Phase 5: Calculation & PDF Report - Research

Researched: 2026-02-19 Domain: Capacity calculation engine + PDF report generation (ReportLab) Confidence: HIGH

Summary

Phase 5 completes the StorePredict pipeline by adding a calculation service that computes per-VM required capacity and workload-grouped totals, a report page displaying summary cards and breakdown tables, and a one-page PDF export using ReportLab.

ReportLab 4.4.10 is already installed. It ships with Bitstream Vera TTF fonts that fully support French characters. The Platypus high-level framework (SimpleDocTemplate, Table, Paragraph) handles content flow, table rendering, and font embedding automatically.

Key Decisions

Decision	Choice	Why
PDF library	ReportLab Platypus (not WeasyPrint)	No system dependencies, already installed
Font	Vera TTF (bundled)	Covers French chars, ships with ReportLab
Download	ui.download with positional src arg	NiceGUI API, no disk I/O
Weighted avg DRR	total_provisioned / total_required	Mathematically correct, simpler than per-group averaging
DRR guard	max(drr, 0.1)	Prevents division by zero

Architecture

pipeline/calculation.py    # Pure data, no UI dependency
services/pdf_report.py     # ReportLab PDF generation
ui/pages/report.py         # Report page with download button

Pitfalls Addressed

1. Division by zero — DRR of 0 guarded with max(drr, 0.1)
2. Empty dataset — Early return for zero rows
3. Vera font in Docker — Path derived from reportlab.__file__ at runtime
4. Large tables — Workload group summary (not per-VM) fits one page
5. Session data shape — .get(key, default) for all field accesses

Sources

• ReportLab docs: Platypus framework, Table styling, TTF font registration
• NiceGUI docs: ui.download, file serving patterns
• Context7: /websites/reportlab, /websites/nicegui_io