I've been doing a lot of research on DDD and have been stumbling a bit on the concept of Aggregates vs Aggregate Roots.
Say I'm trying to model a space game that exists in a strict hierarchy, with a Sector Entity that wraps around everything contained within it. It can have a collection of PlanetarySystem, each of those can have a collection of Star, the stars can have a collection of Planet which themselves can have a collection of Moon
I want all of these objects to either be associated with that Sector, situated nicely in the hierarchy, but it can also be associated with no sector. Each item can have a 0 or 1 relationship with the object in the hierarchy above it... floating in the ether if need be. I want to be able to delete a Planet without deleting all the Moon entities it has, but it can be done optionally.
Am I wrong to think that these should each be their own Aggregate Root in this scenario, with references to the Aggregate Root instances that they contain? Or should the Sector be the actual Aggregate Root, orchestrating everything as the object at the top of the hierarchy?
An aggregate root should never contain a reference to another aggregate root. You should use either an id to reference an associated aggregate or a value object containing the id and some other pertinent data.
Anything that is floating in the ether would be an aggregate root since it has a lifecycle of its own.
I reckon the hierarchy should just work in a unidirectional manner as far as the domain is concerned. If you need to query lower level items based on higher level containment then you either need joins or need to denormalize the relevant higher level identifiers/data into the lower level items.
My advice would be to attempt to keep any aggregate a single level deep but if that isn't practical then try something deeper but it gets unwieldy rather quickly.
Remember something can be an Aggregate in one use case and an Aggregate Root in another. If your use case is AddPlanetarySystemToSector, then Sector is your Aggregate Root. If your use case is AddMoonToPlanet, then Planet is the AR.
The difference is that an AR should not be saved unless it and all Aggregates it owns satisfy all business rules. The higher level your AR, the more business rules you must test and satisfy. Therefore your use cases should be a specific as possible and target the proper AR.
If this poses problems, a work around is to have ARs own value objects instead of aggregates. So a Sector would own PlanetarySystemVOs. If the Sector needs to modify a PlanetarySystem, it can launch a PlanetarySystem use case that performs the proper action.
Related
I saw many different approaches and I am fairly new to domain-driven design approach. What I am struggling with is to understand one complex (at least for me) thing. I know the whole DDD is complex to understand on first but I am trying to find any resources I can on it.
Example: I have an order and order can have operations. Operations can not be accessed without order and they make no sense without an order. So order entity will be my aggregate root. Operations will be entity too because each operation will have an id (am I right on this one?). Each operation can have subitems (array of strings for example and these can be added or removed from any operation).
Now what I am struggling to understand and what I found everywhere is that every modification should be called and set only through aggregate root... But is it okay to have private methods like setters and getters on the Operation entity itself but these would be called only through the aggregate root (order entity)?
Sorry if I missed something basic, as the whole DDD concept for me is new and I am trying to explore it.
Thanks.
A couple of DDD concepts to arrive at the answer:
Aggregates are Transaction Boundaries.
Aggregates act as gatekeepers for all changes to domain elements enclosed within itself.
Data changes to an Aggregate and its enclosed domain elements are committed atomically. Either everything within the Aggregate stays in sync, or the whole state change operation fails.
The rule also means that one should not access Domain Elements within the Aggregate directly. It would be best if you did not manipulate the domain objects outside the context of the Aggregate.
If Operation is an entity under Order aggregate, then Order is responsible for ensuring operations satisfy the business invariants (a.k.a validations).
Aggregates are loaded in entirety.
Since an Aggregate represents the transaction and consistency boundary of a domain concept, its data is loaded in entirety to guarantee that all Business Invariants are satisfied. Data here means data of all underlying entities and value objects.
If you cannot load the entire data, you cannot guarantee that the change satisfies all business invariants. It may also mean that a data-intensive entity within the Aggregate may need to become an Aggregate itself.
You are protecting the data sanctity and operational consistency of the system if you adhere to these rules. Within the Aggregate itself, how you organize state changes is wholly left to you.
IMHO, I would go with your approach of enclosing all Operation related behaviors, data attributes, and invariants within the Operation entity. Order is responsible for protecting the data within its boundary, but it need not own the methods/logic of doing everything.
You can create state change methods within the Operation entity too, just like you would have done in the Order aggregate, but invoke them from the order object.
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.
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)
In my case I have two main concepts: User (main citizen of the system) and Group.
Group has two sub-collections: ranks and roles. Without a group, ranks and roles have no meaning.
When User is assigned to the Group, we also have to pick 1 Role and 1 Rank and assign them to this relationship between User and the Group.
Diagram
Question:
How much aggregate roots do I have here? From the user side its obviously a user (main concept of the system), but what about its relationship with group? AFAIK its forbidden by rules of DDD to reference entities outside of aggregate root.
