# System Architecture (Version 2)

This document describes the V2 production architecture of the personal historical-document transcription system.

## Architecture Objectives

* Preserve source material as immutable transcribed text alongside page-level spatial AI metadata.
* Support batching multi-image and folder uploads cleanly into sequential pages (`page_number`).
* Leverage asynchronous worker pools (`asyncio`) for parallel single-image API execution bounded by rate limiters (`asyncio.Semaphore`).
* Migrate persistence to PostgreSQL using native `UUID`, `TIMESTAMPTZ`, and `JSONB` document storage.
* Standardize all data validation, API parsing, and database models on **Pydantic V2**.
* Support rich historical attribution (multi-author and multi-recipient relationships).

## Runtime Topology

The V2 runtime operates as an asynchronous Python application:

* FastAPI + NiceGUI web application process.
* In-process `asyncio` background task orchestrator for parallel API execution.
* Relational persistence via PostgreSQL (using `asyncpg` or `psycopg3`).
* Pydantic V2 validation layer wrapping API payloads and PostgreSQL `JSONB` schemas.

```mermaid
flowchart LR
U[Browser User] --> A[FastAPI + NiceGUI App]
A --> W[Asyncio Worker Engine]
A --> DB[(PostgreSQL Database)]
W --> P[Vision Provider APIs\nOpenAI / Claude]
W --> DB
```

## Lifecycle Ownership

Application lifespan owns runtime setup/teardown:

* Initialize environment logging and Pydantic configuration.
* Manage asynchronous PostgreSQL connection pools (`asyncpg` / `psycopg3`).
* Execute database migrations and index initialization.
* Recover stale processing jobs on startup.
* Manage graceful shutdown of active `asyncio` worker pools.

## Layered Module Structure

### Interface Layer

* `src/transcription/ui/**` (NiceGUI pages, multi-page renderers, person cards)
* `src/transcription/api/**` (FastAPI routes and JSON error handlers)

### Application & Async Worker Layer

* `src/transcription/services/workflows.py`
* `src/transcription/worker.py`

Responsibilities:

* Batch orchestration and status transitions (`queued` -> `processing` -> `completed` | `partial_success` | `failed`).
* Parallel single-image API execution using `asyncio.gather` bounded by `asyncio.Semaphore`.
* Pydantic schema parsing (`PageAIMetadata`) and validation prior to database storage.

### Domain & Service Layer

* `src/transcription/db/models.py` (SQLModel/Pydantic V2 schema definitions for the current implementation)
* `src/transcription/services/*.py` (Transactional operations for `Document`, `Person`, `Source`, `Job`, and `JobSource`)

### Infrastructure Layer

* `src/transcription/db/**` (PostgreSQL connection pooling and raw parameterized SQL execution)
* `src/transcription/providers/**` (OpenAI & Anthropic Vision SDK adapters)

## Processing Workflow

1. User uploads a folder or batch of images for a `Document`.
2. System creates `Document`, `Job(status='queued')`, and ordered `Source` pages (`page_number = 1..N`).
3. Worker claims job, sets `Job.status = 'processing'`, and spawns parallel `asyncio` tasks bounded by semaphore.
4. Each task calls Vision API for a **single** `Source` image.
5. On task completion:
* Writes a `JobSource` record containing `status='transcribed'`, `raw_transcription`, `ai_metadata` (bounding boxes/confidence), and `raw_api_response`.
* Caches active text to `Source.raw_transcription`.


6. On page failure:
* Writes `JobSource` record with `status='failed'` and `error_detail`.


7. Once all page tasks resolve:
* Marks `Job.status` as `completed` (100% success), `partial_success` (at least 1 success, 1 failure), or `failed` (all failed).



## Domain Ownership & Invariants

* **Immutable AI Outputs:** `source.raw_transcription` and `job_source.raw_transcription` store original, point-in-time machine output and are immutable.
* **Inlined Revisions:** Human corrections occur on `source.revised_text`. UI renders `COALESCE(revised_text, raw_transcription)`.
* **Sequential Integrity:** Multi-page documents are strictly ordered by `source.page_number ASC`.
* **Page Execution Isolation:** A failure on one page image does not invalidate successful transcriptions on sister pages in the same batch job.

## Data Model Summary

* `Document` has many `Source` pages, many `Job` runs, and many `Person` records via `DocumentPerson` junction (`author` or `recipient`).
* `Source` belongs to one `Document` and can be processed across many `JobSource` executions.
* `Job` has many `JobSource` execution records.

## Test Strategy

* Unit tests for Pydantic V2 schemas, custom validators, and JSONB serialization.
* Integration tests for async PostgreSQL connection handling and parameterized queries.
* Async workflow tests using mock AI providers to verify `partial_success` and retry logic.
* UI integration tests for multi-page rendering and person management.

---

## Technology References

- [FastAPI documentation](https://fastapi.tiangolo.com/)
- [NiceGUI documentation](https://nicegui.io/documentation)
- [PostgreSQL documentation](https://www.postgresql.org/docs/)
- [Python asyncio](https://docs.python.org/3/library/asyncio.html#module-asyncio)
- [Pydantic Validation](https://pydantic.dev/docs/validation/latest/get-started/)
- [Pydantic AI](https://pydantic.dev/docs/ai/overview/)
 

## Related Local References

- [System Overview](index_v2.md)
- [System Design Intent](invariant/intent.md)
- [Transcription Methodology](invariant/transcription_methodology.md)
- System Architecture (this document)
- [System Requirements](requirements_v2.md)
- [Data model](schema_v2.md)
- [Error Handling Policy](error_handling_v2.md)
- [Implementation Plan](implementation_plan_v2.md)