DDD states that you should only ever access entities through their aggregate root. So say for instance that you have an aggregate root X which potentially has a lot of child Y entities. Now, for some scenario, you only really care about a subset of these Y entities at a time (maybe you're displaying them in a paged list or whatever).
Is it OK to implement a repository then, so that in such scenarios it returns an incomplete aggregate? Ie. an X object who'se Ys collection only contains the Y instances we're interested in and not all of them? This could for instance cause methods on X which perform some calculation involving the Ys to not behave as expected.
Is this perhaps an indication that the Y entity in question should be considered promoted to an aggregate root?
My current idea (in C#) is to leverage the delayed execution of LINQ, so that my X object has an IQueryable to represent its relationship with Y. This way, I can have transparent lazy loading with filtering... But getting this to work with an ORM (Linq to Sql in my case) might be a bit tricky.
Any other clever ideas?
I consider an aggregate root with a lot of child entities to be a code smell, or a DDD smell if you will. :-) Generally I look at two options.
Split your aggregate into many smaller aggregates. This means that my original design was not optimal and I need to identify some new entities.
Split your domain into multiple bounded contexts. This means that there are specific sets of scenarios that use a common subset of the entities in the aggregate, while there are other sets of scenarios that use a different subset.
Jimmy Nilsson hints in his book that instead of reading a complete aggregate you can read a snapshot of parts of it. But you are not supposed to be able to save changes in the snapshot classes to the database.
Jimmy Nilsson's book Chapter 6: Preparing for infrastructure - Querying. Page 226.
Snapshot pattern
You're really asking two overlapping questions.
The title and first half of your question are philosophical/theoretical. I think the reason for accessing entities only through their "aggregate root" is to abstract away the kinds of implementation details you're describing. Access through the aggregate root is a way to reduce complexity by having a trusted point of access. You're eliminating friction/ambiguity/uncertainty by adhering to a convention. It doesn't matter how it's implemented within the root, you just know that when you ask for an entity it will be there. I don't think this perspective rules out a "filtered repository" as you describe. But to provide a pit of success for devs to fall into, it should be impossible instantiate the repository without being explicit about its "filteredness;" likewise, if shared access to a repository instance is possible, the "filteredness" should be explicit when coding in the caller.
The second half of your question is about implementation on a specific platform. Not sure why you mention delayed execution, I think that's really orthogonal to the filtering question. The filtering itself could be a bit tricky to implement with LINQ. Maybe rather than inlining the Where lambdas, you set up a collection of them and select one depending on the filter you need.
You are allowed since the code will compile anyway, but if you're going for a pure DDD design you should not have incomplete instances of objects.
You should look into LazyLoading if you're afraid to load a huge object of which you will only use a small portion of its child entities.
LazyLoading delays the loading of whatever you decide to lazy-load until the moment they are accessed. They make use of callbacks to call the loading method once the code calls for them.
Is it OK to implement a repository then, so that in such scenarios it
returns an incomplete aggregate?
Not at all. Aggregate is a transnational boundary to change the state of your system. Never use aggregates for querying data. Split the system into Write and Read sides. (read about CQR & CQRS). When we think "CRUD" based, we implement our system, based on some resource. Lets say you have "Appointment" aggregate. Thinking "Crudish" means we should implement usecases Create, Update, Delete, GetAll appointments. That means Appointment[] should be returned for GetAll. When you think usecase based, (HexagonalArchitecture) your usecases would be ScheduleAppointment, RescheduleAppointment, CancelAppointment. But for query side it can be: /myCalendar. We return back all appointments for a specific user in a ClientCalendar object. Create separate DTO's for Query sides. Never use aggregates for this purpose.
Related
No, it is not a duplication question.
I have red many sources on the subject, but still I feel like I don't fully understand it.
This is the information I have so far (from multiple sources, be it articles, videos, etc...) about what is an Aggregate and Aggregate Root:
Aggregate is a collection of multiple Value Objects\Entity references and rules.
An Aggregate is always a command model (meant to change business state).
An Aggregate represents a single unit of (database - because essentialy the changes will be persisted) work, meaning it has to be consistent.
The Aggregate Root is the interface to the external world.
