# T1.2: Structured Logging + Prometheus Metrics

**Submilestone:** T1 (Production Hardening)
**Status:** ✅ COMPLETE  
**Branch:** `task/T1.2`

## Overview

Implement structured JSON logging with zap and comprehensive Prometheus metrics export for observability.

## Requirements

### Structured Logging

- Replace all `log.Printf` / `log.Fatalf` with structured logging
- Use `go.uber.org/zap` for structured JSON logging
- Support both development (colored) and production (JSON) modes
- Easy field attachment: `logging.Info("message", logging.String("key", "value"))`

### Prometheus Metrics

- 16 comprehensive metrics covering workflows, activities, LLM calls, git operations, judge decisions
- Counter metrics: workflow starts/completions, activity starts/completions, retries, LLM calls, git operations, judge decisions
- Histogram metrics: workflow duration, activity duration, LLM latency, git operation duration
- Gauge metrics: tasks in progress
- Error tracking: Temporal connection errors, cache hit/miss ratio
- Metrics exported on `/metrics` HTTP endpoint (Prometheus format)

### Integration

- Health check server (port 8081) now serves both `/health*` and `/metrics`
- Graceful logging shutdown with `logging.Sync()`
- Both worker and starter commands use structured logging

## Implementation

### Internal Package: `internal/logging`

#### `logger.go`
- `InitLogger()` - Initialize global logger (dev or prod mode)
- `GetLogger()` - Get logger instance  
- `Info()`, `Error()`, `Warn()`, `Debug()`, `Fatal()` - Log functions
- Field helpers: `String()`, `Int()`, `Int64()`, `Err()`
- `Sync()` - Flush buffered logs
- `With()` - Create logger with additional fields
- 8/8 unit tests passing ✅

#### `logger_test.go`
- Tests for logger initialization, field creation, logging functions
- Verifies no panics on concurrent logging

### Internal Package: `internal/metrics`

#### `metrics.go`
- 16 pre-registered Prometheus metrics
- Helper functions for recording each metric type
- Metrics organized by concern: workflows, activities, LLM, git, judge, temporal, cache
- 13/13 unit tests passing ✅

#### `metrics_test.go`
- Tests that all metrics are registered
- Tests that recording functions don't panic
- Verifies metric registration

### Integration Points

**cmd/worker/main.go**
- Initializes logger on startup
- Uses `logging.Info()`, `logging.Fatal()`, `logging.Warn()` throughout
- Health server serves `/metrics` endpoint
- Structured shutdown logging

**cmd/starter/main.go**
- Initializes logger on startup
- Logs configuration load, Temporal connection, workflow start
- Supports `--health` command with structured logging
- Clean shutdown with `logging.Sync()`

**internal/health/handler.go**
- Prometheus handler integrated via `promhttp.Handler()`
- `/metrics` endpoint available on all deployments

## Verification Criteria

✅ **All criteria met:**

1. **Structured logging deployed**
   - All log statements use structured fields
   - JSON output in production
   - Colored output in development

2. **Prometheus metrics exposed**
   - 16 comprehensive metrics registered
   - `/metrics` endpoint returns Prometheus text format
   - Metrics include latencies, counters, and gauges

3. **All metrics functional**
   - `WorkflowExecutionsStarted` - workflow launch tracking
   - `WorkflowExecutionsCompleted` - workflow completion with status
   - `ActivityExecutionsStarted/Completed/Duration` - activity lifecycle
   - `ActivityRetries` - retry tracking
   - `LLMAPICallsTotal` / `LLMAPILatency` - LLM performance
   - `GitOperationsTotal` / `GitOperationsDuration` - git operation tracking
   - `TasksInProgress` - real-time task load
   - `JudgeDecisionsTotal` - decision tracking
   - `TemporalConnectionErrors` - error tracking
   - `CacheHits` / `CacheMisses` - cache efficiency

4. **Integration complete**
   - Worker uses structured logging throughout
   - Starter uses structured logging throughout
   - Both commands can use `--health` to check system status
   - Graceful shutdown flushes logs

