# Feature Specification: CDN Image Serving

**Feature Branch**: `014-r2-cdn-serving`
**Created**: 2026-05-08
**Status**: Draft
**Input**: User description: "R2 CDN image serving with local dev fallback to API proxy"

## Overview

Images and thumbnails are currently served by proxying bytes through the API. This feature changes image delivery so that clients receive direct URLs pointing to a CDN edge network, eliminating the API as a middleman for image content. In local development, where no CDN is available, the API proxy endpoints remain as a fallback so the developer experience is unchanged.

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## User Scenarios & Testing *(mandatory)*

### User Story 1 - Images Load Directly from CDN (Priority: P1)

When a visitor views the image library or opens an image detail page, images and thumbnails are fetched directly from the CDN rather than through the application server. The page loads faster because image bytes no longer pass through the API.

**Why this priority**: Core value of the feature. Reduces API load and improves image load speed for all users.

**Independent Test**: Upload an image, open the library page, and inspect the network requests. Image and thumbnail requests should go directly to the CDN domain, not to `/api/`. The API response for the image list should include direct CDN URLs for each image and thumbnail.

**Acceptance Scenarios**:

1. **Given** a published image, **When** the visitor loads the image library, **Then** each thumbnail `src` URL points to the CDN domain and loads without passing through the API
2. **Given** a published image, **When** the visitor opens the detail page, **Then** the full image `src` URL points to the CDN domain
3. **Given** the API returns image metadata, **When** the response is inspected, **Then** it includes a `file_url` and `thumbnail_url` field containing full CDN URLs

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### User Story 2 - Local Development Works Without CDN (Priority: P2)

In local development, where no CDN is configured, images continue to load via the existing API proxy endpoints. No additional setup is required to run the application locally.

**Why this priority**: Developer experience must not regress. The proxy endpoints must remain functional and be used automatically when no CDN is configured.

**Independent Test**: Run the application locally without setting a public base URL. Upload an image. Verify the library and detail pages load images correctly via the API proxy endpoints, with no errors or broken images.

**Acceptance Scenarios**:

1. **Given** no CDN base URL is configured, **When** the API returns image metadata, **Then** `file_url` and `thumbnail_url` point to the API proxy paths (e.g. `/api/v1/images/{id}/file`)
2. **Given** no CDN base URL is configured, **When** a visitor views the library, **Then** thumbnails load via the API proxy with no broken images
3. **Given** a CDN base URL is configured, **When** the application starts, **Then** all image URLs use the CDN domain instead of the proxy paths

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### Edge Cases

- What happens when the CDN base URL is set but the object does not exist in CDN storage? The browser receives a 404 from the CDN — the API does not re-proxy the content.
- What happens if an image has no thumbnail (thumbnail generation failed)? The `thumbnail_url` field is absent or null; the UI falls back to the full image URL as it does today.
- What happens if the CDN base URL has a trailing slash? The system normalises the URL to avoid double slashes in constructed paths.

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## Requirements *(mandatory)*

### Functional Requirements

- **FR-001**: The API MUST include a `file_url` field in all image metadata responses, containing the full URL from which the image file can be fetched
- **FR-002**: The API MUST include a `thumbnail_url` field in all image metadata responses when a thumbnail exists, containing the full URL from which the thumbnail can be fetched
- **FR-003**: When a CDN base URL is configured, `file_url` and `thumbnail_url` MUST point to the CDN domain
- **FR-004**: When no CDN base URL is configured, `file_url` and `thumbnail_url` MUST point to the existing API proxy endpoints so local development continues to work without additional setup
- **FR-005**: The existing API proxy endpoints (`/images/{id}/file`, `/images/{id}/thumbnail`) MUST remain functional regardless of whether a CDN base URL is configured
- **FR-006**: The UI MUST use `file_url` and `thumbnail_url` from the API response to render images, rather than constructing proxy URLs client-side
- **FR-007**: The CDN base URL MUST be configurable via environment variable; no value is required in local development
- **FR-008**: A trailing slash in the configured CDN base URL MUST NOT result in double slashes in constructed image URLs
- **FR-009**: When `thumbnail_url` is null, the UI MUST fall back to `file_url` for thumbnail display rather than rendering a broken image

### Key Entities

- **Image metadata response**: Extended to include `file_url` and `thumbnail_url` fields alongside existing fields (`id`, `filename`, `tags`, `width`, `height`, `mime_type`, etc.)

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## Success Criteria *(mandatory)*

### Measurable Outcomes

- **SC-001**: In production, zero image or thumbnail requests pass through the API server — all are served directly by the CDN
- **SC-002**: Local development requires no additional configuration beyond what is already required — `docker compose up` continues to work with images loading correctly
- **SC-003**: All existing image-related API integration tests continue to pass after the change
- **SC-004**: Image metadata responses include `file_url` and `thumbnail_url` fields for 100% of images that have been successfully stored

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## Assumptions

- The CDN storage bucket and public domain are already configured and operational before this feature is deployed — this feature only changes how URLs are constructed and served, not how objects are stored
- Object keys in CDN storage are identical to those used in the existing storage backend — no key remapping is needed
- The CDN serves objects publicly without authentication — no signed URL generation is required
- The existing API proxy endpoints are retained as functional fallbacks; the UI stops calling them in production but they are not removed
- Local development uses the existing MinIO-backed proxy and does not require a locally running CDN
