Risk Sizing Not Blocking

Experiment Overview

Item	Details
Date	2024-12-24
Goal	Refactor risk/capital managers from blocking trades to sizing them correctly
Environment	scripts/live_trader.py, alpaca_trading/risk/
Status	Success

Context

The trading system had multiple components that could BLOCK trades:

•Capital Manager: check_trade_allowed() returned False if trade exceeded limits
•Portfolio Risk Manager: check_position_limits() returned False for VaR/concentration violations

This caused problems:

•Trades were rejected after signal generation
•Components didn't work in harmony
•Orders sized upstream were blocked downstream

Architectural Principle: All trading system components should work in harmony. There should never be an order that is sized too large for capital management.

Verified Workflow

The Wrong Pattern (Blocking)

python

# WRONG: Check if arbitrary size is allowed, block if not
estimated_qty = cash_available * alloc / price
allowed, reason = capital_mgr.check_trade_allowed(allocation, estimated_qty * price, symbol)
if not allowed:
    logger.warning(f"{symbol}: Capital Manager BLOCKED trade: {reason}")
    return state, {"skip_reason": "capital_limit"}  # Trade rejected!

The Correct Pattern (Sizing)

python

# CORRECT: Get max allowed size, use it as constraint
max_qty_from_capital = capital_mgr.get_max_trade_size(
    allocation=capital_allocation,
    symbol=symbol,
    current_price=price,
    positions=positions_dict
)

# Later, during GARCH sizing:
qty = garch_mgr.calculate_position_size(...)
qty = min(qty, max_qty_from_capital)  # Apply constraint
# Order executes with correct size - no blocking needed

Complete Sizing Flow

python

# 1. Capital Manager provides max qty from allocation
if capital_allocation is not None and capital_mgr is not None:
    max_qty_from_capital = capital_mgr.get_max_trade_size(
        allocation=capital_allocation,
        symbol=symbol,
        current_price=price,
        positions=positions_dict
    )
    if max_qty_from_capital <= 0:
        return state, {"skip_reason": "no_capacity"}  # Only skip if truly zero
else:
    max_qty_from_capital = float('inf')

# 2. Risk Manager provides max qty from position limits
if portfolio_risk_mgr is not None:
    max_value_from_risk = portfolio_metrics.total_value * 0.20  # 20% max
    max_qty_from_risk = max_value_from_risk / price
    max_qty_from_capital = min(max_qty_from_capital, max_qty_from_risk)

# 3. GARCH calculates initial size
qty = garch_mgr.calculate_position_size(symbol, account_value, signal_strength, price, returns)

# 4. Apply all constraints
qty = min(qty, max_qty_from_alloc)      # Allocation limit
qty = min(qty, max_qty_from_capital)     # Capital + risk limits
qty = qty * position_scale               # Drawdown scaling

# 5. Execute with correct size
executor.submit(symbol, qty=qty, side=entry_side, type="limit")

Failed Attempts (Critical)

Attempt	Why it Failed	Lesson Learned
`check_trade_allowed()` blocking	Rejected trades after signal generation	Use `get_max_trade_size()` for constraints
`check_position_limits()` blocking	Portfolio risk rejected sized trades	Apply limits as max position value constraint
Estimate size then check	Estimated size != final size, caused false blocks	Get max size first, use as constraint
Hard-coded $1000 min_cash_buffer	Blocked trades on small accounts	Use percentage-based limits only

Key Insights

Harmony Principle

All trading system components should work together:

code

Signal Generation → Sizing → Risk Constraints → Execution
      ↓               ↓            ↓              ↓
   Direction      Initial qty   Apply limits   Correct size

NOT:

code

Signal Generation → Sizing → Risk Check → BLOCKED!

When to Skip vs When to Size

Situation	Action
No capacity at all (max_qty = 0)	Skip with "no_capacity"
Low confidence signal	Reject signal (not a blocking issue)
Size exceeds limit	Reduce size to limit (don't block)
Risk limit exceeded	Reduce size to risk limit (don't block)

Methods That Size vs Methods That Block

Method	Purpose	Use For
`get_max_trade_size()`	Returns max qty	Sizing constraints
`calculate_allocation()`	Returns capacity info	Sizing constraints
`check_trade_allowed()`	Returns bool	AVOID - causes blocking
`check_position_limits()`	Returns bool	AVOID - causes blocking

Files Modified

code

scripts/live_trader.py:
  - Lines 1375-1391: Capital manager sizing (removed check_trade_allowed)
  - Lines 1393-1411: Risk manager sizing (removed check_position_limits)
  - Lines 1466-1467: Apply max_qty_from_capital constraint

References

•alpaca_trading/risk/capital_manager.py: get_max_trade_size() at lines 284-350
•alpaca_trading/risk/portfolio_risk.py: RiskLimits at lines 20-35
•scripts/live_trader.py: Sizing flow at lines 1375-1505