# FX Trade Execution Flow

## Overview

The FX Engine processes foreign exchange orders through market or limit order types, calculates prices using VWAP/TWAP methods, and executes trades with proper settlement routing. This flow documents the complete order submission and execution process.

## Prerequisites

- Sovereign bank is registered and active
- FX pair exists or can be created
- Valid FX order (market or limit)
- FX service operational
- Market data available (for market orders)

## Visual Flow Diagram

```
┌─────────────┐
│ FX Order    │
│  Request    │
└──────┬──────┘
       │
       │ 1. Get/Create Pair
       ▼
┌─────────────────────────┐
│ Get or Create FX Pair   │
│  - Lookup by pair code   │
│  - Create if not exists  │
└──────┬──────────────────┘
       │
       │ 2. Calculate Price
       ▼
┌─────────────────────────┐
│ Calculate Price         │
│  - Market: getMarketPrice│
│  - Limit: use limitPrice │
└──────┬──────────────────┘
       │
       │ 3. Create Trade
       ▼
┌─────────────────────────┐
│ Create FX Trade Record  │
│  - Trade ID             │
│  - Pair, amount, price   │
│  - Order type           │
│  - Status: pending      │
└──────┬──────────────────┘
       │
       │ 4. Execute Trade
       ▼
┌─────────────────────────┐
│ Execute Trade           │
│  - Update status        │
│  - Set executed time    │
└──────┬──────────────────┘
       │
       │ 5. Complete
       ▼
┌─────────────┐
│ Trade       │
│  Executed   │
└─────────────┘
```

## Step-by-Step Process

### Step 1: Get or Create FX Pair
1. Receive FX order with:
   - Sovereign bank ID
   - FX pair code (e.g., "OMF/USD")
   - Order type (MARKET or LIMIT)
   - Amount
   - Limit price (if LIMIT order)
   - Settlement mode
2. Parse pair code to extract base and quote currencies
3. Lookup FX pair by pair code
4. If pair doesn't exist:
   - Create new FX pair record
   - Set base currency
   - Set quote currency
   - Set pair code
   - Set pricing method: `VWAP` (default)
   - Set status: `active`
5. Return FX pair (existing or newly created)

**Code Reference**: `src/core/fx/fx.service.ts:22-147`

### Step 2: Calculate Price
1. Determine price based on order type:
   - **MARKET order**: Get current market price
     - Call `getMarketPrice(pair)`
     - In production: fetch from market data feeds
     - Currently: returns mock price based on pair
   - **LIMIT order**: Use provided limit price
     - Verify limit price is provided
     - Use limit price directly
2. If MARKET order and price unavailable:
   - Throw error "Market price not available"
3. If LIMIT order and limit price missing:
   - Throw error "Invalid order type or missing limit price"

**Code Reference**: `src/core/fx/fx.service.ts:29-37`

### Step 3: Create FX Trade Record
1. Generate unique trade ID: `FX-{uuid}`
2. Create FX trade record:
   - Trade ID
   - Sovereign bank ID
   - FX pair ID
   - Base currency (from pair)
   - Quote currency (from pair)
   - Trade type: `SPOT` (default)
   - Quantity: order amount
   - Price: calculated price
   - Order type: MARKET or LIMIT
   - Initiator entity: sovereign bank ID
   - Settlement mode: from order
   - Status: `pending`
   - Timestamp: current time
3. Store trade in database
4. Return trade ID and status

**Code Reference**: `src/core/fx/fx.service.ts:39-62`

### Step 4: Execute Trade
1. Lookup trade by trade ID
2. Verify trade exists and is in `pending` status
3. Update trade record:
   - Status: `executed`
   - Executed at: current timestamp
4. Map trade to FxTrade type
5. Return executed trade details

