# products domain — developer notes

DOC-4 (SD-0002 §11): the import/export spine as built in SLICE-5, extended by
SLICE-6–8. Operator-facing material is in [`OPERATIONS.md`](./OPERATIONS.md);
the spec is SD-0002 in the content repo.

## Layout and pipeline

`backend/app/domains/products/` is layered like the rest of the app
(main → domains → platform, enforced by import-linter):

- `models.py` — canonical row model + the column registry.
- `codec.py` — bytes → `ParsedFile`; file-level gates only.
- `validate.py` — rows → `CanonicalProduct` blocks + per-row errors.
- `diff.py` — catalog × canonical products → apply plan + preview records.
- `serialize.py` — the export half: `CatalogProduct` snapshot → canonical CSV
  (the inverse of `codec`/`validate`). DB-free, like `diff.py`.
- `repo.py` — SQL only: catalog snapshot, draft/run CRUD, apply primitives.
  Never commits or rolls back.
- `service.py` — use-case orchestration; owns every transaction boundary and
  emits the TEL events via `backend/app/platform/telemetry.py`.

```mermaid
flowchart LR
    subgraph validate_path [import_validate]
        A[parse_csv] --> B[build_products] --> D[compute_diff] --> E[(import_draft)]
        C[load_catalog] --> D
    end
    subgraph confirm_path [confirm_draft]
        E --> F[re-derive: parse → build → diff] --> G{fingerprint match?}
        G -- yes --> H[one-transaction apply<br/>run + products + delete draft]
        G -- no --> I[PreviewStale<br/>draft kept]
    end
```

Confirm re-derives everything from the draft's stored `file_bytes` against the
live catalog, then checks the fingerprint — so what lands is exactly what the
preview showed, or the confirm refuses (`preview_stale`). A confirm with no
adds and no updates refuses with `nothing_to_apply`.

## Canonical model and blank-vs-absent

`models.py` is the one model every dialect maps to (INV-17). `KNOWN_COLUMNS`
is the registry header detection, unknown-column warnings, and validation all
read; `CLEAR_DEFAULTS` holds the reset values for clearable fields
(`status`, `published`, `product_type`, `tags`).

The §6.5.1 cell semantics, as implemented:

- **Absent column** → the field never enters `fields{}` → untouched by diff
  and apply (never a change).
- **Present-but-empty cell** → `fields[name] = None` (an explicit clear) →
  resolved at diff time to its `CLEAR_DEFAULTS` entry, or NULL where none
  exists.
- **The position exception:** a cleared `Variant Position` has no
  `CLEAR_DEFAULTS` entry — `diff.resolved_variant_fields` resolves it to the
  variant's 1-based file order within its product, never to NULL.

Option names live both in `CanonicalProduct.option_names` (the values) and in
`fields{}` (the file-presence marker the diff needs for the absent-vs-clear
distinction).

## Error granularity

`validate.py` never raises on a row problem: every violation is recorded as a
merchant-language `RowError` and **poisons its whole product block** — the
product previews as `kind="error"` and is excluded from apply, while parsing
continues so one pass yields a complete accounting (BUC-1a). On apply, error
rows are recorded per line in `import_run_error`; `rows_errored` on the run is
the **error-row count**, while the preview's errors tile counts error
**products** — the two numbers legitimately differ.

File-level problems (`not_csv`, `missing_required_column`, `too_many_rows`,
`file_too_large`) raise `FileRejected` in `codec.py` instead: no draft is
created. The BFF adds an early 413 for oversized uploads (`main.py`).

## INV-11 mechanics

`compute_diff` makes **one walk** that produces two views of the same
computation: the typed apply `plan` (resolved natives — `Decimal`, `bool`,
lists) that `confirm_draft` executes, and the JSON-safe preview `records`
stored as draft JSONB and served verbatim to the SPA. Because both derive from
the same walk they cannot diverge. The `fingerprint` is
`sha256(json.dumps(records, sort_keys=True, separators=(",", ":")))`; a
mismatch at confirm means the catalog drifted since preview → `PreviewStale`
(409 `preview_stale`, draft kept for re-validation). The whole apply — run row,
product/variant/image writes, error rows, draft delete — is one transaction;
any exception rolls it back and emits TEL-6.

