# Deep Debugging Report - Port Conflict Resolution

**Date:** December 17, 2025  
**Issue:** Backend service failing to start with "Address already in use" error  
**Status:** ✅ RESOLVED with safeguards implemented

---

## 🔍 ROOT CAUSE ANALYSIS

### The Problem

Backend systemd service (`church-music-backend.service`) was failing repeatedly with error:

```
[ERROR] Connection in use: ('127.0.0.1', 8080)
[ERROR] connection to ('127.0.0.1', 8080) failed: [Errno 98] Address already in use
[ERROR] Can't connect to ('127.0.0.1', 8080)
```

### Investigation Process

1. **Service Status Check**
   - Backend service in failed state after 5 restart attempts
   - Systemd restart limit reached (StartLimitBurst=5)
   - Exit code 1 (FAILURE)

2. **Log Analysis**
   - Error logs showed consistent port 8080 binding failures
   - No application errors - purely infrastructure issue
   - Repeated retry attempts over ~90 seconds

3. **Port Analysis**

   ```bash
   sudo lsof -i :8080
   # Found: python 17329 pts - python app.py
   ```

4. **Process Investigation**

   ```bash
   ps aux | grep 17329
   # Result: python app.py running as development server
   ```

### Root Cause Identified

**A Flask development server (`python app.py`) was running in the background**, occupying port 8080 and preventing the production Gunicorn service from starting.

**How it happened:**

- The `start-dev-mode.sh` script starts `python app.py` in background
- No cleanup when switching to production mode
- No collision detection between dev and production modes
- Process persisted across reboots/sessions

---

## 🛠️ FIXES IMPLEMENTED

### 1. Immediate Fix: Kill Rogue Process

```bash
sudo kill 17329  # Freed port 8080
sudo systemctl reset-failed church-music-backend.service
sudo systemctl start church-music-backend.service
```

**Result:** ✅ Backend service started successfully

### 2. Systemd Service Enhancement

**File:** [church-music-backend.service](church-music-backend.service)

Added pre-start check:

```ini
ExecStartPre=/media/pts/Website/Church_HOP_MusicData/backend/pre-start-check.sh
```

This script:

- Checks if port 8080 is in use before starting
- Kills any rogue processes (except systemd services)
- Prevents startup if port can't be freed
- Logs all actions for debugging

**File:** [backend/pre-start-check.sh](backend/pre-start-check.sh)

### 3. Port Cleanup Utility

**File:** [cleanup-ports.sh](cleanup-ports.sh)

Comprehensive port management script:

- Checks ports 8080 (backend) and 5100 (frontend)
- Identifies processes using each port
- Distinguishes between systemd services and rogue processes
- Safely kills only non-systemd processes
- Cleans up stale PID files
- Color-coded output for clarity

Usage:

```bash
./cleanup-ports.sh
```

### 4. Development Mode Safeguards

**File:** [start-dev-mode.sh](start-dev-mode.sh)

Enhanced with:

- **Production service detection**: Warns if systemd services are running
- **Interactive prompt**: Asks permission to stop production services
- **Old process cleanup**: Kills previous dev mode processes
- **PID file management**: Removes stale PID files
- **Clear status display**: Shows running services and how to stop them

**File:** [stop-dev-mode.sh](stop-dev-mode.sh) (NEW)

Properly stops development mode:

- Kills backend and frontend dev processes
- Removes PID files
- Kills any stray processes
- Prevents port conflicts

### 5. Documentation Updates

- [WEBSOCKET_HTTPS_FIX.md](WEBSOCKET_HTTPS_FIX.md) - WebSocket security fix
- [STATUS.md](STATUS.md) - Updated system status
- This file - Comprehensive debugging documentation

---

## 🔒 SAFEGUARDS ADDED

### 1. Pre-Start Port Validation

- Automatic port conflict detection
- Kills rogue processes before service start
- Prevents "Address already in use" errors
- Logged for audit trail

### 2. Dev/Production Separation

- Development mode checks for production services
- Interactive warning system
- Cannot run both modes simultaneously
- Clear error messages

### 3. Process Tracking

- PID files for development mode
- Automatic cleanup of stale PIDs
- Process state validation

### 4. Monitoring & Diagnostics

- Enhanced logging in service files
- Dedicated cleanup script
- Verification script for WebSocket fix
- Clear error messages with solutions

---

## 🧪 VERIFICATION TESTS

### Test 1: Service Startup

```bash
sudo systemctl status church-music-backend
```

**Result:** ✅ Active (running) with pre-start check successful

### Test 2: API Endpoints

```bash
curl http://localhost:8080/api/health
```

**Result:** ✅ `{"status":"ok","ts":"2025-12-17T07:24:06.301875"}`

### Test 3: HTTPS Access

```bash
curl -I https://houseofprayer.ddns.net/
```

**Result:** ✅ HTTP/2 200

### Test 4: No Port Conflicts

```bash
sudo lsof -i :8080
```

**Result:** ✅ Only gunicorn workers (systemd service)

### Test 5: Pre-Start Check

```bash
sudo systemctl restart church-music-backend
journalctl -u church-music-backend | grep ExecStartPre
```

**Result:** ✅ `ExecStartPre=/media/pts/Website/Church_HOP_MusicData/backend/pre-start-check.sh (code=exited, status=0/SUCCESS)`

---

## 📊 FAILURE POINTS ANALYSIS

### Identified Failure Points

1. **Port Binding**
   - **Risk:** Multiple processes competing for same port
   - **Mitigation:** Pre-start port check, automatic cleanup
   - **Detection:** Service fails immediately with clear error

