’d ask.. 😉

    Given that packages are supposed to be units of reuse, how does that fit into your package schema (given that the Customer type crosses aggregate boundaries) if at all..


  2. thesoftwaresimplist Says:

    Evan,

    Glad you like it.

    Here’s what I think:

    I do not duplicate the package diagram for each aggregate root. The packages called Interfaces, Infrastructure, and NHibernate are used by all.

    I often find this pattern repeating (Service->Domain; Domain Objects->Domain; Persistence->Domain & Domain Objects) between bounded contexts.

    I don’t often see the Customer class in a single bounded context. Rather, there is a different Customer class for each context, representing exactly the part it needs. When it comes to the “Customer Care” context, we see different rules around Orders, than in the “Online Shop” context.

    I don’t quite view packages as units of reuse, but rather as a tool for managing dependencies.

    Does that make sense?


  3. » First principle of design refined Says:

    […] That feels better. It holds for Views interacting with Controllers, MessageHandlers with Messages, even interactions with Domain Classes. […]


  4. » Performant and Explicit Domain Models Says:

    […] Fetching Strategy Design – showing how to separate the concern of eager loading from both your Domain Model and your Service Layer. […]


  5. Fetching Strategy NHibernate Implementation Available Says:

    […] couple of months ago I put out a post discussing one way to implement custom fetching strategies. Anyway, I finally got around to putting my money where my mouth […]


  6. Colin Jack Says:

    I’ve just re-read this post and couldn’t really get what you mean in this bit:

    “One question that you might ask is what if there is more than one class that implements the same interface? How could the infrastructure know which class it should be loading? The answer is simple. You shouldn’t do that. Having two classes fulfilling the same role will get you in trouble even if you don’t use this design. The Single Responsibility Principle should be our guiding light.”

    You are not really saying that SRP is driving you to define interfaces and then to only have one implementation?

    This isn’t the way I read SRP at all, I read it as one reason for change. I also don’t see what you are suggesting as being the (only?) way to do interface based programming, in fact I’d argue its often silly to extract interfaces if you are 100% sure there will only ever be one implementation.

    From the term role here I’m thinking of it being a role in the role-responsibility sense, if so I again would say that if multiple concrete classes can sensibly fulfill a role then a common interface is definitely the way to go?


  7. thesoftwaresimplist Says:

    Colin,

    The idea is not so much to do a “extract interface” refactoring, but rather to represent a use-case a concrete concept in code. Once that interface is there, we can do lots of useful things with it – for instance, different fetching strategies for different use cases.

    I’d also like to refine my statements a bit and say that if there were two (or more) concrete classes that implemented the same interface (role), then, by definition, you shouldn’t care which class you get. If you think the code HAS to care, then you probably have a role struggling to express itself.

    Does that make sense?


  8. Colin Jack Says:

    Completely, and I’m enjoying the series of posts relating to the topic (and thanks for providing the code example of how to apply it!).


  9. awake Says:

    Great job. I was on TechEd 2007 and I saw you presentation about domain models. Good stuff. Could you send me all demos you use on this presentation becouse on post conference dvd i found only presentation and webcast 🙁 thx


  10. Bjorn Says:

    Hi Udi. Great articles all over the place. Now, I was wondering about the package diagram above. What kind of objects does your service layer deliver to the layers above? Objects from the DomainObjects package (or of course rather its interfaces)? Ok, I get that a CalculateCost-method returns a Money-object. But does a method like GetOrder return a Domain.IOrder? But these objects/interfaces are more than just DTOs aren’t they and have methods like Update and so on? Or would you create special Message Objects when the data moves across service boundaries to let’s say a web application?


  11. udidahan Says:

    Bjorn,

    My service layer usually does messaging – those “special Message Objects” you alluded to (which are DTOs).

    You might like this post describing how these two might work together in a broader perspective here:

    Command Query Separation and SOA


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    […] Fetching strategy posts by Simon Segal: […]


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    […] when a role has been specified. The role can be reasoned as a business event of sorts. Udi has written extensively on this and developed a method for using fetching strategies with NHibernate and if you haven’t […]


  17. Stepping onto the Bus | YOOT Says:

    […] to use when querying in a true DDD fashion is the notion of Fetching Strategy (read more about it here & there). We need Fetching Strategies for a very simple […]


  18. Igal Says:

    Hi Udi,

    I’ve Implemented the general Idea of what you’ve written here, my question is a bit on the technical side.

    I have a use-case in my app where I need the first two levels of my object tree.
    I have an object called Player which has a list called CheckIns
    that holds a many-to-one connection to Player as an object reference

    when I execute the Query I get back a seemingly endless chain of
    Player->CheckIns->Player->CheckIns and so on..

    What I need is Player->CheckIns

    the relevant code :

    public ICriteria AddFetchJoinTo(ICriteria criteria)
    {
    criteria.SetFetchMode(“PlayerCheckIns”, FetchMode.Join)
    .SetFetchMode(“PlayerCheckIns.Player”, FetchMode.Lazy);

    return criteria;
    }

    public IList RetrieveEquals(string propertyName, object propertyValue, IFetchingStrategy fetchingStrategy)
    {
    using (ISession session = m_SessionFactory.OpenSession())
    {
    ICriteria criteria = session.CreateCriteria(typeof(T));
    if (fetchingStrategy != null)
    {
    fetchingStrategy.AddFetchJoinTo(criteria);
    }
    criteria.Add(Expression.Eq(propertyName, propertyValue));
    // Get the matching objects
    IList matchingObjects = criteria.List();

    // Set return value
    return matchingObjects;
    }
    }

    Player.hbm.xml

    CheckIn.hbm.xml

    thanks alot!
    Igal


  19. Igal Says:

    My xml files were omitted for some reason..
    I’ll try again..

    Player.hbm.xml

    CheckIn.hbm.xml


  20. Rupesh Kumar Tiwari Says:

    Hi Udi,
    In domain driven design they suggest us to crate reach domain model.
    Therefore, first i will attempt to put the behavior on Order object to calculate Total cost. If you create a Role interface IOrderCalculator then who will implement it ? Is it the same ORDER class ? If Order class will implement it then somewhere I read that ENTITY never implements any interface or multiple interfaces. OR if some other object (OrderCalculator) implements the ROLE then our ORDER class wont have behavior and will become an anemic domain model. And what is the other object (OrderCalculator) is ? Is it an aggregate ?Could you please explain ?


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Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

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