# Payroll System Implementation Summary

## Overview
Complete implementation of a robust weekly payroll generation system for the SwiftOps field service management platform. The system automatically aggregates worker attendance (timesheets) and work completed (ticket assignments) to generate accurate weekly compensation records.

## Architecture Decision
**Approach:** Two separate background jobs (Weekly Batch Generation)
- **Daily Job:** Generate timesheets from ticket_assignments (already implemented)
- **Weekly Job:** Aggregate timesheets → Generate payroll records (newly implemented)

**Rationale:**
- Clean separation of concerns
- No race conditions (timesheets finalized before payroll generation)
- Easy to debug and maintain
- Idempotent operations (safe to re-run)

## Implementation Components

### 1. Data Model (`src/app/models/user_payroll.py`)
**Complete SQLAlchemy model matching schema.sql**

**Key Fields:**
- `user_id`, `project_id`: Links to worker and project
- `period_start_date`, `period_end_date`: Weekly period (Monday-Sunday)
- `tickets_closed`: Count from ticket_assignments
- `hours_worked`, `days_worked`: Aggregated from timesheets
- `flat_rate_amount`: Weekly base rate from project_role
- `ticket_earnings`: Calculated earnings from tickets (base + commission)
- `bonus_amount`, `deductions`: Manual adjustments
- `total_amount`: Calculated total compensation
- `is_paid`: Payment status flag
- `version`: Optimistic locking for concurrent updates

**Business Methods:**
- `calculate_total()`: Computes total_amount from components
- `mark_as_paid()`: Records payment details
- `recalculate_from_data()`: Recalculates from fresh data (timesheet corrections)
- `can_recalculate`: Property to check if recalculation is allowed

**Constraints:**
- Unique constraint: (user_id, project_id, period_start_date, period_end_date)
- All amounts must be non-negative
- Period end >= period start
- Cannot recalculate paid payrolls

### 2. Pydantic Schemas (`src/app/schemas/payroll.py`)
**Complete request/response models with validators**

**Request Schemas:**
- `PayrollGenerateRequest`: Generate for specific user/period (validates Monday-Sunday)
- `PayrollBatchGenerateRequest`: Generate for all workers in a week
- `PayrollUpdateRequest`: Update amounts before payment
- `PayrollMarkPaidRequest`: Mark as paid with payment details

**Response Schemas:**
- `PayrollResponse`: Complete payroll with computed fields (can_recalculate, net_earnings, etc.)
- `PayrollListResponse`: Paginated list
- `PayrollGenerateResponse`: Generation result with success/skip reason
- `PayrollBatchGenerateResponse`: Batch summary with errors

**Validators:**
- period_start_date must be Monday (weekday 0)
- period_end_date must be Sunday (weekday 6)
- Period must be exactly 7 days
- payment_method must be valid (mobile_money, bank_transfer, cash, cheque)

### 3. Business Logic (`src/app/services/payroll_service.py`)
**Comprehensive service layer with robust error handling**

**Core Methods:**

#### `generate_payroll_for_period()`
- Generates payroll for specific user and period
- Checks for existing payroll (idempotent)
- Aggregates data from timesheets and ticket_assignments
- Gets compensation rates from project_roles
- Calculates earnings: `flat_rate + (base_amount * tickets * (1 + commission%))`
- Creates payroll record with detailed calculation_notes
- Returns success/skip reason

#### `generate_weekly_payroll_batch()`
- Generates payroll for all active project_team members
- Processes each worker independently (continues on error)
- Collects errors for review
- Returns comprehensive summary
- Safe for background job (no user context required)

#### `recalculate_payroll()`
- Recalculates payroll from fresh timesheet/ticket data
- Only works on unpaid payrolls
- Preserves manually added bonuses/deductions
- Increments version (optimistic locking)
- Updates calculation_notes with timestamp

#### `mark_as_paid()`
- Marks payroll as paid
- Records payment method, reference, paid_by user
- Prevents recalculation after payment
- Permanent audit trail

**Helper Methods:**
- `get_week_bounds()`: Calculates Monday-Sunday for any date
- `get_previous_week_bounds()`: Gets last completed week
- `aggregate_timesheet_data()`: Sums hours_worked, counts days_present
- `aggregate_ticket_data()`: Counts completed ticket assignments
- `get_compensation_rates()`: Gets rates from project_team + project_role
- `calculate_ticket_earnings()`: Formula: base * count + commission

**Authorization:**
- Platform admins: Full access
- Project managers: Full access to their projects
- Dispatchers: View only
- Workers: View their own payroll only

**Error Handling:**
- Transaction safety (commit per operation, rollback on error)
- Comprehensive logging
- HTTPException with appropriate status codes
- Graceful degradation (batch continues on individual errors)

