# Implementation Plan: Link Preview (Open Graph Meta-Tags)

**Branch**: `012-link-preview` | **Date**: 2026-03-09 | **Spec**: `specs/012-link-preview/spec.md`
**Input**: Feature specification from `/specs/012-link-preview/spec.md`

## Summary

Inject Open Graph and Twitter Card meta-tags into the server-rendered HTML so that shared event links display rich preview cards in messengers and on social media. The Spring Boot backend replaces its current static SPA fallback with a controller that dynamically injects event-specific or generic meta-tags into the cached `index.html` template before serving it.

## Technical Context

**Language/Version**: Java 25 (backend), TypeScript 5.9 (frontend — minimal changes)
**Primary Dependencies**: Spring Boot 3.5.x (existing), Vue 3 (existing) — no new dependencies
**Storage**: PostgreSQL via JPA (existing event data, read-only access)
**Testing**: JUnit 5 + Spring MockMvc (backend), Playwright (E2E)
**Target Platform**: Self-hosted Docker container (Linux)
**Project Type**: Web application (SPA + REST API)
**Performance Goals**: N/A — meta-tag injection adds negligible overhead (<1ms string replacement)
**Constraints**: Meta-tags MUST be in initial HTML response (no client-side JS injection). No external services or CDNs.
**Scale/Scope**: Affects all HTML page responses. No new database tables or API endpoints.

## Constitution Check

*GATE: Must pass before Phase 0 research. Re-check after Phase 1 design.*

| Principle | Status | Notes |
|---|---|---|
| I. Privacy by Design | ✅ PASS | No tracking, analytics, or external services. Meta-tags contain only public event info (title, date, location). No PII exposed. `og:image` is a self-hosted static asset. |
| II. Test-Driven Methodology | ✅ PLAN | Unit tests for meta-tag generation, integration tests for controller, E2E tests for full HTML verification. TDD enforced. |
| III. API-First Development | ✅ N/A | No new API endpoints. This feature modifies HTML serving, not the REST API. Existing OpenAPI spec unchanged. |
| IV. Simplicity & Quality | ✅ PASS | Simple string replacement in cached HTML template. No SSR framework, no prerendering service, no user-agent sniffing. Minimal moving parts. |
| V. Dependency Discipline | ✅ PASS | Zero new dependencies. Uses only Spring Boot's existing capabilities. |
| VI. Accessibility | ✅ N/A | Meta-tags are invisible to users. No UI changes. |

**Gate result: PASS** — No violations. No complexity tracking needed.

## Project Structure

### Documentation (this feature)

```text
specs/012-link-preview/
├── plan.md              # This file
├── research.md          # Phase 0 output — technical decisions
├── data-model.md        # Phase 1 output — meta-tag value objects
├── quickstart.md        # Phase 1 output — implementation guide
├── contracts/
│   └── html-meta-tags.md  # Phase 1 output — meta-tag HTML contract
└── tasks.md             # Phase 2 output (created by /speckit.tasks)
```

### Source Code (repository root)

```text
backend/
├── src/main/java/de/fete/
│   ├── adapter/in/web/
│   │   └── SpaController.java            # NEW — serves index.html with injected meta-tags
│   ├── application/
│   │   └── OpenGraphService.java          # NEW — composes meta-tag values from event data
│   └── config/
│       └── WebConfig.java                 # MODIFIED — remove PathResourceResolver SPA fallback
├── src/main/resources/
│   └── application.properties             # MODIFIED — add forward-headers-strategy
└── src/test/java/de/fete/
    ├── adapter/in/web/
    │   └── SpaControllerTest.java         # NEW — integration tests
    └── application/
        └── OpenGraphServiceTest.java      # NEW — unit tests

frontend/
├── index.html                             # MODIFIED — add <!-- OG_META_TAGS --> placeholder
├── public/
│   └── og-image.png                       # NEW — brand image for og:image (1200x630)
└── e2e/
    └── link-preview.spec.ts               # NEW — E2E tests
```

**Structure Decision**: Web application structure (existing). Backend changes in adapter/web and application layers. Frontend changes minimal (HTML placeholder + static asset).

## Complexity Tracking

> No violations — section intentionally empty.