## Export & the round-trip (SLICE-6)

`serialize.py` is the export half of "one codec, two directions": it turns the
`CatalogProduct` snapshot (the same one `repo.load_catalog` builds for the diff
engine) back into canonical CSV, writing exactly the columns `codec.py` /
`validate.py` parse, in the §6.5.1 row grammar — `HEADER` is the full canonical
column set; product fields + option names sit on the first row; one variant per
row; images interleave; a product with more images than variants emits
image-only rows.

`repo.export_catalog` returns the status-filtered snapshot list (sorted by
handle, deterministic); the `service.export_catalog` generator streams
`serialize.catalog_to_csv` over it and emits TEL-3 (`catalog_exported`) once the
stream is exhausted. The BFF wraps it in a `StreamingResponse`; an empty
(filtered) catalog raises `EmptyCatalog` **eagerly** → `409 empty_catalog`
before any bytes stream.

**INV-12** (`diff(catalog, import(export(catalog))) = ∅`) is locked two ways: a
property test (`test_products_serialize.py`) runs the real
export→parse→diff loop over 200 generated **text-field** catalogs and asserts
every product is `unchanged`; the `e2e_roundtrip_noop` browser scenario does the
same through the UI (export download → re-upload → all-unchanged preview, import
disabled). Numeric/`Decimal` fields — the property test's deliberate blind spot
(string-form vs value-identity) — get explicit round-trip unit tests.

## Image pipeline (SLICE-7)

### `platform/objectstore`

A two-adapter port (`backend/app/platform/objectstore/`):

- `local.py` — writes blobs under a configurable local directory; used in
  tests and local dev (`ECOMM_OBJECTSTORE_KIND=local`).
- `gcs.py` — wraps `google-cloud-storage`; bucket name comes from
  `ECOMM_OBJECTSTORE_BUCKET` (`ECOMM_OBJECTSTORE_KIND=gcs`). Auth is the
  VM's service-account ADC — no credential bytes in the overlay or secrets.

Object keys for product images follow the prefix `product-images/` (the
`provision-bucket` gesture also sets a lifecycle rule on `import-drafts/` for
forward-compat, even though the draft blob remains BYTEA this slice — see
seams below).

### `platform/images`

`backend/app/platform/images.py` decodes an uploaded or fetched image byte
string and produces up to four renditions stored in the objectstore:

- **`original`** — stored verbatim after the resolution bar passes.
- **`thumb`** — 150 px on the shorter side, WebP.
- **`card`** — 400 px on the shorter side, WebP.
- **`detail`** — 800 px on the shorter side, WebP.

**Resolution bar (`MIN_IMAGE_SHORT_SIDE = 500`):** images whose shorter side
is below 500 px are rejected (Q-3). This is the only hard gate; oversized
images are scaled down without rejection. All renditions share the same
objectstore key prefix (`product-images/<image_id>/`).

### `domains/products/imagefetch.py` — the fetch phase

After `confirm_draft` commits, the run enters `fetching_images` (if it has
any pending image URLs) and a `ThreadPoolExecutor(max_workers=4)` processes
them concurrently:

- **SSRF guard (INV-18, extended):** only `http`/`https` schemes are
  permitted; the resolved IP must be public (RFC-1918, loopback, link-local,
  and multicast ranges are blocked); redirects are followed only within the
  same guard — a redirect to a private IP is rejected mid-chain.
- **Bounds:** ≤ 20 MB response body, ≤ 30 s per fetch.
- **Per-image presence/idempotency check (`claim_image_for_fetch`):** before
  processing, each `CatalogImage` row is checked so an already-handled row is
  skipped. This is a presence guard adequate for the **single-worker,
  in-process** model — the deployed app runs one uvicorn worker, recovery runs
  at startup over prior-process runs, and a fresh confirm always targets a new
  run, so two phase invocations never process the same run's images
  concurrently. It is **not** a cross-process lock. A true multi-worker claim
  would need a distinct in-flight status transition (`pending → fetching` with
  `RETURNING`, or `SELECT … FOR UPDATE SKIP LOCKED`) — that is the future seam
  if the fetch phase ever moves multi-process or onto a queue (§6.9).
- **Startup recovery scan:** on process start the app scans for runs stuck in
  `fetching_images` and re-enqueues their pending images (emits TEL-5). This
  covers crash/restart scenarios without a separate scheduler. On a mid-fetch
  process restart (e.g. a deploy) the phase is interrupted and the run is left
  resumable: pending images stay `pending`, fetched ones stay `fetched`, and
  the startup scan completes the run. Any pool-closed error in an in-flight
  worker at shutdown is benign — the uncommitted image stays `pending` and is
  re-fetched on resume. Mid-flight interruption is tolerated by construction
  (§6.9).
- **Progress tracking:** the run row carries `image_progress`,
  `image_counts`, and `image_outcomes` (updated per image); these fields feed
  the run-detail UI panel.