An Aggregate Root must have a globally unique identifier within the system
DDD suggests to have a Repository per Aggregate Root
A simple object from an aggregate can't be changed without its AR(Aggregate Root) knowing it
So with all that in mind, lets get to the part where I get confused:
in this site it says
The Aggregate Root is the interface to the external world. All interaction with an Aggregate is via the Aggregate Root. As such, an Aggregate Root MUST have a globally unique identifier within the system. Other Entites that are present in the Aggregate but are not Aggregate Roots require only a locally unique identifier, that is, an Id that is unique within the Aggregate.
But then, in this example I can see that an Aggregate Root is implemented by a static class called Transfer that acts as an Aggregate and a static function inside called TransferedRegistered that acts as an AR.
So the questions are:
How can it be that the function is an AR, if there must be a globaly unique identifier to it, and there isn't, reason being that its a function. what does have a globaly unique identifier is the Domain Event that this function produces.
Following question - How does an Aggregate Root looks like in code? is it the event? is it the entity that is returned? is it the function of the Aggregate class itself?
In the case that the Domain Event that the function returns is the AR (As stated that it has to have that globaly unique identifier), then how can we interact with this Aggregate? the first article clearly stated that all interaction with an Aggregate is by the AR, if the AR is an event, then we can do nothing but react on it.
Is it right to say that the aggregate has two main jobs:
Apply the needed changes based on the input it received and rules it knows
Return the needed data to be persisted from AR and/or need to be raised in a Domain Event from the AR
Please correct me on any of the bullet points in the beginning if some/all of them are wrong is some way or another and feel free to add more of them if I have missed any!
Thanks for clarifying things out!
I feel like I don't fully understand it.
That's not your fault. The literature sucks.
As best I can tell, the core ideas of implementing solutions using domain driven design came out of the world of Java circa 2003. So the patterns described by Evans in chapters 5 and six of the blue book were understood to be object oriented (in the Java sense) domain modeling done right.
Chapter 6, which discusses the aggregate pattern, is specifically about life cycle management; how do you create new entities in the domain model, how does the application find the right entity to interact with, and so on.
And so we have Factories, that allow you to create instances of domain entities, and Repositories, that provide an abstraction for retrieving a reference to a domain entity.
But there's a third riddle, which is this: what happens when you have some rule in your domain that requires synchronization between two entities in the domain? If you allow applications to talk to the entities in an uncoordinated fashion, then you may end up with inconsistencies in the data.
So the aggregate pattern is an answer to that; we organize the coordinated entities into graphs. With respect to change (and storage), the graph of entities becomes a single unit that the application is allowed to interact with.
The notion of the aggregate root is that the interface between the application and the graph should be one of the members of the graph. So the application shares information with the root entity, and then the root entity shares that information with the other members of the aggregate.
The aggregate root, being the entry point into the aggregate, plays the role of a coarse grained lock, ensuring that all of the changes to the aggregate members happen together.
It's not entirely wrong to think of this as a form of encapsulation -- to the application, the aggregate looks like a single entity (the root), with the rest of the complexity of the aggregate being hidden from view.
Now, over the past 15 years, there's been some semantic drift; people trying to adapt the pattern in ways that it better fits their problems, or better fits their preferred designs. So you have to exercise some care in designing how to translate the labels that they are using.
In simple terms an aggregate root (AR) is an entity that has a life-cycle of its own. To me this is the most important point. One AR cannot contain another AR but can reference it by Id or some value object (VO) containing at least the Id of the referenced AR. I tend to prefer to have an AR contain only other VOs instead of entities (YMMV). To this end the AR is responsible for consistency and variants w.r.t. the AR. Each VO can have its own invariants such as an EMailAddress requiring a valid e-mail format. Even if one were to call contained classes entities I will call that semantics since one could get the same thing done with a VO. A repository is responsible for AR persistence.
The example implementation you linked to is not something I would do or recommend. I followed some of the comments and I too, as one commenter alluded to, would rather use a domain service to perform something like a Transfer between two accounts. The registration of the transfer is not something that may necessarily be permitted and, as such, the domain service would be required to ensure the validity of the transfer. In fact, the registration of a transfer request would probably be a Journal in an accounting sense as that is my experience. Once the journal is approved it may attempt the actual transfer.