AFAIK its forbidden by rules of DDD to reference entities outside of aggregate root.
Well, I would't say it's "forbidden by rules of DDD"... Some times you have no choice. I have to consider the size of the "entities' collection" associated to the Root's Aggregate. Sometime you can maintain the association in the same aggregation and use some kind of "lazy load" to avoid resource consumption. The Vernon's iDDD book[1] has some advises and use cases around this specific case. Take a look on his blog post[2]
[1] https://www.amazon.com.br/Implementing-Domain-Driven-Design-Vaughn-Vernon/dp/0321834577
[2] https://vaughnvernon.co/?p=838
You have at least the following options, depending on your business requirements regarding consistency:
You have 5 aggregate roots: User, Group, Rank, Role and UserAssignment. The last one must protect the invariant "we also have to pick 1 Role and 1 Rank". For lifetime management you use eventual consistency between ARs. For example, when you delete a Group you must delete also the orphan Ranks, Roles and UserAssignments.
You have User (with UserAssignment as nested entity) and Group (with Role and Rank as nested entities). You have strong consistency inside ARs (when you delete a user all its asignments are also deleted) and eventual consistency between User and Group.
What should you use? Only you could decide. For example, if you choose the first option and delete a user then there could be a delay of seconds/minutes/hours before its assignments are also deleted.
Strong consystency should be used to protect only real business invariants because it is not cheap.
P.S. if you need to hold a reference to a nested entity from another AR then you should reconsider your aggregate roots boundaries because your design is most probable wrong.
I'm going to change some of the words and we'll see if it helps (hypothetical):
I have an Order and a Product. When I add a Product to an Order I have to choose a Store and a Colour.
How would you model this?
Colour may very well be a Value Object but Store is not. I would opt for an OrderItem Value Object which contains a Colour Value Object and a StoreId value to capture the relationship. An Order would contain a list of OrderItem entries.
Removing Colour entries is fine since we have denormalized that bit into the OrderItem. We could have another Value Object represent the Store but typically we would not delete the store or have some processing to handle a deletion or, even more typical, use referential integrity constraints to prevent deleting a used Store.
If you consider ever deleting the Order only the OrderItem association is also deleted.
In your case User and Group are probably Aggregate Roots and I'd add a UserGroup (or UserAssignment as Constantin used). The UserGroup contains the association and related bits. You would have to identify the real domain structure though.
i've been assigned a quite simple project as an exam and i had the idea to develop it using the Domain Driven Design.
Many of you might say that the application is so simple that going with repositories and UoW is just a waste of time, and you probably be correct but i think of it as an opportunity to learn something more.
The application is a "Flight tickets" system and from the following image you could probably well guess it's functionality.
The thing is that i am not sure if i am correctly seperating the aggregates and their roots.
EDIT:
I presented the data model so anyone can spot the whole functionality easily.
The thing is that from an employe perspective the flight as "Rad" said encapsulates the whole functionality and is the aggregate root.
However from an admin perspective, flights are none his bussiness.
He just want to update or add new planes-companies, etc..
So then there is a new aggregate root which is the Airplane which encapsulates the Airplane seats(Entity), the seatType(value object) and the company(Entity) as a new aggregate.
This tends to confuses me as i have an aggregate root(Airplane) inside another aggregate(Flight Aggregate).
Since the aggregate root is consider to be the "CORE" entity which without it the other entities inside it will not make any sense without it, i am thinking about Company. And i conclude that company makes sense without the airplane.
To explain more i think of the scenario where the admin want to just insert a new Company, or want to first load a company and then its airplanes.
DDD principles say that any entities inside the aggregate may only be loaded from the root itself.
So here is the confusion.
Mmm, where is the Aggregate and Aggregate roots here ? This is only Data Model... Not Domain Model.
Aggregate is a cluster of items (Domain Object) that are gathered together, and Aggregate Root are the entity root... (If you consider the Flight Aggregate encapsulates Seats, Location... The Aggregate Root should be Flight entity).
[Edit]
You have to ignore the persistent. In your app you can have many aggregate it depends in your Domain, maybe Flight is an Aggregate and Company another one ;), don't confuse entity and Aggregate...
An aggregate is a group of entities (objects with identity) and maybe value objects (objects without identity, immutable). There is exactly one entity in an aggregate that is the aggregate root. You can easily identify it by checking if the other objects in the aggregate depend on it, for example, if you delete an object of the aggregate root type, the remaining objects don't make sense anymore (in database terms, you'd cascade delete the dependent objects).
The aggregate root is the sole object in the aggregate that gives access to the other types in the aggregate, hence you'll have one repository per aggregate and it returns instances of aggregate root type.