# ADR-004: In-Process PDFBox Thumbnails (not ocr-service)

## Status

Accepted

## Context

The archive lists documents as text-only rows everywhere (home search, person detail, conversation timeline, Chronik). For a fundamentally visual archive — letters, scans, handwritten pages — this is a real discoverability problem. Issue #307 introduces a small JPEG thumbnail for every document.

A viable alternative to rendering in Spring Boot is delegating to the existing `ocr-service` (Python), which already has PyMuPDF/PIL available and is the project's designated place for PDF pixel work. The comparison is not obvious: either place works.

## Decision

Render thumbnails in-process in Spring Boot using **Apache PDFBox 3.0.4** (already a dependency for training-data export). A dedicated `thumbnailExecutor` pool isolates the work from the shared task pool used by OCR.

- PDF first page rendered via `PDFRenderer.renderImageWithDPI(0, 100, ImageType.RGB)`, scaled to 240 px width (bilinear) and encoded as JPEG quality 85.
- Non-PDF image types (JPEG, PNG, TIFF) decoded via `javax.imageio` — TIFF requires the `twelvemonkeys-imageio-tiff` plugin on the classpath.
- Upload paths fire-and-forget via `ThumbnailAsyncRunner.dispatchAfterCommit(docId)`; a `ThumbnailBackfillService` covers anything the async task missed or that pre-dates this feature.

## Alternatives Considered

| Alternative | Why rejected |
|---|---|
| Delegate to `ocr-service` (PyMuPDF) | Adds a network hop and a failure mode to every document upload. `ocr-service` is not guaranteed healthy at upload time (model-loading start period is 60 s). PDFBox is already a backend dependency — delegating is a net complexity increase. |
| Render on the frontend with `pdfjs-dist` at display time | Would work for PDFs but not for scans / images; list pages would need to render dozens of PDFs on first paint; no server-side caching. |
| Thumbor / imaginary / a dedicated thumbnail service | Overkill for a single-operator household tool; new container to operate and secure. |

## Consequences

**Easier:**
- Zero new infrastructure. `thumbnails/` is a prefix in the existing MinIO bucket — production migration to Hetzner Object Storage works identically.
- Backfill is a plain sequential loop; no inter-service retry semantics.
- Integration test runs against real MinIO without needing `ocr-service` to be healthy.

**Harder:**
- PDFBox is a parser attack surface. Mitigated by a 30-second watchdog timeout in `ThumbnailAsyncRunner` and by the fire-and-forget contract (failures never break upload).
- Memory ceiling: the `thumbnailExecutor` is capped at 2 threads on memory-constrained hosts. A busy backfill alongside OCR can approach the 3 GB heap on an 8 GB server — acceptable but not comfortable. The current production server (64 GB) has ample headroom. Streaming via `FileService.downloadFileStream` keeps this bounded for PDFs up to 50 MB.

### Operational caveats (intentional)

**Backfill state is in-memory and single-node.** `ThumbnailBackfillService.currentStatus` is a volatile reference updated on the thumbnail executor thread. Restarting the backend mid-run loses progress and the next `runBackfillAsync()` starts over. This mirrors `MassImportService.ImportStatus` and is acceptable because the household archive runs as a single Spring Boot process, backfill is a rare one-shot admin action, and re-running the backfill is idempotent (`findByFilePathIsNotNullAndThumbnailKeyIsNull()` naturally skips completed documents).

**`ThumbnailService` and `ThumbnailBackfillService` inject `DocumentRepository` directly.** This is a deliberate exception to the project's "services never reach into another domain's repository" rule. Treating thumbnails as a cross-cutting aspect of `Document` rather than a sub-domain avoids a circular dependency (`DocumentService` → `ThumbnailAsyncRunner` → `DocumentService` would close the loop). If thumbnail state grows beyond two columns into its own domain model, extract a proper `ThumbnailRepository` at that point — not before.

## Future Direction

- If a second image-processing job (OCR region crops, sharing previews) arrives, revisit moving all image work to `ocr-service` so the two share a single PyMuPDF instance.
- If thumbnails ever need to be generated at multiple sizes, switch the key pattern from `thumbnails/{docId}.jpg` to `thumbnails/{docId}/{width}.jpg` — the endpoint and cache-bust URL are already structured to accommodate that.
