domain-fintech

Layer 3: Domain Constraints

Domain Constraints → Design Implications

| Domain Rule | Design Constraint | Rust Implication | |-------------|-------------------|------------------| | Audit trail | Immutable records | Arc<T>, no mutation | | Precision | No floating point | rust_decimal | | Consistency | Transaction boundaries | Clear ownership | | Compliance | Complete logging | Structured tracing | | Reproducibility | Deterministic execution | No race conditions |

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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

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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

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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))
    }
}

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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 |

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Trace to Layer 1

| Constraint | Layer 2 Pattern | Layer 1 Implementation | |------------|-----------------|------------------------| | Immutable records | Event sourcing | Arc<T>, Clone | | Transaction scope | Aggregate | Owned children | | Precision | Value Object | rust_decimal newtype | | Thread-safe sharing | Shared immutable | Arc (not Rc) |

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Related Skills

| When | See | |------|-----| | Value Object design | m09-domain | | Ownership for immutable | m01-ownership | | Arc for sharing | m02-resource | | Error handling | m13-domain-error |