At some point in my DDD journey I thought that there has to be something wrong since it shouldn't be so difficult to understand aggregates. There are many opinions and interpretations w.r.t. to DDD and aggregates which is why it can get confusing. The other aspect is, in IMHO, that there is a fair amount of design involved that requires some creativity and which is based on an understanding of the domain itself. Creativity cannot be taught and design falls into the realm of tacit knowledge. The popular example of tacit knowledge is learning to ride a bike. Now, we can read all we want about how to ride a bike and it may or may not help much. Once we are on the bike and we teach ourselves to balance then we can make progress. Then there are people who end up doing absolutely crazy things on a bike and even if I read how to I don't think that I'll try :)
Keep practicing and modelling until it starts to make sense or until you feel comfortable with the model. If I recall correctly Eric Evans mentions in the Blue Book that it may take a couple of designs to get the model closer to what we need.
Keep in mind that Mike Mogosanu is using a event sourcing approach but in any case (without ES) his approach is very good to avoid unwanted artifacts in mainstream OOP languages.
How can it be that the function is an AR, if there must be a globaly unique identifier to it, and there isn't, reason being that
its a function. what does have a globaly unique identifier is the
Domain Event that this function produces.
TransferNumber acts as natural unique ID; there is also a GUID to avoid the need a full Value Object in some cases.
There is no unique ID state in the computer memory because it is an argument but think about it; why you want a globaly unique ID? It is just to locate the root element and its (non unique ID) childrens for persistence purposes (find, modify or delete it).
Order A has 2 order lines (1 and 2) while Order B has 4 order lines (1,2,3,4); the unique identifier of order lines is a composition of its ID and the Order ID: A1, B3, etc. It is just like relational schemas in relational databases.
So you need that ID just for persistence and the element that goes to persistence is a domain event expressing the changes; all the changes needed to keep consistency, so if you persist the domain event using the global unique ID to find in persistence what you have to modify the system will be in a consistent state.
You could do
var newTransfer = New Transfer(TransferNumber); //newTransfer is now an AG with a global unique ID
var changes = t.RegisterTransfer(Debit debit, Credit credit)
persistence.applyChanges(changes);
but what is the point of instantiate a object to create state in the computer memory if you are not going to do more than one thing with this object? It is pointless and most of OOP detractors use this kind of bad OOP design to criticize OOP and lean to functional programming.
Following question - How does an Aggregate Root looks like in code? is it the event? is it the entity that is returned? is it the function
of the Aggregate class itself?
It is the function itself. You can read in the post:
AR is a role , and the function is the implementation.
An Aggregate represents a single unit of work, meaning it has to be consistent. You can see how the function honors this. It is a single unit of work that keeps the system in a consistent state.
In the case that the Domain Event that the function returns is the AR (As stated that it has to have that globaly unique identifier),
then how can we interact with this Aggregate? the first article
clearly stated that all interaction with an Aggregate is by the AR, if
the AR is an event, then we can do nothing but react on it.
Answered above because the domain event is not the AR.
4 Is it right to say that the aggregate has two main jobs: Apply the
needed changes based on the input it received and rules it knows
Return the needed data to be persisted from AR and/or need to be
raised in a Domain Event from the AR
Yes; again, you can see how the static function honors this.
You could try to contat Mike Mogosanu. I am sure he could explain his approach better than me.
I'm struggling with how/if to define "a set of aggregates". Aggregates are supposed to be stand alone and isolated but it's easy to think of a bigger set of aggregates that belong together. But is this a trap?
Using this "set of aggregates" it would be possible to for instance enumerate and index aggregates on a unique property within the set and have other domain rules that could be validated across all aggregates in the set. It's tempting but also feels a bit wrong.
Another approach would be to avoid this thinking completely and not allow/define a set of aggregates and not allow enumerating aggregates but only load/save on aggregate-id. Using this option if would be necessary to reference aggregates from other aggregates and by doing this build up an interconnected graph of aggregates.
The approaches are similar to having aggregates in a folder on disk or having an "internet" of aggregates where the references between them are defining the bigger set of aggregates. In any case I'm really stuck on this problem. I have never read anywhere about this and I guess nobody really cares that much? I'm not sure I explain this very good but my question is if there are any definitions of the "set of aggregates" or if we should think of aggregates as totally isolated/on its own and with only a unique aggregate-id (UUID)?
The set of aggregates could for instance be the database being used under the surface. But what I'm wondering is if this database as in the information about what aggregates it contains has any definition in DDD or if we should think about a set of aggregates as an interconnected graph where only traversal of this graph can be used to enumerate all "associated" aggregates.
