# API Stabilization Report

## Executive Summary
Phase 4 focused on stabilizing the core API handlers by replacing brittle error handling logic with robust sentinel errors, ensuring consistency across services, and verifying cross-layer interactions with micro-E2E tests.

## Key Accomplishments

### 1. Handler Audits & Repairs
- **PlaylistHandler**: Replaced string literal checks (`"playlist not found"`) with sentinel errors (`services.ErrPlaylistNotFound`).
- **BitrateHandler**: Standardized error responses to use `services.ErrInvalidTrackID`, `ErrInvalidBitrate`, etc.
- **CommentHandler**: Implemented specific error codes (404, 403) for `ErrCommentNotFound`, `ErrParentCommentNotFound`, `ErrForbidden`.
- **RoomHandler**: Fixed "Blind 404" issue where internal errors were masked. Now distinguishes `ErrRoomNotFound` from other errors.

### 2. Service Layer Refactoring
- **Centralized Errors**: Created `internal/services/errors.go` to consolidate common errors and prevent duplication.
- **Updated Services**: `PlaylistService`, `BitrateAdaptationService`, `CommentService`, `RoomService` now return consistent, exported sentinel errors wrapping low-level DB errors.

### 3. Verification & Testing
- **Unit/Integration Tests**: Updated all affected service and handler tests to assert new error types.
- **Micro-E2E Test Suite**: Created `internal/handlers/api_flow_test.go` (`TestAPIFlow_UserJourney`) simulating a complete user session:
  1. Artist uploads Track.
  2. Listener streams (Bitrate Adaptation).
  3. Listener comments on Track.
  4. Artist replies.
  5. Listener attempts unauthorized delete (Fail).
  6. Listener creates Playlist and adds Track.

## Status Checklist
- [x] All defined handlers audit for HTTP semantics.
- [x] Brittle string matching replaced with `errors.Is`.
- [x] Cross-layer error consistency verified.
- [x] Regression testing via E2E flow.

## Recommendations for Phase 5 (Frontend Integration)
- The API is now stable and returns predictable error codes (400, 401, 403, 404).
- Frontend clients should handle `403` for permission issues specifically.
- `404` reliably indicates resource missing, not internal error.