### 4. REST API (`src/app/api/v1/payroll.py`)
**Complete API endpoints with comprehensive documentation**

**Endpoints:**

#### `POST /payroll/generate`
- Generate payroll for specific user/period
- Validates Monday-Sunday period
- Returns PayrollGenerateResponse with success/skip reason
- Audit logging

#### `POST /payroll/generate-batch`
- Generate payroll for all workers in a week
- Optional project_id filter
- Returns batch summary with errors
- Audit logging

#### `GET /payroll/{payroll_id}`
- Get payroll by ID
- Authorization: admins/managers view all, workers view own
- Returns complete payroll with joined data

#### `GET /payroll`
- List payrolls with filtering and pagination
- Filters: user_id, project_id, is_paid, period dates
- Sorting: period_start_date, total_amount, created_at, paid_at
- Authorization: workers auto-filtered to own records

#### `POST /payroll/{payroll_id}/recalculate`
- Manually trigger recalculation
- Only works on unpaid payrolls
- Returns updated payroll
- Audit logging

#### `POST /payroll/{payroll_id}/mark-paid`
- Mark payroll as paid
- Requires payment_method and optional payment_reference
- Returns updated payroll
- Audit logging

**All endpoints include:**
- Comprehensive OpenAPI documentation
- Authorization via `@require_permission("manage_payroll")`
- Error handling with appropriate HTTP status codes
- Audit logging via AuditService

### 5. Router Registration (`src/app/api/v1/router.py`)
**Integrated into main API router**
- Imported payroll module
- Registered with prefix="/payroll" and tags=["Payroll"]
- Positioned after timesheets router (logical flow)

### 6. Timesheet Integration (`src/app/services/timesheet_service.py`)
**Auto-recalculation on timesheet correction**

**Implementation in `update_timesheet()`:**
1. Update timesheet fields
2. Commit changes
3. Find payroll for this period (if exists and unpaid)
4. Trigger recalculation
5. Continue even if recalculation fails (logged warning)

**Benefits:**
- Payroll stays in sync with attendance data
- No manual recalculation needed
- Only affects unpaid payrolls (safe)
- Non-blocking (timesheet update succeeds even if payroll recalc fails)

### 7. Background Job (`src/app/tasks/payroll_tasks.py`)
**Simple cron-based approach (no Redis/Celery)**

**Main Function: `generate_weekly_payroll_job()`**
- Runs Monday morning (e.g., 6 AM)
- Processes previous week (Monday-Sunday)
- Uses PayrollService.generate_weekly_payroll_batch()
- Comprehensive logging to file + console
- Dry-run mode for testing
- Returns summary dict

**Schedule Options:**

**Cron (Linux/Mac):**
```bash
# Every Monday at 6 AM
0 6 * * 1 cd /path/to/project && python -m app.tasks.payroll_tasks
```

**Windows Task Scheduler:**
- Trigger: Weekly, Monday, 6:00 AM
- Action: `python -m app.tasks.payroll_tasks`
- Working Directory: Project root

**CLI Interface:**
```bash
# Generate for previous week (default)
python -m app.tasks.payroll_tasks

# Dry run (test without committing)
python -m app.tasks.payroll_tasks --dry-run

# Specific date
python -m app.tasks.payroll_tasks --date 2024-12-09

# Specific project
python -m app.tasks.payroll_tasks --project-id <uuid>

# Ad-hoc week (catch-up)
python -m app.tasks.payroll_tasks --start 2024-12-02 --end 2024-12-08
```

**Safety Features:**
- Transaction-safe (commits per user, rollback on error)
- Idempotent (safe to run multiple times)
- Continues on error (doesn't fail entire batch)
- Comprehensive logging to file
- Dry-run mode for testing
- Ad-hoc mode for catch-up

## Data Flow

### Weekly Payroll Generation (Automated)
```
Monday 6 AM: Background Job Runs
    ↓
Calculate Previous Week (Mon-Sun)
    ↓
For Each Active Project Team Member:
    ↓
    1. Aggregate Timesheets
       - Sum hours_worked (PRESENT days)
       - Count days_worked (PRESENT count)
    ↓
    2. Aggregate Ticket Assignments
       - Count completed assignments in period
    ↓
    3. Get Compensation Rates
       - flat_rate_amount from project_role
       - base_amount, commission_percentage from project_role
    ↓
    4. Calculate Earnings
       - flat_rate (weekly base)
       - ticket_earnings = (base * tickets) + commission
       - total_amount = flat_rate + ticket_earnings + bonus - deductions
    ↓
    5. Create/Update Payroll Record
       - Skip if already paid
       - Update if unpaid (recalculate)
       - Create if doesn't exist
    ↓
Summary: X generated, Y skipped, Z errors
```

