Aurora-Core/docs/architecture.md

# Aurora Core — Architecture

Aurora Core is a production-grade **modular monolith** (section 1/9 of
`docs/aurora-core-spec.md`) — one deployable API codebase with clear internal
module boundaries, not a set of microservices. It is the canonical metadata
layer for the Aurora ecosystem; Plex, Overseerr, Radarr, Sonarr, and future
Aurora applications are consumers of it, never dependencies of it.

```
Internet
    |
Traefik
    |
Aurora Core API replicas (stateless)
    |
    +--- PostgreSQL (authoritative)
    |
    +--- PgBouncer (production connection pooling)
    |
    +--- Redis (cache / rate limit / job queue — disposable)
    |
    +--- Background Workers (arq, scale independently)
            |
            +--- Metadata ingestion
            +--- Search index maintenance (implicit: Postgres FTS/trigram indexes)
```

## Module boundaries (`app/`)

| Module | Responsibility |
|---|---|
| `app/core/` | Config, DB engine, Redis client, structured logging, security/rate-limiting, job queue connection. No business logic. |
| `app/models/` | SQLAlchemy 2.0 ORM models — the schema. See `docs/database.md`. |
| `app/schemas/` | Pydantic **response** models. API responses are always built from these, never from serialized ORM objects directly (section 18). |
| `app/repositories/` | Raw data access (SQLAlchemy queries) per aggregate, plus `polymorphic.py` for the shared credit/artwork/video/external-id tables. No response shaping. |
| `app/services/` | Business logic: assembling API responses from repositories (`movie_service.py`, ...), caching (`cache.py`), search (`search/`), and the ingestion pipeline (`metadata/`, `ingestion/`). |
| `app/providers/` | Provider adapters (`MetadataProvider` ABC + `TMDbProvider`). Fetch **raw** data only — never write to the database. |
| `app/api/v1/` | FastAPI routers. Thin: parse request, call a service, translate service exceptions to HTTP errors. |
| `app/workers/` | arq worker process + task functions, thin wrappers around `app/services/ingestion/runner.py`. |

Data flows in one direction: `providers` → `services/metadata` (normalize) →
`services/metadata/matcher` (match) → `repositories` (persist) →
`services/*_service` (assemble response) → `schemas` (validate/serialize) →
`api/v1` (HTTP). Nothing skips a layer; in particular, provider adapters
never touch the database, and routers never touch SQLAlchemy models
directly.

## Provider abstraction & ingestion pipeline (sections 13–15)

```
Provider.get_movie(external_id)   # raw dict, provider-shaped
    -> normalize_movie()          # -> NormalizedMovie, provider-agnostic
    -> match_entity()             # exact external-ID match, or NEW
    -> ingest_movie()             # upsert Movie + satellite tables
    -> upsert_provider_record()   # provenance: raw payload + match confidence
```

`MetadataProvider` (`app/providers/base.py`) is the only thing that knows
TMDb-specific request shapes. A second provider (IMDb dataset export,
Wikidata, ...) means writing a new adapter that returns the same raw shape
TMDb's endpoints return, or a new `normalize_*` sibling if its shape differs
— nothing in `app/services/ingestion/` or downstream needs to change.

**Matching policy is deliberately conservative** (section 14): only an exact
external-ID match reuses an existing Aurora entity. Everything else creates
a new entity. Aurora never guesses via fuzzy title matching that two
providers' records are "the same movie" — that risk (wrongly merging two
different films) is worse than the cost of a duplicate until a human-
reviewable matching workflow exists (tracked in `docs/database.md#known-limitations`).

Ingestion always runs through workers, never inline in an HTTP request
(section 15). The admin API (`POST /api/v1/admin/ingest/movie`) only
enqueues an arq job and returns immediately; `app/workers/tasks.py` picks it
up and calls `app/services/ingestion/runner.py`, which tracks the job's
lifecycle in `ingestion_jobs` (idempotent via a unique `idempotency_key` —
re-triggering ingestion for the same provider + external ID reuses the same
job record rather than double-processing).

Failures actually get retried, not just labeled retryable: arq only
auto-retries a job on its own `arq.Retry` exception (or `CancelledError`) —
never on an arbitrary exception a task raises — so `_run_ingestion` catches
the failure, records it, and if `ingestion_jobs.attempts` hasn't hit
`max_attempts` yet, raises `Retry(defer=...)` (capped exponential backoff)
so arq actually puts the job back on the queue instead of the DB saying
"retrying" while nothing retries it. Series ingestion cascading into
concurrent season jobs (`include_seasons`) is exactly what surfaces
transient failures worth retrying this way: two seasons sharing a guest
star can race to create the same `Person` row (see
`app/repositories/reference_repository.py:_create_or_recover`, which
recovers from that specific race directly; the arq retry here is the
backstop for whatever it doesn't).