On completion the phase sets the run status to `complete` and emits TEL-4.

### Image-serving route

`GET /api/products/images/{id}/{rendition}` — storefront-authorized,
streams the objectstore blob. The response sets
`Cache-Control: public, immutable, max-age=31536000` (the object key encodes
the image id, so the URL is stable for the lifetime of the image). INV-16:
an image belongs to exactly one storefront; the route enforces storefront
scope before reading from the objectstore.

### Hosted-URL recognition and INV-12 over images

`domains/products/hosted.py` provides a predicate that recognises whether an
image URL is already served by this app (i.e., a `GET /api/products/images/…`
URL at the current `APP_URL`). The diff pre-pass `_resolve_hosted_images`
uses it to convert hosted URLs back to their `CatalogImage` FK before hashing
the diff fingerprint — this closes the INV-12 round-trip guarantee over
image columns (a re-imported export previews as all-unchanged even when
images are hosted here).

## Named seams (what later slices replace)

- **`import_draft.file_bytes` remains BYTEA (deferred past SLICE-7).** Moving
  the draft blob to object storage was scoped out of SLICE-7. The media
  bucket holds only `product-images/` objects this slice; the `provision-bucket`
  script sets the `import-drafts/` lifecycle rule for forward-compat so the
  migration will be clean when it lands.
- **`codec.detect_dialect` → Shopify (SLICE-8).** Today it always returns
  `"canonical"`; SLICE-8 recognizes Shopify's exact header set here (INV-17).
- **Run-status complete shortcut → shipped in SLICE-7.**
  `confirm_draft` now inserts the run as `applying`, then transitions to
  `fetching_images` (if the run has pending image URLs) or directly to
  `complete`. The fetch phase fills `image_progress` / `image_counts` /
  `image_outcomes` and sets `complete` when done.

## Test map

| File | Covers |
| --- | --- |
| `backend/tests/test_products_codec.py` | file-level gates: parse, caps (INV-18), required columns, dialect |
| `backend/tests/test_products_validate.py` | every §6.5.1 row-error rule, one fixture each |
| `backend/tests/test_products_diff.py` | classification, blank-vs-absent, option matching, fingerprint |
| `backend/tests/test_products_serialize.py` | serializer grammar + INV-12 property test (round-trip no-op over generated catalogs) + decimal round-trip |
| `backend/tests/test_products_export.py` | status-filtered snapshot, streamed export, EmptyCatalog, TEL-3 |
| `backend/tests/test_products_service.py` | draft lifecycle: validate/preview/discard, expiry, TEL-1 |
| `backend/tests/test_products_invariants.py` | INV-10 (never deletes), INV-14 (storefront isolation), apply transactionality, TEL-6 |
| `backend/tests/test_products_endpoints.py` | §6.4 API scenarios + auth/storefront gates |
| `e2e/tests/import-preview-confirm.spec.ts` | happy path: upload → preview → confirm → history |
| `e2e/tests/import-errors.spec.ts` | actionable row errors at preview and on the run report |
| `e2e/tests/import-file-rejected.spec.ts` | file-level rejection, picker stays live, no trace |
| `e2e/tests/import-cancel.spec.ts` | cancel at preview leaves no trace |
| `e2e/tests/export-download.spec.ts` | export downloads canonical CSV, status filter respected |
| `e2e/tests/roundtrip-noop.spec.ts` | export → re-import → all-unchanged, import disabled (PUC-10) |