2. **Development vs Production Conflict**
   - **Risk:** Running both modes simultaneously
   - **Mitigation:** Interactive warnings, automatic detection
   - **Detection:** start-dev-mode.sh checks systemd services

3. **Zombie Processes**
   - **Risk:** Background processes persisting after crashes
   - **Mitigation:** PID tracking, automatic cleanup
   - **Detection:** cleanup-ports.sh finds and kills

4. **Service Restart Limits**
   - **Risk:** Hitting StartLimitBurst causing permanent failure
   - **Mitigation:** Pre-start checks prevent repeated failures
   - **Recovery:** Manual reset with `systemctl reset-failed`

5. **Missing Dependencies**
   - **Risk:** Backend starts before database ready
   - **Mitigation:** `After=postgresql.service` in service file
   - **Detection:** Backend logs show connection errors

### Monitoring Recommendations

1. **Port Monitoring**

   ```bash
   # Add to cron for automated monitoring
   */5 * * * * /media/pts/Website/Church_HOP_MusicData/cleanup-ports.sh
   ```

2. **Service Health Checks**

   ```bash
   curl http://localhost:8080/api/health
   ```

3. **Log Monitoring**

   ```bash
   sudo journalctl -u church-music-backend -f
   ```

---

## 📝 USAGE GUIDE

### Production Mode (Recommended)

```bash
# Start services
sudo systemctl start church-music-backend
sudo systemctl start church-music-frontend

# Check status
sudo systemctl status church-music-backend
sudo systemctl status church-music-frontend

# View logs
sudo journalctl -u church-music-backend -f
```

### Development Mode

```bash
# Start (will check for conflicts)
./start-dev-mode.sh

# Stop
./stop-dev-mode.sh

# View logs
tail -f /tmp/church-*.log
```

### Troubleshooting

```bash
# Clean up port conflicts
./cleanup-ports.sh

# Reset failed services
sudo systemctl reset-failed church-music-backend

# Verify WebSocket fix (for frontend)
./verify-websocket-fix.sh
```

---

## 📈 IMPROVEMENTS SUMMARY

### Before

- ❌ Port conflicts caused service failures
- ❌ No detection of dev/prod conflicts
- ❌ Manual cleanup required
- ❌ Difficult to diagnose issues
- ❌ Zombie processes persisted

### After

- ✅ Automatic port conflict resolution
- ✅ Dev/prod conflict detection and warnings
- ✅ Automated cleanup scripts
- ✅ Clear error messages and logs
- ✅ Automatic zombie process cleanup
- ✅ Pre-start validation
- ✅ Comprehensive documentation

---

## 🎯 LESSONS LEARNED

1. **Always validate port availability before binding**
   - Implement pre-start checks in systemd services
   - Log port conflicts with process details

2. **Separate development and production environments**
   - Never mix dev and prod processes
   - Implement conflict detection
   - Clear documentation of each mode

3. **Track background processes properly**
   - Use PID files for all background processes
   - Clean up PIDs on exit
   - Validate process state before operations

4. **Provide clear error messages**
   - Log what's wrong and how to fix it
   - Include process details in errors
   - Offer automated solutions

5. **Document everything**
   - Usage guides for operators
   - Troubleshooting steps
   - Architecture decisions

---

## 🔗 RELATED FILES

### Created/Updated

1. [cleanup-ports.sh](cleanup-ports.sh) - Port conflict resolution
2. [backend/pre-start-check.sh](backend/pre-start-check.sh) - Service pre-start validation
3. [start-dev-mode.sh](start-dev-mode.sh) - Enhanced with safeguards
4. [stop-dev-mode.sh](stop-dev-mode.sh) - Proper cleanup
5. [church-music-backend.service](church-music-backend.service) - Added pre-start check
6. [WEBSOCKET_HTTPS_FIX.md](WEBSOCKET_HTTPS_FIX.md) - WebSocket security fix
7. [STATUS.md](STATUS.md) - Updated system status

### Configuration Files

- [nginx-ssl.conf](nginx-ssl.conf) - HTTPS proxy configuration
- [frontend/.env](frontend/.env) - WebSocket security settings
- [frontend/.env.production](frontend/.env.production) - Production build settings

---

## ✅ FINAL STATUS

**Backend Service:** ✅ Running (with pre-start protection)  
**Frontend Service:** ✅ Running (production build)  
**WebSocket Error:** ✅ Fixed (no dev server in production)  
**Port Conflicts:** ✅ Prevented (automatic cleanup)  
**Documentation:** ✅ Complete  
**Safeguards:** ✅ Implemented  

**System Status:** FULLY OPERATIONAL with enhanced reliability

---

## 🆘 EMERGENCY PROCEDURES

If services fail to start:

1. **Quick Fix**

   ```bash
   ./cleanup-ports.sh
   sudo systemctl reset-failed church-music-backend
   sudo systemctl start church-music-backend
   ```

2. **Check Logs**

   ```bash
   sudo journalctl -u church-music-backend --no-pager | tail -50
   ```

3. **Manual Port Check**

   ```bash
   sudo lsof -i :8080
   sudo kill -9 <PID>  # If rogue process found
   ```

4. **Restart All**

   ```bash
   ./stop-dev-mode.sh
   sudo systemctl restart church-music-backend
   sudo systemctl restart church-music-frontend
   ```

---

**Author:** GitHub Copilot (Claude Sonnet 4.5)  
**Date:** December 17, 2025  
**Status:** Production Ready ✅