## Artwork self-hosting (section 10)

```
ingest_movie/ingest_series/ingest_person succeeds
    -> tasks.py: get_primary_artwork_for_download(entity_type, [entity_id, ...])
    -> one download_artwork_task enqueued per primary artwork id
    -> download_artwork(): fetch original_url, verify it's an image, cap
       size, sha256 it, storage.save(), set Artwork.local_path + checksum
```

Every image API responses reference is stored with its provider CDN URL
(`original_url`) from the moment it's ingested; `local_path` starts empty
and is filled in by this pipeline running asynchronously afterward, so
responses transparently point at TMDb until a local copy exists, then
switch over. Only artwork marked `is_primary` (the one image TMDb itself
designates for an entity, not the dozens of alternates also ingested) gets
downloaded automatically — see `docs/database.md#artwork-storage-self-hosting`.

Storage is behind an `ArtworkStorage` interface
(`app/services/artwork/storage.py`) with one implementation,
`LocalDiskStorage` — a shared Docker volume between the `api` and `worker`
containers (`docker-compose.yml`'s `aurora_media_data`), which is enough
for this project's actual deployment shape (one Docker host, principle 9:
avoid premature complexity) but assumes exactly that. A deployment that
outgrew a single host would implement the same interface against S3/R2
instead, without the download pipeline or callers changing.

This is also why `app/repositories/polymorphic.py:sync_artwork` upserts
artwork by `provider_asset_id` instead of the delete-then-recreate pattern
every other satellite table uses (movie/series/person credits, videos,
genre/keyword/company links): a delete-then-recreate would generate a fresh
`Artwork.id` with `local_path=None` on every re-ingestion, silently
discarding every downloaded copy and forcing a full re-download each sync.

## Search (section 16)

`app/services/search/base.py:SearchProvider` is the abstraction; API
handlers and `SearchService` only depend on it, never on SQL directly.
`PostgreSQLSearchProvider` blends `ts_rank_cd` (full-text) with `pg_trgm`
`similarity()` (typo tolerance) across primary titles/names and alternate
titles. Swapping in `OpenSearchProvider` later means implementing the same
`SearchProvider.search()` contract — no caller changes.

## Caching (section 21)

`app/services/cache.py` wraps Redis with fail-open semantics: every read/
write function catches Redis errors and behaves as a cache miss / no-op
rather than raising, so Redis being down degrades performance, not
correctness. `app/api/deps.py:cached_response()` is the read-through helper
used by the movie/person detail endpoints (the read-heaviest, per section
21). Cache keys are versioned (`aurora:v1:...`) so a response-shape change
(bumping `CACHE_VERSION`) doesn't require a manual flush.

Cache invalidation on write: 0.1 has no write API yet (ingestion writes
directly to Postgres via workers, not through the cached read endpoints), so
there is nothing to invalidate today. The moment a write endpoint exists,
it must call `app/services/cache.py:cache_delete` for the affected keys —
tracked as a Known limitation.

## Security (section 22)

- CORS restricted to `CORS_ORIGINS` (comma-separated env var), not `*`.
- Every response carries `X-Content-Type-Options`, `X-Frame-Options`,
  `Referrer-Policy` (see `app/main.py` middleware).
- Request bodies over 2 MiB are rejected before parsing.
- Redis-backed fixed-window rate limiting (`app/core/security.py`), fails
  open if Redis is unreachable (same reasoning as caching).
- Admin/provider endpoints (`/api/v1/admin/*`) require `X-API-Key` matching
  `AURORA_ADMIN_API_KEYS`; public read endpoints require no auth but are
  rate-limited.
- No provider credentials are hard-coded; `TMDB_API_KEY` is read from the
  environment and the TMDb adapter raises a clear
  `ProviderNotConfiguredError` (not a fake success) if it's missing.
- Secrets never committed — `.env.example` holds placeholders only.

Not yet implemented (tracked): encryption-at-rest for provider credentials
beyond "not committed to Git" (there's currently exactly one credential,
`TMDB_API_KEY`, sourced from the environment — a secrets manager integration
is the natural next step once there are more); audit logging for admin
actions beyond structured request logs.

## Observability (section 23)

Structured logging via `structlog`, JSON in non-development environments.
Every request gets a correlation ID (`X-Request-ID`, generated if the client
doesn't send one) bound into the log context for that request's lifetime
(`app/core/logging.py`, `app/main.py:request_context_middleware`).
`GET /metrics` exposes Prometheus counters/histograms (HTTP request count,
latency by route) — compatible with Prometheus/Grafana; nothing in local
development requires the observability stack to be running.

## Health checks (section 24)

`GET /health` — liveness only (process is up). `GET /ready` — checks
PostgreSQL connectivity (always required) and Redis (required only if
`REDIS_REQUIRED=true`, since Redis is disposable — see `docs/database.md`).
Both are mounted twice: unprefixed (`/health`, `/ready`, for
Docker/Traefik health checks) and under `/api/v1` (per section 17's endpoint
list).

## Statelessness & horizontal scaling (section 1/2)

The API holds no in-process state across requests — no server-side
sessions, no in-memory caches keyed by request. Every dependency
(`get_db`, `get_redis`, `get_queue`) is request-scoped and backed by a
shared pool. This is what makes `docker-compose.prod.yml`'s `api: replicas:
3` behind Traefik correct without sticky sessions. Workers
(`docker-compose.prod.yml`'s `worker: replicas: 2`) scale independently of
the API — arq's Redis-backed queue ensures each job is claimed by exactly
one worker.