Domain-Driven Design: Tackling Complexity in the Heart of Software

Figure 6.9. Over-cautious locking is interfering with people's work.
The inconvenience is mounting, because there is a lot of contention for the instruments (the
"parts"). And then:
Figure 6.10. Deadlock
Those three will be waiting a while.
At this point we can begin to improve the model by incorporating the following knowledge of the
1.

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business:
Parts are used in many POs (high contention).
1.
There are fewer changes to parts than there are to POs.
2.
Changes to part prices do not necessarily propagate to existing POs. It depends on the time
of a price change relative to the status of the PO.
3.
Point 3 is particularly obvious when we consider archived POs that have already been delivered.
They should, of course, show the prices as of the time they were filled, rather than current prices.
Figure 6.11. Price is copied into Line Item. A
GGREGATE
 invariant can now
be enforced.
An implementation consistent with this model would guarantee the invariant relating PO and its
items, while changes to the price of a part would not have to immediately affect the items that
reference it. Broader consistency rules could be addressed in other ways. For example, the system
could present a queue of items with outdated prices to the users each day, so they could update or
exempt each one. But this is not an invariant that must be enforced at all times. By making the
dependency of line items on parts looser, we avoid contention and reflect the realities of the
business better. At the same time, tightening the relationship of the PO and its line items
guarantees that an important business rule will be followed.
The 
AGGREGATE
imposes an ownership of the PO and its items that is consistent with business
practice. The creation and deletion of a PO and items are naturally tied together, while the creation
and deletion of parts is independent.
AGGREGATES
mark off the scope within which invariants have to be maintained at every stage of
the life cycle. The following patterns, 
FACTORIES
and 
REPOSITORIES
, operate on 
AGGREGATES
,
encapsulating the complexity of specific life cycle transitions. . . .

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[ Team LiB ]

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[ Team LiB ]