# System Architecture (Version 4)

This document describes the production architecture of the document transcription system.

## Architecture Objectives

- Preserve original source material and immutable machine transcription output.
- Support batching one or more images into ordered multi-page documents.
- Capture complete submission-time prompt provenance and per-page provider response evidence.
- Execute page transcription concurrently with bounded `asyncio` workers.
- Maintain relational portability across SQLite and PostgreSQL.
- Keep operator workflows cross-platform and Python-driven.
- Support many-to-many document-person relationships with extensible roles.
- Support registry-driven document type classification.
- Enforce relationship-role exclusivity rules consistently across UI, API, and persistence boundaries.

## Runtime Topology

The runtime operates as an asynchronous Python application:

- FastAPI + NiceGUI web application process.
- In-process `asyncio` worker engine for transcription execution.
- Relational persistence via SQLModel / SQLAlchemy.
- Pydantic V2 validation across API payloads, prompt configuration, and structured metadata.

^^^mermaid
flowchart LR
U[Browser User] --> A[FastAPI + NiceGUI App]
A --> W[Asyncio Worker Engine]
A --> DB[(Relational DB)]
W --> P[Vision Provider APIs]
W --> DB
^^^

## Lifecycle Ownership

Application lifespan owns runtime setup and teardown:

- Initialize logging, settings, directories, and prompt configuration.
- Manage asynchronous database engine connection pools.
- Execute database bootstrap or migrations.
- Recover stale or interrupted jobs on startup.
- Manage graceful shutdown of active background tasks.

## Layered Module Structure

### Interface Layer

- `src/transcription/ui/**`
- `src/transcription/api/**`

Responsibilities:

- Render document, source, person, job, and classification views.
- Accept user input for uploads, editing, linking, and revisions.
- Present structured validation and conflict feedback.

### Application and Async Worker Layer

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

Responsibilities:

- Orchestrate uploads, job creation, and status transitions.
- Execute per-page provider calls through bounded concurrency.
- Persist page-level outcomes and update aggregate job state.

### Domain and Service Layer

- `src/transcription/db/models.py`
- `src/transcription/services/*.py`

Responsibilities:

- Manage transactional operations for documents, people, types, links, sources, jobs, and job sources.
- Apply deterministic conflict handling for relationship-role writes.
- Use set-based synchronization for many-to-many relationship updates.
- Resolve and validate registry-backed document types.

### Infrastructure Layer

- `src/transcription/db/**`
- `src/transcription/providers/**`

Responsibilities:

- Provide async database sessions and engine configuration.
- Provide provider adapters for vision model execution.

## Core Workflows

### 1. Multi-Page Transcription

1. User uploads one or more images for a `Document`.
2. System stores files, hashes them, creates ordered `Source` rows, and creates a `Job`.
3. Worker claims the job, marks it `processing`, and executes page calls concurrently.
4. Each page writes a `JobSource` result with raw output, metadata, and full provider response evidence.
5. Aggregate status becomes `completed`, `partial_success`, or `failed`.

### 2. Document-Person Relationship Management

1. User opens a document or person edit flow.
2. UI loads existing links grouped by role.
3. User adds or removes people within one or more roles.
4. Service computes add/remove deltas rather than replacing all links blindly.
5. Conflict checks enforce exclusivity policy before persistence commits.

### 3. Document Type Management

1. User selects a registry-backed document type for a document.
2. Service resolves the stable type code or id.
3. Persistence stores the `document_type_id` reference.
4. Inactive types remain valid for historical rows but are excluded from default selectors.

## Domain Invariants

- `Source.raw_transcription` stores immutable machine output.
- Human corrections occur only in `Source.revised_text`.
- Prompt and parameter provenance is frozen on `Job` at submission time.
- Provider output evidence is stored on `JobSource` for each page execution.
- `DocumentPerson` links are unique for `(document_id, person_id, role_id)`.
- Configured exclusive role pairs cannot coexist for the same `(document_id, person_id)`.
- Relationship mutations are deterministic and set-based.
- `DocumentType.code` is stable; `DocumentType.label` may evolve.

## Data Model Summary

- `Document` has one `DocumentType`, many `Source` pages, many `Job` runs, and many `Person` records through `DocumentPerson`.
- `Source` belongs to one `Document` and may participate in many `JobSource` executions.
- `Job` has many `JobSource` rows.
- `PersonRole` defines available relationship roles.
- `RoleExclusivity` defines role pairs that cannot coexist for the same document-person pair.

## Test Strategy

- Unit tests for models, validation, hashing, and registry resolution.
- Service tests for CRUD, set-based sync, and exclusivity enforcement.
- Async workflow tests for page isolation, partial failure handling, and stored evidence.
- UI integration tests for multi-page rendering, role grouping, and document type selection.

## Related Local References

- [System Overview](index_v4.md)
- [System Requirements](requirements_v4.md)
- [Data Model](schema_v4.md)
- [Error Handling Policy](error_handling_v4.md)