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ADR-0007: ReportLab for PDF report generation

Status: Accepted
Date: 2026-06-20

Context

Each patient's AI Summary tab offers a downloadable clinical summary PDF containing the AI narrative, patient metadata, risk scores, and referenced sources. Options for server-side PDF generation in Python:

  • WeasyPrint — HTML/CSS → PDF; excellent for web-to-print but adds a heavy libpango/libcairo dependency (difficult in slim containers)
  • Puppeteer / Playwright headless Chrome — HTML → PDF via Chrome; adds ~300 MB to the container image
  • fpdf2 — lightweight but limited layout control; no frame/template system
  • ReportLab (reportlab) — mature, pure-Python, ships a full platypus layout engine with BaseDocTemplate, Frame, PageTemplate, Canvas callbacks

Decision

Use reportlab with BaseDocTemplate and per-page canvas callbacks for header/footer.

Rationale

  • Pure Python with no native library dependencies — works in the existing python:3.12-slim base image
  • BaseDocTemplate + PageTemplate + canvas onPage callbacks give full control over the header band (indigo bar with Pulse logo drawn via canvas primitives), page numbers, and confidentiality footer — none of which are easily achievable with fpdf2
  • The logo is drawn as a polyline via canvas.beginPath(), eliminating any SVG/image dependency at PDF-generation time
  • Risk score cells can be colour-coded programmatically via TableStyle — not possible in an HTML-to-PDF workflow without CSS hacks

Consequences

  • The PDF layout is defined in Python, not in a template file — design changes require code changes
  • ReportLab's free open-source version (reportlab) does not include CMYK colour or some advanced typography features; those are not needed here
  • PDF bytes are streamed back via a FastAPI Response(content=..., media_type="application/pdf") and downloaded client-side using fetch() + URL.createObjectURL() — never via a URL query parameter (see ADR-0008)