1.
@mauroservienti | #EDDD
all our
aggregates
are wrong
Mauro Servienti
@mauroservienti

2.
@mauroservienti | #EDDD
All I wanna do
when I wake up in
the morning is…
Rosanna, Toto. Toto IV

3.
@mauroservienti | #EDDD
Buy a "Banana Protector"

4.
@mauroservienti | #EDDD
Buy a "Banana Protector"

5.
@mauroservienti | #EDDD
Sounds like a nice looking aggregate
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart

6.
@mauroservienti | #EDDD
It even has behaviors

7.
@mauroservienti | #EDDD
please, welcome the
business

8.
@mauroservienti | #EDDD
Business requirements (#1)

9.
@mauroservienti | #EDDD
Straightforward, isn’t it?
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart
- Item ID
- Price
Sales

10.
@mauroservienti | #EDDD
Straightforward, isn’t it?
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart
- Item ID
- Price
Sales
copy

11.
@mauroservienti | #EDDD
Business requirements (#2)

12.
@mauroservienti | #EDDD
Still not a big deal…
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart
copy
- Item ID
- Inventory
Warehouse
copy

13.
@mauroservienti | #EDDD
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart
- Item ID
- Inventory
Warehouse
However, availability changes over time
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
copy

14.
@mauroservienti | #EDDD
Business requirements (#3)

15.
@mauroservienti | #EDDD
- Item ID
- Inventory
Warehouse
…uhm…
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- Quantity
- Inventory
- Name
- Description
Shopping Cart

16.
@mauroservienti | #EDDD
- Item ID
- Inventory
Warehouse
…uhm…
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
Shopping Cart

17.
@mauroservienti | #EDDD
- Item ID
- Inventory
Warehouse
…uhm…
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
Shopping Cart
Is item XYZ in a cart?
Swap & Insert Prices

18.
@mauroservienti | #EDDD
- Item ID
- Inventory
Warehouse
…uhm…
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- Price
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
Shopping Cart
Is item XYZ in a cart?
Swap & Insert Prices
Move to Saved for…

19.
@mauroservienti | #EDDD
- Item ID
- Inventory
Warehouse
It’s becoming messy, isn’t it?
- Item ID
- Price
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
Shopping Cart
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Update XYZ inventory
Is item XYZ in a cart?
Swap & Insert Prices
Move to Saved for…
Is item XYZ in a cart?
Swap & Insert Prices
Move to Saved for…

20.
@mauroservienti | #EDDD
The big picture is even messier!
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
Shopping Cart
- Item ID
- Inventory
Warehouse
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
- Item ID
- Name
- Description
Marketing
copy
Is item XYZ in a cart?
Swap & Insert Prices
Move to Saved for…

21.
@mauroservienti | #EDDD
…and messier!
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
- Estimated
Delivery
Shopping Cart
- Item ID
- Inventory
Warehouse
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
Is item XYZ in a cart?
Swap & Insert Prices
- Item ID
- Name
- Description
Marketing
copy
- Item ID
- Delivery
Type
Shipping
Is item XYZ in a cart?
Update XYZ delivery
Move to Saved for…

22.
@mauroservienti | #EDDD
autonomy?
puff, it’s gone

23.
@mauroservienti | #EDDD
Can we get rid of all this coupling?
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
- Inventory
- Name
- Description
- Estimated
Delivery
Shopping Cart
- Item ID
- Inventory
Warehouse
Is item XYZ in a cart?
Update XYZ inventory
- Item ID
- Price
Sales
- Item ID
- Name
- Description
Marketing
copy
- Item ID
- Delivery
Type
Shipping
Is item XYZ in a cart?
Update XYZ delivery
Is item XYZ in a cart?
Swap & Insert Prices
Move to Saved for…

24.
@mauroservienti | #EDDD
shopping cart
doesn’t exist

25.
@mauroservienti | #EDDD
Sales shopping cart
[ list-of ]
- Item ID
- Quantity
- Inventory
- Name
- Description
- Delivery Est.
Shopping Cart Warehouse
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
- Item ID
- Inventory
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
- Item ID
- Delivery
Type
Shipping

26.
@mauroservienti | #EDDD
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
Warehouse shopping cart
[ list-of ]
- Item ID
- Quantity
- Name
- Description
- Delivery Est.
Shopping Cart
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

27.
@mauroservienti | #EDDD
Shipping shopping cart
[ list-of ]
- Item ID
- Quantity
- Name
- Description
Shopping Cart
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

28.
@mauroservienti | #EDDD
Marketing tends to be stable
[ list-of ]
- Item ID
- Quantity
Shopping Cart
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

29.
@mauroservienti | #EDDD
Do we need the Shopping Cart at all?
[ list-of ]
- Item ID
- Quantity
Shopping Cart
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

30.
@mauroservienti | #EDDD
Can Sales be responsible for that concept?
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
Cart ID
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

31.
@mauroservienti | #EDDD
We followed the coupling
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
Cart ID
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

32.
@mauroservienti | #EDDD
information
structure doesn’t fit users

33.
@mauroservienti | #EDDD
Can Read/View Models be a solution?
Marketing Sales Shipping Warehouse others
ViewModel
Storage
Shopping Cart

34.
@mauroservienti | #EDDD
that’s a
cache

35.
@mauroservienti | #EDDD
We cannot cache everything

36.
@mauroservienti | #EDDD
Shared identifiers
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
Cart ID
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

37.
@mauroservienti | #EDDD
ViewModel Composition
/products/
Browser
1

38.
@mauroservienti | #EDDD
ViewModel Composition
/products/
Marketing Sales Shipping Warehouse others
Browser
IDIDIDIDID
1
ViewModel

39.
@mauroservienti | #EDDD
How can we build such a page?

