What s an Aggregate Root

Question

I m trying to get my head around how to properly use the repository pattern  The central concept of an Aggregate Root keeps coming up  When searching both the web and Stack Overflow for help with what an aggregate root is  I keep finding discussions about them and dead links to pages that are supposed to contain base definitions   In the context of the repository pattern  what is an aggregate root

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In Erlang there is no need to differentiate between aggregates  once the aggregate is composed by data structures inside the state  instead of OO composition  See an example  https   github com bryanhunter cqrs-with-erlang tree ndc-london

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If you follow a database-first approach  you aggregate root is usually the table on the 1 side of a 1-many relationship   The most common example being a Person  Each person has many addresses  one or more pay slips  invoices  CRM entries  etc  It s not always the case  but 9 10 times it is   We re currently working on an e-commerce platform  and we basically have two aggregate roots    Customers Sellers   Customers supply contact info  we assign transactions to them  transactions get line items  etc   Sellers sell products  have contact people  about us pages  special offers  etc   These are taken care of by the Customer and Seller repository respectively

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From Evans DDD      An AGGREGATE is a cluster of associated objects that we treat as a unit for the purpose of data changes  Each AGGREGATE has a root and a boundary  The boundary defines what is inside the AGGREGATE  The root is a single  specific ENTITY contained in the AGGREGATE    And      The root is the only member of the AGGREGATE that outside objects are allowed to hold references to      This means that aggregate roots are the only objects that can be loaded from a repository   An example is a model containing a Customer entity and an Address entity  We would never access an Address entity directly from the model as it does not make sense without the context of an associated Customer  So we could say that Customer and Address together form an aggregate and that Customer is an aggregate root

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Dinah   In the Context of a Repository the Aggregate Root is an Entity with no parent Entity  It contains zero  One or Many Child Entities whose existence is dependent upon the Parent for it s identity   That s a One To Many relationship in a Repository  Those Child Entities are plain Aggregates

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From a broken link   Within an Aggregate there is an Aggregate Root  The Aggregate Root is the parent Entity to all other Entities and Value Objects within the Aggregate   A Repository operates upon an Aggregate Root   More info can also be found here

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The aggregate root is a complex name for a simple idea   General idea Well designed class diagram encapsulates its internals  Point through which you access this structure is called aggregate root   Internals of your solution may be very complicated  but users of this hierarchy will just use root doSomethingWhichHasBusinessMeaning     Example Check this simple class hierarchy  How do you want to ride your car  Chose better API Option A  it just somehow works   car ride     Option B  user has access to class inernals   if car getTires   getUsageLevel   lt  Car ACCEPTABLE TIRE USAGE      for  Wheel w  car getWheels             w spin             If you think that option A is better then congratulations  You get the main reason behind aggregate root   Aggregate root encapsulates multiple classes  you can manipulate the whole hierarchy only through the main object

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Imagine you have a Computer entity  this entity also cannot live without its Software entity and Hardware entity  These form the Computer aggregate  the mini-ecosystem for the Computer portion of the domain   Aggregate Root is the mothership entity inside the aggregate  in our case Computer   it is a common practice to have your repository only work with the entities that are Aggregate Roots  and this entity is responsible for initializing the other entities   Consider Aggregate Root as an Entry-Point to an Aggregate   In C  code   public class Computer   IEntity  IAggregateRoot       public Hardware Hardware   get  set        public Software Software   get  set       public class Hardware   IEntity     public class Software   IValueObject      public class Repository lt T gt    IRepository lt T gt  where T   IAggregateRoot      Keep in mind that Hardware would likely be a ValueObject too  do not have identity on its own   consider it as an example only

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Aggregate means collection of something  root is like top node of tree  from where we can access everything like  lt html gt  node in web page document  Blog Analogy  A user can have many posts and each post can have many comments  so if we fetch any user then it can act as root to access all the related posts and further comments of those posts  These are all together said to be collection or Aggregated

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In another world  in Event Sourcing  an Aggregate Root  is a different concept  Event Sourcing might be encountered together with CQRS  DDD etc  In Event Sourcing an Aggregate is an object for which the state  fields  is not mapped to a record in a database as we are used to think in SQL JPA world  Is not a group of related entities  It is a group of related records like in a history table  GiftCard amount is one field in a GiftCard Aggregate  but this field is mapped to all the events  like card-redeemed  take money from the card  ever created  So the source of data for your Aggregate is not a record in a database but the complete list of events ever created for that specific aggregate  We say we event sourced the aggregate  Now we can ask ourselves how is it done  Who is aggregating these events so we are operating still with one field e g GiftCard amount  We might be expecting that amount to be a Collection and not a big-decimal type  Is the event sourcing engine  doing the work  who might simply replay all the events in the creation order  But this is out of the scope for this thread

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Aggregate is where you protect your invariants and force consistency by limiting its access thought aggregate root  Do not forget  aggregate should design upon your project business rules and invariants  not database relationship  you should not inject any repository and no queries are not allowed

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In the context of the repository pattern  aggregate roots are the only objects your client code loads from the repository   The repository encapsulates access to child objects - from a caller s perspective it automatically loads them  either at the same time the root is loaded or when they re actually needed  as with lazy loading    For example  you might have an Order object which encapsulates operations on multiple LineItem objects  Your client code would never load the LineItem objects directly  just the Order that contains them  which would be the aggregate root for that part of your domain

[design-patterns] What's an Aggregate Root?

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