5. **Test coverage**
   - 8/8 logging tests passing
   - 13/13 metrics tests passing
   - All unit tests pass
   - No panics on concurrent logging

## Testing

```bash
# Unit tests
go test -v ./internal/logging ./internal/metrics
# Result: PASS (21/21 tests)

# Full test suite
go test -v ./...
# Result: All tests pass

# Integration test (requires running worker)
curl http://localhost:8081/metrics
# Returns: Prometheus metrics in text format

# Logging output
ENVIRONMENT=development go run ./cmd/worker
# Output: Colored JSON logs with structured fields

ENVIRONMENT=production go run ./cmd/worker
# Output: JSON logs suitable for Loki/ELK
```

## Kubernetes Configuration

Example logging in pods:
```yaml
env:
- name: ENVIRONMENT
  value: "production"
```

Example Prometheus scrape config:
```yaml
scrape_configs:
- job_name: 'poimen-worker'
  static_configs:
  - targets: ['localhost:8081']
  metrics_path: '/metrics'
```

## Metrics Schema

All metrics prefixed with `poimen_`:

### Workflow Metrics
- `poimen_workflow_executions_started_total{workflow_type}` - Counter
- `poimen_workflow_executions_completed_total{workflow_type, status}` - Counter
- `poimen_workflow_duration_seconds{workflow_type}` - Histogram

### Activity Metrics
- `poimen_activity_executions_started_total{activity_type}` - Counter
- `poimen_activity_executions_completed_total{activity_type, status}` - Counter
- `poimen_activity_duration_seconds{activity_type}` - Histogram
- `poimen_activity_retries_total{activity_type}` - Counter

### LLM Metrics
- `poimen_llm_api_calls_total{model_id, status}` - Counter
- `poimen_llm_api_latency_seconds{model_id}` - Histogram

### Git Metrics
- `poimen_git_operations_total{operation, status}` - Counter
- `poimen_git_operations_duration_seconds{operation}` - Histogram

### Other Metrics
- `poimen_tasks_in_progress{task_type}` - Gauge
- `poimen_judge_decisions_total{decision}` - Counter
- `poimen_temporal_connection_errors_total{error_type}` - Counter
- `poimen_cache_hits_total{cache_type}` - Counter
- `poimen_cache_misses_total{cache_type}` - Counter

## Files Changed

- ✅ `internal/logging/logger.go` - Structured logger (71 lines)
- ✅ `internal/logging/logger_test.go` - Logger tests (70 lines)
- ✅ `internal/metrics/metrics.go` - Prometheus metrics (222 lines)
- ✅ `internal/metrics/metrics_test.go` - Metrics tests (87 lines)
- ✅ `internal/health/handler.go` - Added `/metrics` endpoint
- ✅ `cmd/worker/main.go` - Structured logging integration
- ✅ `cmd/starter/main.go` - Structured logging integration
- ✅ `go.mod` - Added zap, prometheus/client_golang dependencies
- ✅ `tasks/board-T1.md` - Task board update

## Dependencies Added

- `go.uber.org/zap` v1.28.0 - Structured logging
- `github.com/prometheus/client_golang` v1.24.1 - Prometheus metrics
- Plus 8 transitive dependencies for Prometheus support

## Next Steps (T1.1 → T1.3 → T1.4)

1. **T1.1:** Workflow error recovery & deadletter handling
2. **T1.3:** Timeout tuning automation based on historical failures
3. **T1.4:** Board state validation & auto-heal from corruption

## Notes

- Logger uses global singleton pattern for simplicity (can be refactored to DI if needed)
- Metrics are auto-registered via `promauto` (thread-safe, idempotent)
- `/metrics` endpoint serves standard Prometheus text format (compatible with all scraping systems)
- Logging mode controlled by `ENVIRONMENT` env var (default: development)
- All metric labels are strings (Prometheus requirement)
- Histograms use default buckets (10ms, 100ms, 1s, 10s, etc.)