40.
@mauroservienti | #EDDD
Sales Product Requests Handler

41.
@mauroservienti | #EDDD
Sales Request matching

42.
@mauroservienti | #EDDD
Sales Composition

43.
@mauroservienti | #EDDD
what
about
writes?

44.
@mauroservienti | #EDDD
ViewModel Decomposition
[ list-of ]
- Cart ID
- Item ID
- Quantity
- Delivery Est.
- Item ID
- Price
- Item ID
- Name
- Description
Marketing
Sales
Cart ID
[ list-of ]
- Item ID
- CurrentPrice
- LastPrice
- Quantity
[ list-of ]
- Cart ID
- Item ID
- Inventory
- Quantity
Warehouse
- Item ID
- Inventory
- Item ID
- Delivery
Type
Shipping

45.
@mauroservienti | #EDDD
ViewModel Decomposition
POST -> /shopping-cart/
Browser
{
ItemID
Quantity
}

46.
@mauroservienti | #EDDD
ViewModel Decomposition
POST -> /shopping-cart/
Browser
Sales Shipping Warehouse
ID
Q.ty
ID
Q.ty
ID
Q.ty
{
ItemID
Quantity
}

47.
@mauroservienti | #EDDD
Shipping Shopping Cart Requests Handler

48.
@mauroservienti | #EDDD
Shipping Request Matching

49.
@mauroservienti | #EDDD
Shipping Decomposition

50.
@mauroservienti | #EDDD
things might
go wrong

51.
@mauroservienti | #EDDD
Shipping Decomposition

52.
@mauroservienti | #EDDD
Shipping Shopping Cart Requests Handler

53.
@mauroservienti | #EDDD
Shipping Error Handling

54.
@mauroservienti | #EDDD
Append Only Model

55.
@mauroservienti | #EDDD
there is no such thing as orchestration

56.
@mauroservienti | #EDDD
Full Vertical Slices
MarketingSales Warehouse Shipping
behavior & databehavior & data behavior & databehavior & data
ViewModel Composition ViewModel Composition ViewModel CompositionViewModel Composition

57.
@mauroservienti | #EDDD
services need to
communicate

58.
@mauroservienti | #EDDD
Business requirements (#4)
• Annoy Notify users of 1 week stale carts
• Wipe 1 month stale carts

59.
@mauroservienti | #EDDD
Services communication channel
MarketingSales Warehouse Shipping
behavior & databehavior & data behavior & databehavior & data
ViewModel Composition ViewModel Composition ViewModel CompositionViewModel Composition

60.
@mauroservienti | #EDDD
Services communication channel
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart

61.
@mauroservienti | #EDDD
Services communication channel
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout

62.
@mauroservienti | #EDDD
Services communication channel
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout

63.
@mauroservienti | #EDDD
1 week later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
Cart Got Stale

64.
@mauroservienti | #EDDD
1 week later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
busCart Got Stale

65.
@mauroservienti | #EDDD
1 week later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
bus Cart Got Stale

66.
@mauroservienti | #EDDD
1 week later
MarketingSales Warehouse Shipping
behavior & data
behavior & data behavior & data
behavior & data
ViewModel Composition
ViewModel Composition ViewModel Composition
ViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
bus
Notify User
Cart Got Stale

67.
@mauroservienti | #EDDD
1 month later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
bus
Cart Expired

68.
@mauroservienti | #EDDD
busCart Expired
1 month later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
Cart Expired

69.
@mauroservienti | #EDDD
1 month later
MarketingSales Warehouse Shipping
behavior & data
behavior & data
behavior & databehavior & data
ViewModel Composition
ViewModel Composition ViewModel CompositionViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
busCart Expired Cart Expired

70.
@mauroservienti | #EDDD
1 month later
MarketingSales Warehouse Shipping
behavior & databehavior & data
behavior & data
behavior & data
ViewModel Composition ViewModel Composition ViewModel Composition
ViewModel Composition
Add Item to Cart
1. Set 1 week timeout
2. Set 1 month timeout
bus
Wipe cart Wipe cart
Cart Expired Cart Expired

71.
@mauroservienti | #EDDD
cross services events
can be as thin as
event name and identifiers

72.
@mauroservienti | #EDDD
bus
The final big picture
MarketingSales Warehouse Shipping
behavior & databehavior & data behavior & databehavior & data
UI Shell
ViewModel Composition ViewModel Composition ViewModel CompositionViewModel Composition

73.
@mauroservienti | #EDDD
the 45’ tyranny
Demos
bit.ly/explore-ddd-demos
Udi Dahan talking about boundaries
go.particular.net/explore-ddd-boundaries

74.
@mauroservienti | #EDDD
Takeaways
• Boundaries are key to success
• Do not bring in more technology to solve non-
technical problems

75.
@mauroservienti | #EDDD
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Users/Business analysts tend to think in term
of data presentation

76.
@mauroservienti | #EDDD
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Do not name things prematurely
• Premature names stick and drive data
aggregation

77.
@mauroservienti | #EDDD
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Do not name things prematurely
• Behaviors define how to aggregate data
• Group data that change together and that influence each
other
• Use anti-requirements techniques to validate data
grouping
• Follow the coupling

78.
@mauroservienti | #EDDD
Takeaways
• Boundaries are key to success
• Mental model can badly influence design
• Do not name things prematurely
• Behaviors define how to aggregate data
• Use ViewModel composition to present data
• No need for complex projections and read
models

79.
@mauroservienti | #EDDD
Thank you!
I’ll be around to answer your
questions the whole day