Aggregates are connected
In any application with sufficient complexity, Aggregates end up referencing one-another. And it is perfectly reasonable to use their unique identifiers as reference IDs to refer to each other.
But take care to load and persist aggregates outside the domain layer, typically in repositories. If you want to traverse links across aggregates and load them into memory, you will be doing that upfront before handing over control to the domain layer for the actual processing.
Traversing the graph to get all related aggregates is correct, but this rarely spans across too many aggregate boundaries. You rarely find a single change or rule to be applied throughout the application. If you do have such a transaction, it is probably a sign of the domain design needing improvement, simply because you are spreading one responsibility/change amongst many aggregates.
The connectivity is so usual that you should watch out for aggregates that have no linkages with the rest of the system. They are either standalone libraries, or they probably belong to a different bounded context.
Aggregates can morph into different forms
They are aggregates because they form a clear invariant boundary, with their primary responsibility being to enforce invariants across state changes for all the entities within themselves. But they can morph into different kinds of DDD objects based on the requirement.
A good example is of a single Currency note. In most applications, they are value objects. But for the federal bank, they are aggregates with clear cut invariant rules. They are aggregates when they are created and referenced, but in a transaction that ships printed notes to banks, they may become value objects.
So you may have to evaluate whether you are talking about a domain entity in its aggregate form, or as a value object when you consider each linkage.
Aggregates are invariant boundaries
It is wrong to validate domain rules across aggregates.
Your aggregate boundary is an invariant boundary, meaning all the domain rules within it should be satisfied at all time. By that logic, you are going to incorrectly build up a structure that will need to ensure that all domain rules across aggregates are valid at all time. Doing so will impose considerable performance burden, not to mention the complexity in business logic.
But this is not to say that there may be domain rules that span across aggregates. The correct way to accomplish this would be using eventual consistency and an Event-driven approach.
The primary changing aggregate would validate and persist the data, and bubble up an event containing the state change. Other aggregates would then act on the event and bring themselves up-to-date. If an aggregate's domain rules break because of the change, there is usually a supplementary mechanism that allows correction of the problem (a preferred mechanism) or a rollback of the first state change (happens very rarely).
Perhaps you can find a common denominator the agg sets have in common and use that to work with?
A simplified example; there is a set of Books and a set of Users that have nothing in common except you want to know whenever they were first registered? What might be an option is to have an interface FirstRegistration and then you can choose to either expand Books/Users or create a specific entity instead.
I'm struggling with how/if to define "a set of aggregates". Aggregates
are supposed to be stand alone and isolated but it's easy to think of
a bigger set of aggregates that belong together. But is this a trap?
I think you're struggling because indeed the idea of a set of aggregates (instances) is very generic, and the uses of such things are contextual and domain-specific. People don't talk specifically about it because of course you may have behaviors that operate on a collection of multiple aggregates, but that doesn't give such collections any particular common properties or requirements that would allow you, from a general DDD perspective, to characterize such collections more specifically than "a set of aggregates", "a list of distinct aggregates", or similar.
Using this "set of aggregates" it would be possible to for instance
enumerate and index aggregates on a unique property within the set and
have other domain rules that could be validated across all aggregates
in the set. It's tempting but also feels a bit wrong.
Tempting why? You've couched the question in very abstract terms, so it's pretty much impossible to contradict you about the "it would be possible", but just because something may be possible doesn't mean it would be useful. In practice, I think you'll find that rules or behaviors that operate on collections of aggregates most naturally belong not to collections of aggregates in an abstract sense, but rather to other aggregate types in your domain model, to domain repositories, or to domain services.
It is entirely plausible that your domain model might want to handle particular sets of aggregates characterized by some rule. For example, if you're an airline, then one of the aggregates in your domain model might a single seat on a flight, since that's the unit you sell. It makes sense in that case that there would be operations on all the seats on a particular flight, for example, but whatever rules and behaviors you might have about that are specifically about that kind of aggregate, selected in that particular way.
Another approach would be to avoid this thinking completely and not
allow/define a set of aggregates and not allow enumerating aggregates
but only load/save on aggregate-id.
It's surely counterproductive to forbid working with sets of aggregates. Just don't attribute more significance to it than is warranted. There is nothing particularly special about sets of aggregates in general.
