Domain Primitives In Action - Explore DDD 2017

Domain Primitives in Action
- Making it Secure by Design
@DanielDeogun @danbjson
Explore DDD, Denver 2017

@DanielDeogun @danbjson #SecureByDesign #EDDD
About Us…
Dan Bergh Johnsson
Secure Domain Philosopher
Omegapoint, Sweden
Umeå
Malmö
Göteborg
Falun
New York
Stockholm
Daniel Deogun
Coder and Quality Defender

Key Take Aways
Domain Primitives
- are native to your domain
- form a conceptual whole
- easy way to improve and secure existing code

Modeling a Hotel Room

Peano's Axioms
1. Zero is a number
2. If n is a number, the successor of n is a number
3. Zero isn’t the successor of a number
4. Two numbers of which the successors are equal are themselves equal
5. If a set S of numbers contains zero and also the successor of every
number in S, then every number is in S
Abelian group
• Closure: a + b = integer
• Associativity: a + (b + c) = (a + b) + c
• Commutativity: a + b = b + a
• Identity: a + 0 = a
• Inverse: a + (−a) = 0
It’s “just” an Integer?

…or is it a String?
• What characters are legal?
• Which operations are allowed?
• Does this make sense?

Math and Hotel are
different Contexts
Math Domain
Integer
Hotel Domain
Room Number
Hotel Domain
Math Domain
Integer
(a.k.a Room Number)

Room Number
public final class RoomNumber {
private final int value;
public RoomNumber(final int value) {
isTrue(Floor.isValid(value));
inclusiveBetween(1, 50, value % 100);
this.value = value;
}
public Floor floor() {
return new Floor(value);
}
// other logic …
}

Definition of
Domain Primitive
• Building block that’s native to your domain
• Valid in your context
• Immutable and resemble a value object
“A value object so precise in its deﬁnition that it, by its mere
existence, manifests its validity is called a Domain Primitive.”
- Secure by Design

Less Simple
Domain Primitive
public void pay(final double money, final int recipientId) {
final String currency = CurrencyService.currencyFor(recipientId);
BankService.transfer(money, currency, recipientId);
}
public void pay(final Money money, final Recipient recipient) {
notNull(money);
notNull(recipient);
BankService.transfer(money, recipient);
}
But Money is a conceptual whole and
should be modeled as a domain primitive

Intelligent Machines -
not just values
class Rate { 
private final Currency from; 
private final Currency to; 
 
Rate(Currency from, Currency to) { 
this.from = notNull(from); 
this.to = notNull(to); 
}
 
Money exchange(Money from) {
notNull(from);
isTrue(this.from
.equals(from.currency));
BigDecimal converted = . . . 
return new Money(converted, to); 
} 
}

Standing on the
Shoulders of Giants
• Domain Primitives act as building blocks
• Guy L. Steele Jr. “Growing a Language”
• Abelson, Sussman “Structure and
Interpretation of Computer Programs”
https://ﬂic.kr/p/8cc44h https://creativecommons.org/licenses/by/2.0/

… In Action
https://ﬂic.kr/p/Q7zV https://creativecommons.org/licenses/by-sa/2.0/
vs
https://ﬂic.kr/p/djYc9H https://creativecommons.org/licenses/by/2.0/
Green Field Brown Field

“Draw the Line”
- Find a Semantic Border
https://ﬂic.kr/p/nEZKMd https://creativecommons.org/licenses/by/2.0/
public void checkout(final int roomNumber) {
new RoomNumber(roomNumber); //throws exception if invalid
houseKeepingService.registerForCleaning(roomNumber);
minibarService.replenish(roomNumber);
// other operations ...
}
public void checkout(final int roomNumber) {
if (!RoomNumber.isValid(roomNumber)) {
reporter.logInvalidRoomNumber(roomNumber);
}
houseKeepingService.registerForCleaning(roomNumber);
minibarService.replenish(roomNumber);
// other operations ...
}

Hardening your APIs
- Enforce Data Quality
Generic
Speciﬁc
public void checkout(final int roomNumber)
public void checkout(final RoomNumber roomNumber)
Enforce data quality

Cluttered Entity
class Order {
private ArrayList<Object> items;
private boolean paid; 
 
public void addItem(String isbn, int qty) {
if(this.paid == false) {
notNull(isbn);
inclusiveBetween(10, 10, isbn.length());
isTrue(isbn.matches("[0-9X]*"));
isTrue(isbn.matches("[0-9]{9}[0-9X]")); 
Book book = bookCatalogue.findByISBN(isbn); 
if (inventory.availableBooks(isbn) >= qty) {
items.add(new OrderLine(book, qty));
}
}
}
//Other logic...
}

De-Cluttered Entity
class Order {
private ArrayList<Object> items;
private boolean paid; 
public void addItem(ISBN isbn, Quantity qty) {
notNull(isbn);
notNull(qty);
if(this.paid == false) {
Book book = bookCatalogue.findByISBN(isbn); 
if (inventory.availableBooks(isbn).greaterOrEqualTo(qty)) {
items.add(new OrderLine(book, qty));
}
}
}
//Other logic...
}

Renovating Services

Renovating Services
public interface ExchangeService { 
double rate(String from, String to); 
} 
private double convertForeignPrice( 
double priceForeignCurrency, 
String foreignCurrency) { 
 
double rate = exchange.rate(foreignCurrency, "USD"); 
double priceUSD = priceForeignCurrency * rate; 
// … and some rounding
return priceUSD; 
} 
public interface ExchangeService { 
Rate rate(Currency from, Currency to); 
} 
private Money convertForeignPrice( 
Money priceForeignCurrency, 
Currency foreignCurrency) { 
 
Rate rate = exchange.rate(foreignCurrency, USD); 
Money priceUSD = rate.exchange(priceForeignCurrency); 
return priceUSD; 
}

But…
https://flic.kr/p/eGYhMw
https://creativecommons.org/licenses/by/2.0/
… what about performance?
https://flic.kr/p/2pvb2T
… it becomes a lot of classes!
… isn’t it overly complex?
https://flic.kr/p/7Ro4HU

… Making it
Secure by Design
OWASP:
• Injection Flaw
• Cross-site scripting (XSS)
Seals lots of small holes
https://ﬂic.kr/p/85qctm
Conﬁdentiality
Integrity
Availability

Contains several
concepts from DDD
40% Discount code
ctweddd17
Design secure
software without
“thinking” about
security
There’s a Book…
Shameless plug…

Q&A
https://ﬂic.kr/p/9ksxQa https://creativecommons.org/licenses/by-nc-nd/2.0/

Thanks

Domain Primitives In Action - Explore DDD 2017

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