# ocr-service

Python FastAPI microservice that performs the actual handwritten text recognition (HTR) and OCR. The Spring Boot backend orchestrates jobs; this service executes them.

## What this service owns

- Text recognition: Surya (typewritten text) and Kraken (Kurrent/Sütterlin historical handwriting)
- Baseline layout analysis: Kraken BLLA model
- Sender recognition: trained per-archive sender models
- HTTP API at port 8000 (internal Docker network — no external port)

## What this service does NOT own

- Job lifecycle — tracked in the backend's `ocr/` domain
- MinIO storage — the service fetches PDFs via presigned URLs generated by the backend; it does not hold credentials
- Transcription block storage — results are streamed back to the backend, which writes them to PostgreSQL

## API endpoints

| Endpoint | Auth | Purpose |
|---|---|---|
| `POST /ocr` | None (internal network only) | Run OCR on a PDF (presigned MinIO URL in request body) |
| `POST /train` | `X-Training-Token` header | Trigger sender-model training |
| `POST /segtrain` | `X-Training-Token` header | Trigger segmentation training |
| `GET /health` | None | Health check |

## Environment variables

| Variable | Default | Required? | Sensitive? | Purpose |
|---|---|---|---|---|
| `TRAINING_TOKEN` | — | YES (prod) | YES | Guards `/train` and `/segtrain`. Do not leave empty in production. |
| `ALLOWED_PDF_HOSTS` | `minio,localhost,127.0.0.1` | YES | — | SSRF protection — comma-separated allowed PDF source hosts. Never set to `*`. |
| `KRAKEN_MODEL_PATH` | `/app/models/` | — | — | Directory where Kraken HTR models are stored (populated by `download-kraken-models.sh`) |
| `BLLA_MODEL_PATH` | `/app/models/blla.mlmodel` | — | — | Kraken baseline layout analysis model. Auto-downloaded via `ensure_blla_model.py` on startup if missing. |
| `HF_HOME` | `/app/cache` | — | — | HuggingFace model cache root. Keeps model downloads on the persistent cache volume. |
| `XDG_CACHE_HOME` | `/app/cache` | — | — | XDG cache root (used by some Surya components alongside `HF_HOME`). |
| `TORCH_HOME` | `/app/models/torch` | — | — | PyTorch model cache. Kept on the persistent models volume. |
| `TMPDIR` | `/app/cache/.tmp` | — | — | Download-staging directory for GB-scale Surya model files. Must point to a disk-backed path, not the 512 MB `/tmp` tmpfs — see ADR-021. |

## Key files

| File | Purpose |
|---|---|
| `main.py` | FastAPI app, endpoint definitions, SSRF validation |
| `engines/` | Surya and Kraken engine wrappers |
| `models.py` | Pydantic request/response models |
| `preprocessing.py` | PDF-to-image conversion before OCR |
| `confidence.py` | Per-block confidence scoring |
| `spell_check.py` | Post-OCR spell correction using historical dictionaries |
| `ensure_blla_model.py` | Startup script that downloads the BLLA model if missing |
| `entrypoint.sh` | Docker entrypoint — runs `ensure_blla_model.py` then starts the server |

## Backend counterpart

`backend/src/main/java/org/raddatz/familienarchiv/ocr/README.md`