Using this option if would be
necessary to reference aggregates from other aggregates and by doing
this build up an interconnected graph of aggregates.
I don't follow that. One certainly must be able to retrieve and store individual aggregates from persistence, as that's more or less the defining property of aggregates -- they are the unit of persistence. But that doesn't mean that you must reject the ability to work with collections of aggregates. However, sets of aggregates do not have identity in the same way that individual aggregates do, so yes, relationships between aggregates need to be modeled in terms of individual aggregates. Nevertheless, that does not inherently preclude 1:m or n:m relationships among aggregates.
I'm really stuck on this problem. I have never read anywhere about this and I guess nobody really cares that much?
You'll find all sorts of uses of various sets of aggregates in applications built and maintained based on DDD ideas, but there's not much to talk about at the level of abstraction of your question, and what there is is already summed up in the words "set" and "aggregate".
The set of aggregates could for instance be the database being used
under the surface. But what I'm wondering is if this database as in
the information about what aggregates it contains has any definition
in DDD
Not to my knowledge. I suspect most DDD practitioners would just call it "the data", or something similar.
or if we should think about a set of aggregates as an
interconnected graph where only traversal of this graph can be used to
enumerate all "associated" aggregates.
I'm still not seeing why you set that up as a thing. Sure, depending on the domain model, you might be able to traverse all or substantial chunks of the data by traversing associations between aggregates, and that might be appropriate for some purposes, but DDD doesn't have to give a special name or special rules for sets of aggregates for you to work with them.
Like any useful methodology, DDD exists to solve problems. Its bread & butter is complex applications with complex data and evolving requirements. It is not to be interpreted as a straight jacket preventing designers and developers from (thoughtfully) writing designs and code that incorporate aspects of other design approaches, much less designs and code that provide for the application's idiosyncratic needs.
If we are working on a sub-domain where we're only dealing with a read-only scenario, meaning that our entities and value objects will not be changed, does it make sense to create aggregates composed by roots and its children or should each entity of this context map to a single aggregate?
Imagine that we've entity A and entity B.
In a context where modifications are made, we create an aggregate composed by entity A and entity B, where A is the aggregate root (let's say that B can't live without A and there are some invariants involved).
If we move the same entities to a different context where no modifications are made, does it make sense to keep this aggregate or should we create an aggregate for entity A and a different one for entity B?
In 2019, there's fairly large support for the idea that in a read only scenario, you don't bother with the domain model at all.
Just load the data directly into whatever read only data structure makes sense to support the use case.
See also: cqrs.
The first thing is if B cant live without A and there are some invariants involved, to me A is an Aggregate root, with B being an entity that belongs to it.
Aggregate roots represent a real world concept and dont just exist for the convenience of modification. In many of our applications, we don't modify state of our aggregate roots once created - i.e. we in effect have immutable aggregate roots. These would have some logic for design by contract checks/invariant checks etc but they are in effect anaemic as there is no "Update" methods due to its immutability. Since the "blue book" was written by Eric Evans, alot of things have changed, e.g. the concept of NoSql database have become very popular, functional programming concepts have become very influential rising to more advanced DDD style architectures being recommended such as CQRS. So for example, rather than doing updates to a database I can append (i.e. insert) instead. This leads to aggregates no longer having to be "updated". This leads to leaner anaemic types but this is what we want in this context. The issue before with anaemic types was that "update logic" for a given type was put elsewhere in the codebase instead of being put into the type itself. However if you do not require "update logic" in the first place then you dont have that problem!
If for example there is an Order with many OrderItems, we would create an Order aggregate root and an OrderItem entity. Its a very important concept to distill your domain to properly identify what are aggregates, entities and value types.
Then creation of domain services, repositories etc just flows naturally. For example, aggregate roots and repositories are 1 to 1 i.e. in the example above we would have an Order repository and not have an OrderItem repository. That way your main domain concepts are spread throughout your code in a predictable and easy to understand way.
Finally, in your specific question I would not treat them as the same entities. In one context, you seem to need modification logic - in the other they you dont - they are separate domain concepts to me.
In context where modifications are made: A=agg root, B=entity.
In context without modifications: A=agg root (immutable), B=entity(immutable)
Ive heard a lots that aggregate roots depend on the use case. But what does that mean in coding context ?
You have a service class which offcourse hold methods (use cases) that gonna accomplish something in a repository. Great, so you use a repository which is equal to an aggregate root to perform your querying.
