Aggregate In DDD

1
Aggregate In DDD
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What makes an ENTITY AGGREGATE root

• Has global identity.
• Expected to be directly accessed in typical
  business scenarios.
• If it has other objects to take care of and
• Manage lifecycle of internal objects within its
  boundary.
• Control all access to internal objects.
• Enforce invariants/integrity in transactions.

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AGGREGATE in modeling process(1)

• Separate domain LAYERS.
• Identify ENTITY and VALUE objects in domain
  layer.
• Identify AGGREGATE root of ENTITIES and define
  its boundary.
• Create REPOSITORY for AGGREGATE root.

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AGGREGATE in modeling process(2)

• Consider creation of objects (constructor,
  AGGREGATE root, FACTORY).
• Clarify responsibilities and put objects into
  proper MODULES and LAYERS.
• Iteratively discover and refine domain model.

5
AGGREGATE in codes(1)

• AGGREGATE root creates internal objects.
• Part part root.AddPart(part-1)
• Part part new Part(root, part-1)
• root.AddPart(part)

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AGGREGATE in codes(2)

• ONLY provide accessibility through REPOSITORY for
  AGGREGATE root. Other objects must be found by
  traversal of associations.
• Nothing outside the aggregate boundary can hold a
  reference to anything inside, except the root.
  (deleting operation has no side effects.)
• ENTITYES inside boundary have local identity,
  unique within AGGREGATE.

7
AGGREGATE in codes(3)

• AGGREGATE root holds internal object related
  logic for some cases.
• Objects within AGGREGATE can hold references to
  other AGGREGATE roots.

8
Defining boundary(1)

• Listen to typical business scenarios, be
  pragmatic on defining boundary.
• Be referenced by an AGGREGATE root does not mean
  being aggregated.
• Entity accessed through different paths
  (AGGREGATE roots) in different scenarios usually
  worth making itself its own AGGREGATE root.

9
Defining boundary(2)

• Use this pattern
• "No one would look up A directly without
  wanting the B itself.
• instead of the below one
• the A is not integrated/valid conceptually
  without an B.

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Not mentioned topics

• NHibernate cascading strategy for AGGREGATE.
• Typical trade-offs when implementing AGGREGATE.