### Timesheet Correction Flow
```
Manager Corrects Timesheet
    ↓
TimesheetService.update_timesheet()
    ↓
Update timesheet fields
    ↓
Commit changes
    ↓
Find payroll for this period (if exists and unpaid)
    ↓
If found: Trigger PayrollService.recalculate_payroll()
    ↓
    - Re-aggregate timesheets
    - Re-aggregate tickets
    - Recalculate earnings
    - Preserve bonuses/deductions
    - Increment version
    - Update calculation_notes
    ↓
Payroll now reflects corrected timesheet
```

## Calculation Formula

### Total Compensation
```
total_amount = flat_rate_amount + ticket_earnings + bonus_amount - deductions
```

### Ticket Earnings
```
base_earnings = base_amount * tickets_closed
commission = base_earnings * (commission_percentage / 100)
ticket_earnings = base_earnings + commission
```

### Example Calculation
```
Worker: John Doe
Role: Technician
Period: Dec 09-15, 2024

From project_role:
- flat_rate_amount: KES 5,000 (weekly base)
- base_amount: KES 500 per ticket
- commission_percentage: 10%

From timesheets (Dec 09-15):
- days_worked: 5 (Monday-Friday present)
- hours_worked: 40 hours

From ticket_assignments (Dec 09-15):
- tickets_closed: 12 tickets

Calculation:
flat_rate = 5,000
base_earnings = 500 * 12 = 6,000
commission = 6,000 * 0.10 = 600
ticket_earnings = 6,000 + 600 = 6,600
bonus_amount = 0 (none added)
deductions = 0 (none added)

total_amount = 5,000 + 6,600 + 0 - 0 = KES 11,600
```

## Key Features

### 1. Robustness
- ✅ Transaction safety (atomic operations)
- ✅ Optimistic locking (version column)
- ✅ Comprehensive error handling
- ✅ Idempotent operations (safe to re-run)
- ✅ Graceful degradation (continues on error)
- ✅ Audit logging (full trail)

### 2. Data Integrity
- ✅ Unique constraint (no duplicate payrolls)
- ✅ Validation (Monday-Sunday, non-negative amounts)
- ✅ Cannot modify paid payrolls
- ✅ Auto-recalculation on timesheet correction
- ✅ Preserves manual adjustments (bonuses/deductions)

### 3. Performance
- ✅ Batch processing (all workers at once)
- ✅ Efficient queries (joinedload, filters)
- ✅ Pagination support (API)
- ✅ Indexed columns (period dates, user_id, project_id)

### 4. Maintainability
- ✅ Clean separation of concerns (Model/Schema/Service/API)
- ✅ Comprehensive documentation (docstrings)
- ✅ Logging (debug, info, error levels)
- ✅ Simple approach (no external dependencies)

### 5. Flexibility
- ✅ Manual generation (ad-hoc periods)
- ✅ Project-specific filtering
- ✅ Dry-run mode (testing)
- ✅ CLI interface (operations)
- ✅ API endpoints (programmatic access)

## Testing Checklist

### Unit Tests
- [ ] PayrollService.generate_payroll_for_period()
- [ ] PayrollService.generate_weekly_payroll_batch()
- [ ] PayrollService.recalculate_payroll()
- [ ] PayrollService.aggregate_timesheet_data()
- [ ] PayrollService.aggregate_ticket_data()
- [ ] PayrollService.calculate_ticket_earnings()
- [ ] UserPayroll.calculate_total()
- [ ] UserPayroll.mark_as_paid()
- [ ] UserPayroll.recalculate_from_data()

### Integration Tests
- [ ] Generate payroll for worker with timesheets and tickets
- [ ] Generate batch payroll for multiple workers
- [ ] Recalculate payroll after timesheet correction
- [ ] Mark payroll as paid
- [ ] Prevent recalculation of paid payroll
- [ ] Handle worker with no timesheets (0 days worked)
- [ ] Handle worker with no tickets (flat rate only)
- [ ] Handle worker with no role (0 compensation)
- [ ] Idempotency (re-run generation)
- [ ] Optimistic locking (concurrent updates)

### API Tests
- [ ] POST /payroll/generate (success, skip, error)
- [ ] POST /payroll/generate-batch (success, errors)
- [ ] GET /payroll/{id} (authorization)
- [ ] GET /payroll (filtering, pagination, sorting)
- [ ] POST /payroll/{id}/recalculate (unpaid only)
- [ ] POST /payroll/{id}/mark-paid (once only)