Now you need to perform some other kind of operation which use totally different use case than the first service class but use the same entities.
Here the representation :
Entities: Customer, Orders, LineOrder
Service 1: Add new customers, Delete some customers, retrieve customer orders
Here the aggregate root seem to be Customer because you need this repository to perform thoses use cases.
Service 2: Retrieve customer from an actual order
Here the aggregate root seem to be Order because you need this repository to perform this use case.
If i am wrong please correct me. Now that mean you have 2 aggregates roots.
Now my question is, since aggregate roots depend on the use case does that mean that we might end up with really a lots of repositories if you end up having lots of use cases ?
The above example was probably not the best example... so lets say we have a Journal which hold JournalEntries which each entries hold Tasks, Problems and Notes. (This is in the context of telling to a system what have been done to a project)
Does that mean that im gonna end up with 2 repository ? (Journal, JournalEntry)
In the use cases where i need to add new tasks, problems and notes from an journal entry ?
(Can be seen as a service)
Or might end up with 4 repository. (Journal, Task, Problems, Notes)
In the use cases where i need to access directment task, problems and notes ?
(Can be seen as another service)
But that would mean if i need both of theses services (that actually hold the use cases) that i actually need 5 repository to be able to perform use cases in both of them ?
Thanks.
Hi I saw your post and thought I may give you my opion. First I must say I've been doing DDD in project for three years now, so I'm not an expert. But I'm currently working in a project as an architect an coaching developers in DDD, and I must say it isn't a walk in the park... I don't know how many times I've refactored the model and Entity relationships.
But my experience is that you endup with some repositories (more than few but not many). My Aggregates usually contains a few classes and the Aggregate object graph isn't that deep (if you know what I mean).
But I try to be concrete:
1) Aggregate roots are defined by your needs. I mean if you feel that you need that Tasks object through Journal to often, then maybe thats a sign for it to be upgraded as a aggregate root.
2) But everything cannot be aggregate roots, so try to capsulate object that are tight related. Notes seems like a candidate for being own by a root object. You'd probably always relate Notes to the root or it loses its context. Notes cannot live by itself.
3) Remember that Aggregates are used for splitting up large complex domains into smaller "islands" that take care of thier inhabbitants. Its important to not make your domain more complex than it is.
4) You don't know how your model look likes before you've reached far into the project implementation phase. If you realize that some repositories aren't used that much, they may be candidates for merging into other root object (if they have that kind of relationship). You can break out objects that are used so much through root object without its context. I mean for example if Journal are aggregate root and contains Notes and Tasks. After a while you model grows and maybe Tasks
have assoications to Action and ActionHistory and User and Rule and Permission. Now I just throw out a bunch om common objects in a rule/action/user permission functionality. Maybe this result in usecases that approach Tasks from another angle, "View all Tasks performed by this User" etc. Tasks get more involved in some kind of State/Workflow engine and therefor candidates for being an aggregate root itself.
Okey. Not the best example but it maybe gives you the idea. A root object can contain children where some of its children can also be root object because we need it in another context (than journal).
But I have myself banged my head against the wall everytime you startup with a fresh model. Just go with the flow and let the model evolve itself through its clients/subsribers. You refine the model through its usage. The Services (application services and not domain services) are of course extended with methods that respond to UI and usecases (often one-to-one).
I hope I helped you in someway...or not :D
Yes, you would most likely end up with 5 repositories (Journal, JournalEntry, Task, Problems, Notes). Your services would then use these repositories to perform CRUD for each type of entity.
Your reaction of "wow so many repositories" is not uncommon for developers new to DDD.
However, your repositories are usually light weight assuming your model and DB schema are fairly evenly matched which is often the case. If you use an ORM such as nHibernate or a tool such as codesmith generator then it gets even easier to create your repositories.
At first you need to define what is aggregate. I don't know about use case aggregates.
I know about aggregates following...
Aggregates are union of several entities. One of the entities is the aggregate root, the rest entities (or value types) have sense only in selected aggregate root context. For example you can define Order and OrderLine as an aggregate if you don't need to do any independent actions with OrderLine entities. It means that OrderLine makes sense in Order context only.
Why to define aggregates at all? It is required to reduce references between objects. That will simplify you domain model.
And of course you don't need to have OrderLineRepository if OrderLine is a part of Order aggregate.
