Domain-Driven Design:Large-Scale Structure*To-ju HuangComputer Science Department

University of California, San Diego

April 23, 2010

1*Evans 2003 Domain-Driven Design: Tacking Complexity in the Heart of Software (Ch. 16). Addison-Wesley Longman Publishing Co., Inc.

Outline

• Motivating Example – Online Bookstore• Background

• Domain-Driven Design• Role of Large-Scale Structure

• Large-Scale Structure• Responsibility Layer• Knowledge Level• Pluggable Component Framework

• Conclusion

Motivating Example - Online Bookstore

• Requirements• Customers can place orders online• Customers can pay orders online (credit cards or PayPal)• Customers can access order, shipping, payment information• Warehouse staff can prepare and ship orders• Warehouse staff can contact distributors to restock

• Assumptions (some will change later)• Single warehouse and single seller

(i.e. the owner company itself)• No physical store (only online-purchase)

Motivating Example - Online Bookstore

order

orderline book

shipment payment

bookcatalog

stock distributor

credit cardtransaction

PayPaltransaction

courier

customer

warehouse

* *

1

1

1

*

*1

1..*

11

Motivating Example - Online Bookstore

• Modeling Challenges• We may miss the big picture when the model grows big

• Relations among entities can change on the fly; we need flexibility while obeying high-level domain restrictions

• The same high-level abstraction recurs among projects which do not work closely together; the similarity ends right here and prevents further reuse

Outline

• Motivating Example – Online Bookstore• Background

• Domain-Driven Design• Role of Large-Scale Structure

• Large-Scale Structure• Responsibility Layer• Knowledge Level• Pluggable Component Framework

• Conclusion

Domain-Driven Design

• What Domain-Driven Design is about?• Shared models serve both design and implementation teams• Communication via unified domain languages, whose

vocabulary is used to through the models• Efficient communication helps stakeholders gain and accumulate

insights, which in turn enter the unification domain language; this forms a positive feedback loop

• Relation to Model-Driven Engineering initiatives (e.g. MDA)• Both are domain engineering approaches• Both advocate the central role of models in software

development• DDD focuses more on modeling practices;

MDE initiatives are more about the development processes

7

Role of Large-Scale Structure

• Help stakeholders to grasp the high-level of the model; everybody can identify and locate his responsibility easily

• Efficient communication is assured: different teams can understand their roles and relations to the others

• Prevents teams from diverging to the point that the whole project breaks into remotely related pieces

8

Outline

• Motivating Example – Online Bookstore• Background

• Domain-Driven Design• Role of Large-Scale Structure

• Large-Scale Structure• Responsibility Layer• Knowledge Level• Pluggable Component Framework

• Conclusion

Responsibility Layer

• Layers help us track dependencies more easily, but we need more guidance on making layers

10

why 3 layers?

could this be on layers lower?

why in the same layer?

Responsibility Layer

• Some meaningful partitions may manifest themselves, by the responsibilities assumed by a group of entities

11

order

orderline book

shipment payment

bookcatalog

stock distributor

credit cardtransaction

PayPaltransaction

courier

customer

warehouse

* *

1

1

1

*

*1

1..*

1

AND, beware of the dependencies among layers

operation

capability

Responsibility Layer

• Lower layers should not know upper layers

12

order

orderline book

shipment payment

bookcatalog

stock distributor

credit cardtransaction

PayPaltransaction

courier

customer

warehouse

* *

1

1

1

*

*1

1..*

1

operation

capability

product

Responsibility Layer

• We can have more layers according to responsibilities

13

order

orderline book

shipment payment

bookcatalog

stock

distributorcredit cardtransaction

PayPaltransaction

courier

customer

warehouse

* *

1

1

1

*

*1

1..*

1

product

capability

operation

policy

decision

promotionpolicy

discount policy

pricing

discount/promotionenabler

sales report,business analysis

Outline

• Motivating Example – Online Bookstore• Background

• Domain-Driven Design• Role of Large-Scale Structure

• Large-Scale Structure• Responsibility Layer• Knowledge Level• Pluggable Component Framework

• Conclusion

Knowledge Level

• Now, the online bookstore allows 3rd-party booksellers

15

book

stock distributor

warehouse

1..*

1product

book

stock distributor

warehouse

1..*

1product

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

manufacturinginfo

Knowledge Level

• However, the company decided that some individual sellers could be bought in, so owned warehouse can be associated both kind of stock …

16

stock

warehouse

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

TOO RESTRICTED

manufacturinginfo distributor

1..*

Knowledge Level

• However, the company decided that some individual sellers could be bought in, so owned warehouse can be associated both kind of stock …

17

stock distributor

warehouse

1..*

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

stock

distributor

warehouse

1..*

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

manufacturinginfo

manufacturinginfo

Knowledge Level

• But, this is a weird that individual seller can have regular stock or owned-warehouse have manufacturing info

18

TOO LOOSE

stock

distributor

warehouse

1..*

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

manufacturinginfo

Knowledge Level

• But, this is a weird that individual seller can have regular stock

19

stock

distributor

warehouse

1..*

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

manufacturinginfo

Knowledge Level

• We introduce the warehouse type which indirectly associate stock, distributor, manufacturing info to warehouse while can only be modified by certain entities

20

stock

distributor

warehouse

1..*

individualwarehouse

owned warehouse

self-suppliedstock

regularstock

manufacturinginfo

warehouse

manufacturinginfo

distributor

warehousetype

regularstock

self-suppliedstock

1

stock

• We introduce the warehouse type which indirectly associate stock, distributor, manufacturing info to warehouse while can only be modified by certain entities

21warehouse

manufacturinginfo

distributor

warehousetype

regularstock

self-suppliedstock

1

stock

Knowledge Level

Operation Level

Outline

• Motivating Example – Online Bookstore• Background

• Domain-Driven Design• Role of Large-Scale Structure

• Large-Scale Structure• Responsibility Layer• Knowledge Level• Pluggable Component Framework

• Conclusion

Pluggable Component Framework

• Entities can be substituted by compatible plugins• However, agreed-upon standards are required…

23

order

orderline<<interface >>

product

shipment<<interface >>

payment

<<interface >>

catalog

stock<<interface >>

distributor

credit cardtransaction

PayPaltransaction

<<interface >>

courier

customer

<<interface >>

warehouse

* *

1

1

1

*

*1

1..*

1

Conclusion

• Large-scale structure helps stakeholders to understand their roles, improve communication, and keep project coherent

• Dependencies can be turned into responsibility layers with clear meanings

• Knowledge level allows flexible relations among entities while keeping the model consistent with the business logic

• Pluggable component framework, if feasible standards employed, can make high-level architecture fully utilized

24

Thank You

25