### Background Job Tests
- [ ] Run with default (previous week)
- [ ] Run with specific date
- [ ] Run with project filter
- [ ] Run in dry-run mode
- [ ] Ad-hoc week generation
- [ ] Error handling (DB connection, invalid data)

## Production Deployment

### 1. Database Migration
```sql
-- Verify table exists
SELECT * FROM user_payroll LIMIT 1;

-- Check indexes
\d user_payroll

-- Verify constraints
SELECT conname, contype FROM pg_constraint WHERE conrelid = 'user_payroll'::regclass;
```

### 2. Schedule Background Job

**Linux/Mac (Cron):**
```bash
# Edit crontab
crontab -e

# Add line (Monday 6 AM)
0 6 * * 1 cd /var/www/swiftops-backend && /usr/bin/python3 -m app.tasks.payroll_tasks >> /var/log/payroll_jobs.log 2>&1
```

**Windows (Task Scheduler):**
1. Open Task Scheduler
2. Create Task: "Weekly Payroll Generation"
3. Trigger: Weekly, Monday, 6:00 AM
4. Action: Start Program
   - Program: `python.exe`
   - Arguments: `-m app.tasks.payroll_tasks`
   - Working Directory: `D:\atomio\swiftops-backend`
5. Settings: Run whether user is logged on or not

### 3. Monitoring
- Check logs: `payroll_job_YYYYMMDD_HHMMSS.log`
- Monitor errors in batch summary
- Alert on job failure (email/SMS)
- Track metrics: generated count, skipped count, errors

### 4. First Run
```bash
# Dry run first (test)
python -m app.tasks.payroll_tasks --dry-run

# Run for specific week (backfill)
python -m app.tasks.payroll_tasks --start 2024-12-02 --end 2024-12-08

# Verify results
# Check database for generated payrolls
# Review logs for errors
```

## Maintenance

### Weekly Operations
1. Monday morning: Job runs automatically
2. Review logs for errors
3. Check skipped workers (investigate why)
4. Fix any errors (missing roles, invalid data)
5. Re-run for specific workers if needed

### Manual Operations
```bash
# Generate for specific worker
curl -X POST http://localhost:8000/api/v1/payroll/generate \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "user_id": "uuid",
    "project_id": "uuid",
    "period_start_date": "2024-12-09",
    "period_end_date": "2024-12-15"
  }'

# List unpaid payrolls
curl http://localhost:8000/api/v1/payroll?is_paid=false

# Mark as paid
curl -X POST http://localhost:8000/api/v1/payroll/{id}/mark-paid \
  -H "Authorization: Bearer $TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "payment_method": "mobile_money",
    "payment_reference": "ABC123XYZ"
  }'
```

## Future Enhancements

### Phase 2 (Optional)
- [ ] Email notifications to workers (payroll generated)
- [ ] PDF payslip generation
- [ ] Payment gateway integration (M-Pesa B2C)
- [ ] Payroll approval workflow
- [ ] Payroll adjustments (corrections after payment)
- [ ] Payroll reports (project, period, worker)
- [ ] Payroll analytics (cost trends, efficiency)

### Phase 3 (Advanced)
- [ ] Multi-currency support
- [ ] Tax calculations (PAYE, NHIF, NSSF)
- [ ] Payroll export (accounting systems)
- [ ] Payroll forecasting
- [ ] Bonus calculation rules engine
- [ ] Overtime tracking and compensation

## Summary

**Implementation Status:** ✅ COMPLETE

**Files Created/Modified:**
1. ✅ `src/app/models/user_payroll.py` (NEW)
2. ✅ `src/app/schemas/payroll.py` (UPDATED)
3. ✅ `src/app/services/payroll_service.py` (UPDATED)
4. ✅ `src/app/api/v1/payroll.py` (UPDATED)
5. ✅ `src/app/api/v1/router.py` (UPDATED)
6. ✅ `src/app/services/timesheet_service.py` (UPDATED)
7. ✅ `src/app/tasks/payroll_tasks.py` (UPDATED)
8. ✅ `src/app/models/project.py` (UPDATED - added payrolls relationship)

**Key Achievements:**
- ✅ Robust weekly payroll generation from timesheets
- ✅ Automatic recalculation on timesheet correction
- ✅ Simple background job (no Redis/Celery)
- ✅ Comprehensive error handling and logging
- ✅ Idempotent operations (production-safe)
- ✅ Complete REST API with documentation
- ✅ Authorization and audit logging
- ✅ Optimistic locking for concurrency
- ✅ CLI interface for operations

**Production-Ready Features:**
- Transaction safety
- Data integrity constraints
- Comprehensive logging
- Error recovery
- Dry-run mode
- Manual override capabilities
- Complete audit trail

The system is now ready for deployment and testing! 🚀