Here is a link with more information. You can read Eric Evans DDD book. He explains aggregates very well.
After reading Evan's and Nilsson's books I am still not sure how to manage Data access in a domain driven project. Should the CRUD methods be part of the repositories, i.e. OrderRepository.GetOrdersByCustomer(customer) or should they be part of the entities: Customer.GetOrders(). The latter approach seems more OO, but it will distribute Data Access for a single entity type among multiple objects, i.e. Customer.GetOrders(), Invoice.GetOrders(), ShipmentBatch.GetOrders() ,etc. What about Inserting and updating?
CRUD-ish methods should be part of the Repository...ish. But I think you should ask why you have a bunch of CRUD methods. What do they really do? What are they really for? If you actually call out the data access patterns your application uses I think it makes the repository a lot more useful and keeps you from having to do shotgun surgery when certain types of changes happen to your domain.
CustomerRepo.GetThoseWhoHaventPaidTheirBill()
// or
GetCustomer(new HaventPaidBillSpecification())
// is better than
foreach (var customer in GetCustomer()) {
/* logic leaking all over the floor */
}
"Save" type methods should also be part of the repository.
If you have aggregate roots, this keeps you from having a Repository explosion, or having logic spread out all over: You don't have 4 x # of entities data access patterns, just the ones you actually use on the aggregate roots.
That's my $.02.
DDD usually prefers the repository pattern over the active record pattern you hint at with Customer.Save.
One downside in the Active Record model is that it pretty much presumes a single persistence model, barring some particularly intrusive code (in most languages).
The repository interface is defined in the domain layer, but doesn't know whether your data is stored in a database or not. With the repository pattern, I can create an InMemoryRepository so that I can test domain logic in isolation, and use dependency injection in the application to have the service layer instantiate a SqlRepository, for example.
To many people, having a special repository just for testing sounds goofy, but if you use the repository model, you may find that you don't really need a database for your particular application; sometimes a simple FileRepository will do the trick. Wedding to yourself to a database before you know you need it is potentially limiting. Even if a database is necessary, it's a lot faster to run tests against an InMemoryRepository.
If you don't have much in the way of domain logic, you probably don't need DDD. ActiveRecord is quite suitable for a lot of problems, especially if you have mostly data and just a little bit of logic.
Let's step back for a second. Evans recommends that repositories return aggregate roots and not just entities. So assuming that your Customer is an aggregate root that includes Orders, then when you fetched the customer from its repository, the orders came along with it. You would access the orders by navigating the relationship from Customer to Orders.
customer.Orders;
So to answer your question, CRUD operations are present on aggregate root repositories.
CustomerRepository.Add(customer);
CustomerRepository.Get(customerID);
CustomerRepository.Save(customer);
CustomerRepository.Delete(customer);
I've done it both ways you are talking about, My preferred approach now is the persistent ignorant (or PONO -- Plain Ole' .Net Object) method where your domain classes are only worried about being domain classes. They do not know anything about how they are persisted or even if they are persisted. Of course you have to be pragmatic about this at times and allow for things such as an Id (but even then I just use a layer super type which has the Id so I can have a single point where things like default value live)
The main reason for this is that I strive to follow the principle of Single Responsibility. By following this principle I've found my code much more testable and maintainable. It's also much easier to make changes when they are needed since I only have one thing to think about.
One thing to be watchful of is the method bloat that repositories can suffer from. GetOrderbyCustomer.. GetAllOrders.. GetOrders30DaysOld.. etc etc. One good solution to this problem is to look at the Query Object pattern. And then your repositories can just take in a query object to execute.
I'd also strongly recommend looking into something like NHibernate. It includes a lot of the concepts that make Repositories so useful (Identity Map, Cache, Query objects..)
Even in a DDD, I would keep Data Access classes and routines separate from Entities.
Reasons are,
Testability improves
Separation of concerns and Modular design
More maintainable in the long run, as you add entities, routines
I am no expert, just my opinion.
The annoying thing with Nilsson's Applying DDD&P is that he always starts with "I wouldn't do that in a real-world-application but..." and then his example follows. Back to the topic: I think OrderRepository.GetOrdersByCustomer(customer) is the way to go, but there is also a discussion on the ALT.Net Mailing list (http://tech.groups.yahoo.com/group/altdotnet/) about DDD.