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observability.md

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Observability

Structured logging and diagnostics for mt.

Crate Stack

Crate Role
tracing Structured event macros (info!, warn!, etc.)
tracing-subscriber Subscriber registry, formatting, env-filter
tracing-appender Non-blocking daily-rotated file writer

Subscriber Architecture

logging::init_tracing() sets a global subscriber with two layers:

  1. Stdout -- compact format with ANSI colors, useful during task tauri:dev
  2. File -- daily-rotated files via tracing-appender::rolling::daily

Both layers share a single EnvFilter.

File Locations

Database

Platform Path
macOS ~/Library/Application Support/com.mt.desktop/mt.db
Linux $XDG_DATA_HOME/com.mt.desktop/mt.db

Log Files

Platform Path
macOS ~/Library/Logs/com.mt.desktop/
Linux $XDG_DATA_HOME/com.mt.desktop/logs/

Files are named mt.log.YYYY-MM-DD.

Log Levels and MT_LOG Override

Default levels:

  • Release builds: info
  • Debug builds: debug

Override with the MT_LOG environment variable (uses tracing-subscriber EnvFilter syntax):

# Show all debug output
MT_LOG=debug task tauri:dev

# Trace-level for the watcher module only
MT_LOG=mt_tauri::watcher=trace task tauri:dev

# Silence everything except errors
MT_LOG=error task tauri:dev

Rotation and Retention

  • Rotation: daily (new file each calendar day)
  • Retention: 3 days. Files older than 3 days are deleted on startup in cleanup_old_logs().

Export Diagnostics

Settings > Export Logs produces a text file containing:

  1. App version, build ID, platform
  2. Rust version, working directory
  3. Log directory path
  4. Up to 3 most recent log files (newest first), capped at 5 MB total

IPC Instrumentation

All 94 #[tauri::command] functions have #[tracing::instrument] annotations. High-frequency polling commands (audio_get_status, audio_get_volume, queue_get_playback_state) use level = "trace" to stay out of normal logs.

logging::log_slow_command() emits a warn when any command exceeds 500 ms.

Frontend Error Capture

error-reporter.js installs two global handlers at startup:

  • window.addEventListener('error', ...) -- uncaught exceptions
  • window.addEventListener('unhandledrejection', ...) -- unhandled promise rejections

Both route to the log_frontend_error Tauri command, which logs at target: "mt::frontend". Use reportError(level, message, context) for explicit error reporting from frontend code.

Reading Structured Output

Each log line contains:

2025-01-15T10:23:45.123Z  INFO mt_tauri::watcher: Folder scan complete folder_id=1 added=12 updated=3 deleted=0
  • Timestamp -- UTC
  • Level -- TRACE, DEBUG, INFO, WARN, ERROR
  • Target -- Rust module path (e.g. mt_tauri::watcher)
  • Message -- human-readable summary
  • Fields -- structured key=value pairs from tracing macros