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