FastAPI Backend Deployment Troubleshooting

Systematic approach to deploying and fixing FastAPI + SQLAlchemy backends.

Overview

This skill documents the process of deploying a FastAPI backend with SQLAlchemy ORM, including common pitfalls and troubleshooting patterns encountered during a real-world quantitative trading system deployment.

Common Issues & Solutions

1. Dependency Version Conflicts

Problem: FastAPI pins specific versions of starlette, causing conflicts when requirements.txt specifies both.

ERROR: Cannot install -r requirements.txt (line 7) and starlette==0.41.0 because these package versions have conflicting dependencies

Solution: Remove the explicit starlette version from requirements.txt and let FastAPI pull in its compatible version.

# Check FastAPI's required starlette version
pip show fastapi | grep Requires

2. Module Import Errors

Pattern: Core modules don't export expected symbols

Config Module Fix

core/config.py may need a global settings instance:

@lru_cache()
def get_settings() -> Settings:
    """Get global config singleton"""
    return Settings()

# Add this for direct import compatibility
settings = get_settings()

Exceptions Module Fix

core/exceptions.py needs a base exception class:

class JuyidaException(Exception):
    """System base exception"""
    code: int = 500
    message: str = "System internal error"
    
    def __init__(self, message: str = None, code: int = None):
        self.message = message or self.message
        self.code = code or self.code
        super().__init__(self.message)

class BusinessException(JuyidaException):
    """Business exception base class"""
    # ... existing code

Auth Module Fix

core/auth.py needs decode_token alias:

# Alias for backward compatibility
decode_token = verify_token

Dependencies Module Fix

core/dependencies.py needs pagination dependency:

class PaginationDep:
    """Pagination dependency"""
    def __init__(self, page: int = 1, page_size: int = 20):
        self.page = max(1, page)
        self.page_size = min(100, max(1, page_size))
        self.offset = (self.page - 1) * self.page_size
        self.limit = self.page_size

async def get_current_user_id():
    """Get current user ID"""
    return None

3. Database Index Name Conflicts (CRITICAL!)

Problem: SQLite does not allow duplicate index names across tables. When multiple tables define indexes with same generic names like idx_symbol_time, table creation fails.

Error:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) index idx_symbol_time already exists

Solution: Prefix all indexes with table-specific names.

Step-by-Step Fix Process:

1. Identify all duplicate indexes:

cd backend
grep -hr "Index(" apps/ | sort | uniq -d

2. Rename patterns systematically: | Generic Name | Table-Specific Names | |--------------|---------------------| | idx_symbol_time | idx_orders_symbol_time, idx_trades_symbol_time, idx_quotes_symbol_time | | idx_user_time | idx_orders_user_time, idx_trades_user_time, idx_flows_user_time, idx_signals_user_time, idx_alerts_user_time | | idx_user_status | idx_rules_user_status, idx_alerts_user_status, idx_strat_user_status |

3. Fix each model file:

apps/market/models.py:

# Before
Index('idx_symbol_time', 'symbol', 'quote_time')
Index('idx_symbol_date', 'symbol', 'trade_date', unique=True)

# After
Index('idx_quotes_symbol_time', 'symbol', 'quote_time')
Index('idx_daily_symbol_date', 'symbol', 'trade_date', unique=True)

apps/trading/models.py:

# Orders table
Index('idx_orders_user_time', 'user_id', 'order_time')
Index('idx_orders_symbol_time', 'symbol', 'order_time')

# Trades table
Index('idx_trades_user_time', 'user_id', 'trade_time')
Index('idx_trades_symbol_time', 'symbol', 'trade_time')

# Capital flows table
Index('idx_flows_user_time', 'user_id', 'flow_time')

apps/risk/models.py:

# Risk rules table
Index('idx_rules_user_type', 'user_id', 'rule_type')
Index('idx_rules_user_status', 'user_id', 'status')

# Risk alerts table
Index('idx_alerts_user_status', 'user_id', 'status')
Index('idx_alerts_user_time', 'user_id', 'alert_time')

apps/strategy/models.py:

Index('idx_strat_user_status', 'user_id', 'status')
Index('idx_signals_user_time', 'user_id', 'signal_time')

4. Verify fixes:

grep -hr "Index('idx_" apps/ | sort | uniq -d
# Should return nothing if all conflicts fixed

4. Missing Optional Dependencies

Problem: Some packages are listed as optional but required in code:

ModuleNotFoundError: No module named 'psutil'

Solution: Install them separately:

pip install psutil

Deployment Verification Checklist

• All dependencies install without version conflicts
• All Python modules import without errors
• Database initialization completes successfully
• All tables created (verify count matches expected)
• Health check endpoint returns 200
• API docs accessible at /docs
• Test user login works

Quick Start After Fixes

cd backend
source venv/bin/activate

# Initialize database
python init_db.py

# Start server
python main.py

# Verify
curl http://localhost:8000/health
open http://localhost:8000/docs

Key Lessons Learned

1. SQLite is strict about index names - Always prefix indexes with table names in multi-table schemas
2. Module imports need consistent exports - Aliases help with backward compatibility during refactoring
3. Verify after each major fix - Don't wait until the end to test database initialization
4. Use grep to find duplicates - Systematically identify and fix naming conflicts

Typical Time Estimates