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+# 2026-05-21-musicbrainz-provider
+
+## Overview
+Implement a MusicBrainz API provider with strict 1 request/second rate limiting, 24-hour caching of Release Group data, and filtering capabilities as per specification. The provider will fetch artist discographies from MusicBrainz, normalize the data, and store it in the external_releases table for use by the scanner engine.
+
+## Context (from discovery)
+- Files/components involved: internal/musicbrainz/ package (new), database schema updates, main.go integration
+- Related patterns found: Follows the internal/navidrome/ pattern with client.go, sync.go, and model separation
+- Dependencies identified: Will add golang.org/x/time/rate for rate limiting, use net/http for API calls
+
+## Development Approach
+- **Testing approach**: Regular (code first, then tests)
+- Complete each task fully before moving to the next
+- Make small, focused changes
+- **CRITICAL: every task MUST include new/updated tests** for code changes in that task
+  - tests are not optional - they are a required part of the checklist
+  - write unit tests for new functions/methods
+  - write unit tests for modified functions/methods
+  - add new test cases for new code paths
+  - update existing test cases if behavior changes
+  - tests cover both success and error scenarios
+- **CRITICAL: all tests must pass before starting next task** - no exceptions
+- **CRITICAL: update this plan file when scope changes during implementation**
+- Run tests after each change
+- Maintain backward compatibility
+
+## Testing Strategy
+- **Unit tests**: required for every task (see Development Approach above)
+- **E2E tests**: if project has UI-based e2e tests (Playwright, Cypress, etc.):
+  - UI changes → add/update e2e tests in same task as UI code
+  - Backend changes supporting UI → add/update e2e tests in same task
+  - Treat e2e tests with same rigor as unit tests (must pass before next task)
+  - Store e2e tests alongside unit tests (or in designated e2e directory)
+
+## Progress Tracking
+- Mark completed items with `[x]` immediately when done
+- Add newly discovered tasks with ➕ prefix
+- Document issues/blockers with ⚠️ prefix
+- Update plan if implementation deviates from original scope
+- Keep plan in sync with actual work done
+
+## What Goes Where
+- **Implementation Steps** (`[ ]` checkboxes): tasks achievable within this codebase - code changes, tests, documentation updates
+- **Post-Completion** (no checkboxes): items requiring external action - manual testing, changes in consuming projects, deployment configs, third-party verifications
+- **Checkbox placement**: Checkboxes belong only in Task sections (`### Task N:` or `### Iteration N:`). Do not put checkboxes in Success criteria, Overview, or Context — they cause extra loop iterations.
+
+## Implementation Steps
+
+### Task 1: Create MusicBrainz client and data models
+- [ ] create `internal/musicbrainz/client.go` with MusicBrainzClient struct wrapping net/http.Client
+- [ ] implement constructor taking config and rate limiter
+- [ ] create `internal/musicbrainz/model.go` with structs for MusicBrainz API responses (ReleaseGroup, Artist, etc.)
+- [ ] implement XML parsing functions for MusicBrainz responses
+- [ ] write tests for XML parsing (success + error cases)
+- [ ] write tests for client constructor and basic API call structure
+- [ ] run tests - must pass before next task
+
+### Task 2: Implement rate limiting and caching layer
+- [ ] add golang.org/x/time/rate dependency to go.mod
+- [ ] implement rate limiter using golang.org/x/time/rate.NewLimiter(1, 1) for 1 req/sec
+- [ ] create wrapper method for rate-limited HTTP GET requests
+- [ ] implement caching check: query database for existing Release Group data within TTL
+- [ ] write tests for rate limiting behavior (timing tests)
+- [ ] write tests for cache hit/miss logic
+- [ ] run tests - must pass before next task
+
+### Task 3: Implement MusicBrainz API endpoints and filtering
+- [ ] implement GetArtistReleaseGroups(artistMBID string) method
+- [ ] apply filters: exclude Bootleg/Promotion/Pseudo-Release status
+- [ ] apply type filters: include Album/Single/EP/Compilation only
+- [ ] implement per-artist type filtering hooks (placeholder for Web UI integration)
+- [ ] normalize artist names and titles (remove special characters, years, brackets)
+- [ ] write tests for filtering logic (table-driven test cases)
+- [ ] write tests for normalization functions
+- [ ] run tests - must pass before next task
+
+### Task 4: Implement database integration and sync orchestration
+- [ ] create `internal/musicbrainz/sync.go` with SyncArtistDiscography function
+- [ ] implement upsert logic: INSERT OR REPLACE into external_releases table
+- [ ] add cached_at column to external_releases table via migration
+- [ ] implement context.Context support for cancellation
+- [ ] write tests for database upsert operations
+- [ ] write integration tests with in-memory SQLite
+- [ ] run tests - must pass before next task
+
+### Task 5: Wire up provider in application entry point
+- [ ] update `cmd/naviwatcher/main.go` to initialize MusicBrainz client
+- [ ] add MusicBrainz client to application context/dependencies
+- [ ] ensure graceful shutdown includes closing HTTP client connections
+- [ ] update config validation to ensure MusicBrainz.UserAgent is set
+- [ ] write tests for main.go integration (startup/shutdown)
+- [ ] run tests - must pass before next task
+
+### Task 6: Verify acceptance criteria and run full test suite
+- [ ] verify all requirements from Overview are implemented
+- [ ] verify edge cases are handled (network errors, invalid responses, rate limit blocking)
+- [ ] run full test suite (unit tests)
+- [ ] run linter - all issues must be fixed
+- [ ] verify test coverage meets project standard (80%+)
+
+## Technical Details
+### Data Structures
+- MusicBrainzClient: wraps *http.Client with rate limiter and config
+- ExternalRelease: matches existing database struct with addition of CachedAt time.Time
+- MusicBrainz API Response Models: ReleaseGroup, Artist, etc. based on XML schema
+
+### Parameters and Formats
+- Rate Limiter: 1 request per second burst size of 1 (strict limit)
+- Cache TTL: configurable via MusicBrainzConfig.CacheTTL (default 24h)
+- API Endpoint: https://musicbrainz.org/ws/2/ with proper User-Agent header
+- Response Format: XML parsing of MusicBrainz Web Service responses
+
+### Processing Flow
+1. SyncArtistDiscography called with MusicBrainz Artist ID
+2. Check cache: query external_releases for RGIDs with cached_at within TTL
+3. If cache miss or expired: call MusicBrainz API with rate limiting
+4. Parse XML response into ReleaseGroup models
+5. Apply status/type filtering (Bootleg/Promotion/Pseudo-Release excluded)
+6. Apply per-artist type filtering ( Singles/Compilations toggle via Web UI)
+7. Normalize strings (remove special chars, years, brackets for fuzzy matching)
+8. Upsert each Release Group to external_releases with current timestamp
+9. Return list of Release Groups for scanner consumption
+
+## Post-Completion
+*Items requiring manual intervention or external systems - no checkboxes, informational only*
+
+**Manual verification** (if applicable):
+- Manual testing of rate limiting under load
+- Verify cache expiration behavior over time
+- Test with real MusicBrainz API to ensure compliance with their usage policy
+- Performance testing of XML parsing and filtering logic
+
+**External system updates** (if applicable):
+- None - this is a standalone provider implementation
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