FinTech Domain
Layer 3: Domain Constraints
Domain Constraints → Design Implications
| Domain Rule | Design Constraint | Rust Implication |
|---|
| Audit trail | Immutable records | Arc, no mutation |
| Precision | No floating point | rust_decimal |
| Consistency | Transaction boundaries | Clear ownership |
| Compliance | Complete logging | Structured tracing |
| Reproducibility | Deterministic execution | No race conditions |
Critical Constraints
Financial Precision
RULE: Never use f64 for money
WHY: Floating point loses precision
RUST: Use rust_decimal::Decimal
Audit Requirements
RULE: All transactions must be immutable and traceable
WHY: Regulatory compliance, dispute resolution
RUST: Arc<T> for sharing, event sourcing pattern
Consistency
RULE: Money can't disappear or appear
WHY: Double-entry accounting principles
RUST: Transaction types with validated totals
Trace Down ↓
From constraints to design (Layer 2):
"Need immutable transaction records"
↓ m09-domain: Model as Value Objects
↓ m01-ownership: Use Arc for shared immutable data
"Need precise decimal math"
↓ m05-type-driven: Newtype for Currency/Amount
↓ rust_decimal: Use Decimal type
"Need transaction boundaries"
↓ m12-lifecycle: RAII for transaction scope
↓ m09-domain: Aggregate boundaries
Key Crates
| Purpose | Crate |
|---|
| Decimal math | rust_decimal |
| Date/time | chrono, time |
| UUID | uuid |
| Serialization | serde |
| Validation | validator |
Design Patterns
| Pattern | Purpose | Implementation |
|---|
| Currency newtype | Type safety | struct Amount(Decimal); |
| Transaction | Atomic operations | Event sourcing |
| Audit log | Traceability | Structured logging with trace IDs |
| Ledger | Double-entry | Debit/credit balance |
Code Pattern: Currency Type
use rust_decimal::Decimal;
#[derive(Clone, Debug, PartialEq)]
pub struct Amount {
value: Decimal,
currency: Currency,
}
impl Amount {
pub fn new(value: Decimal, currency: Currency) -> Self {
Self { value, currency }
}
pub fn add(&self, other: &Amount) -> Result<Amount, CurrencyMismatch> {
if self.currency != other.currency {
return Err(CurrencyMismatch);
}
Ok(Amount::new(self.value + other.value, self.currency))
}
}
Common Mistakes
| Mistake | Domain Violation | Fix |
|---|
| Using f64 | Precision loss | rust_decimal |
| Mutable transaction | Audit trail broken | Immutable + events |
| String for amount | No validation | Validated newtype |
| Silent overflow | Money disappears | Checked arithmetic |
Trace to Layer 1
| Constraint | Layer 2 Pattern | Layer 1 Implementation |
|---|
| Immutable records | Event sourcing | Arc, Clone |
| Transaction scope | Aggregate | Owned children |
| Precision | Value Object | rust_decimal newtype |
| Thread-safe sharing | Shared immutable | Arc (not Rc) |
Related Skills
| When | See |
|---|
| Value Object design | m09-domain |
| Ownership for immutable | m01-ownership |
| Arc for sharing | m02-resource |
| Error handling | m13-domain-error |