ADR-027: PDF In-Memory via BytesIO (No Temp Files)

Status: Accepted Date: 2026-02-19

Context

The generated PDF must be delivered to the user's browser. It can be written to a temp file on disk or kept in memory.

Decision

Generate the PDF into a BytesIO buffer and serve it via ui.download(bytes). No temp files are created.

Rationale

• Eliminates temp file cleanup logic and the risk of orphaned files
• No race conditions between file creation and file serving
• BytesIO is garbage-collected automatically when the function returns
• NiceGUI's ui.download() accepts bytes directly

Alternatives Considered

• Write to /tmp and serve via static endpoint: Requires a cleanup strategy (TTL, delete-on-download); introduces race conditions in concurrent sessions

Consequences

• PDF generation must complete before the download starts (no streaming)
• Memory usage scales with PDF size; for a one-page sizing report this is negligible
• No persistent copy of the generated PDF exists server-side after download