**Code Reference**: `src/core/fx/fx.service.ts:67-87`

### Step 5: Market Price Calculation (for MARKET orders)
1. Parse pair to extract base and quote currencies
2. Get market price:
   - In production: fetch from market data feeds
   - Use VWAP (Volume Weighted Average Price) or TWAP (Time Weighted Average Price)
   - Currently: return mock prices for known pairs
3. Return price as string

**Code Reference**: `src/core/fx/fx.service.ts:92-105`

### Step 6: VWAP/TWAP Calculation (optional)
1. **VWAP Calculation**:
   - Get trade history for pair within time window (default 3600s)
   - Calculate: `VWAP = Σ(price × volume) / Σ(volume)`
   - Return VWAP price
2. **TWAP Calculation**:
   - Get price history for pair within time window
   - Calculate: `TWAP = Σ(price × time_weight) / Σ(time_weight)`
   - Return TWAP price

**Code Reference**: `src/core/fx/fx.service.ts:110-123`

## Error Handling

### Error: Invalid Order Type
- **Detection**: Order type not MARKET or LIMIT
- **Action**: Throw validation error
- **Recovery**: Correct order type, retry

### Error: Missing Limit Price
- **Detection**: LIMIT order without limit price
- **Action**: Throw error "Invalid order type or missing limit price"
- **Recovery**: Provide limit price, retry

### Error: Market Price Unavailable
- **Detection**: MARKET order but price unavailable
- **Action**: Throw error "Market price not available"
- **Recovery**: Wait for market data, retry, or use limit order

### Error: Trade Not Found
- **Detection**: Trade ID doesn't exist
- **Action**: Throw error "FX trade not found"
- **Recovery**: Verify trade ID, retry

### Error: Trade Already Executed
- **Detection**: Trade status is not `pending`
- **Action**: Return existing trade (idempotent)
- **Recovery**: Use existing trade result

## Integration Points

### Related Services
- **FX Service**: `src/core/fx/fx.service.ts`
- **Market Data Service**: (in production - external feeds)
- **Settlement Service**: For trade settlement routing
- **SIRE Routing**: For optimal settlement path

### API Endpoints
- `POST /api/v1/fx/orders` - Submit FX order
- `POST /api/v1/fx/trades/:tradeId/execute` - Execute trade
- `GET /api/v1/fx/trades/:tradeId` - Get trade details
- `GET /api/v1/fx/pairs/:pairCode/price` - Get market price

### Database Models
- `FxTrade` - FX trade records
- `FxPair` - FX pair definitions

## Performance Metrics

- **Pair Lookup/Creation**: < 10ms target
- **Price Calculation**: < 20ms target (market data dependent)
- **Trade Creation**: < 10ms target
- **Trade Execution**: < 10ms target
- **Total End-to-End**: < 50ms target
- **Throughput**: 20,000+ orders/second
- **Availability**: 99.99% uptime target

## Security Considerations

### Order Validation
- Verify sovereign bank is active
- Validate order parameters
- Check amount limits

### Price Verification
- Market prices from trusted sources
- Limit prices validated against market
- Price manipulation detection

### Trade Execution
- Idempotent execution (can retry safely)
- Trade status tracking
- Audit trail for all trades

## Testing Scenarios

### Happy Path - Market Order
1. Valid market order request
2. FX pair exists or created
3. Market price available
4. Trade created successfully
5. Trade executed successfully

### Happy Path - Limit Order
1. Valid limit order request
2. FX pair exists or created
3. Limit price provided
4. Trade created successfully
5. Trade executed successfully

### Error Scenarios
1. Invalid order type
2. Missing limit price
3. Market price unavailable
4. Trade not found
5. Trade already executed

### Edge Cases
1. Maximum order amount
2. Minimum order amount
3. New FX pair creation
4. Concurrent orders for same pair
5. Price calculation during high volatility

---

**Related Flows**:
- [SSU Mint Burn Flow](./ssu-mint-burn-flow.md)
- [FX/SSU Integration Flow](./fx-ssu-integration-flow.md)
- [GSS Settlement Flow](./gss-settlement-flow.md)