# Ledger Service - Prometheus Metrics

## Overview

The Ledger service exposes Prometheus metrics on the metrics endpoint (default: `:9401/metrics`). This provides operational visibility into ledger operations, performance, and errors.

## Metrics Endpoint

- **URL**: `http://localhost:9401/metrics`
- **Format**: Prometheus exposition format
- **Configuration**: Set via `config.yml` → `metrics.address`

## Available Metrics

### 1. Journal Entry Operations

#### `ledger_journal_entries_total`
**Type**: Counter
**Description**: Total number of journal entries posted to the ledger
**Labels**:
- `entry_type`: Type of journal entry (`credit`, `debit`, `transfer`, `fx`, `fee`, `adjust`, `reverse`)
- `status`: Operation status (`success`, `error`, `attempted`)

**Example**:
```promql
# Count of successful credit entries
ledger_journal_entries_total{entry_type="credit", status="success"}

# Rate of failed transfers
rate(ledger_journal_entries_total{entry_type="transfer", status="error"}[5m])
```

---

#### `ledger_journal_entry_duration_seconds`
**Type**: Histogram
**Description**: Duration of journal entry posting operations
**Labels**:
- `entry_type`: Type of journal entry

**Buckets**: `[.001, .005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10]` seconds

**Example**:
```promql
# 95th percentile latency for credit postings
histogram_quantile(0.95, rate(ledger_journal_entry_duration_seconds_bucket{entry_type="credit"}[5m]))

# Average duration for all entry types
rate(ledger_journal_entry_duration_seconds_sum[5m]) / rate(ledger_journal_entry_duration_seconds_count[5m])
```

---

#### `ledger_journal_entry_errors_total`
**Type**: Counter
**Description**: Total number of journal entry posting errors
**Labels**:
- `entry_type`: Type of journal entry
- `error_type`: Error classification (`validation`, `insufficient_funds`, `db_error`, `not_implemented`, etc.)

**Example**:
```promql
# Errors by type
sum by (error_type) (ledger_journal_entry_errors_total)

# Validation error rate for transfers
rate(ledger_journal_entry_errors_total{entry_type="transfer", error_type="validation"}[5m])
```

---

### 2. Balance Operations

#### `ledger_balance_queries_total`
**Type**: Counter
**Description**: Total number of balance queries
**Labels**:
- `status`: Query status (`success`, `error`)

**Example**:
```promql
# Balance query success rate
rate(ledger_balance_queries_total{status="success"}[5m]) / rate(ledger_balance_queries_total[5m])
```

---

#### `ledger_balance_query_duration_seconds`
**Type**: Histogram
**Description**: Duration of balance query operations
**Labels**:
- `status`: Query status

**Example**:
```promql
# 99th percentile balance query latency
histogram_quantile(0.99, rate(ledger_balance_query_duration_seconds_bucket[5m]))
```

---

### 3. Reversal Operations

#### `ledger_reversals_total`
**Type**: Counter
**Description**: Total number of journal entry reversals
**Labels**:
- `status`: Reversal status (`success`, `error`)

**Example**:
```promql
# Reversal error rate
rate(ledger_reversals_total{status="error"}[5m])
```

---

### 4. Transaction Amounts

#### `ledger_transaction_amount`
**Type**: Histogram
**Description**: Distribution of transaction amounts (normalized)
**Labels**:
- `currency`: Currency code (`USD`, `EUR`, `GBP`, etc.)
- `entry_type`: Type of journal entry

**Buckets**: `[1, 10, 50, 100, 500, 1000, 5000, 10000, 50000, 100000]`

**Example**:
```promql
# Average transaction amount in USD for credits
rate(ledger_transaction_amount_sum{currency="USD", entry_type="credit"}[5m]) /
rate(ledger_transaction_amount_count{currency="USD", entry_type="credit"}[5m])

# 90th percentile transaction amount
histogram_quantile(0.90, rate(ledger_transaction_amount_bucket[5m]))
```

---

### 5. Account Operations

#### `ledger_account_operations_total`
**Type**: Counter
**Description**: Total number of account-level operations
**Labels**:
- `operation`: Operation type (`create`, `freeze`, `unfreeze`)
- `status`: Operation status (`success`, `error`)

**Example**:
```promql
# Account creation rate
rate(ledger_account_operations_total{operation="create"}[5m])
```

---

### 6. Idempotency

#### `ledger_duplicate_requests_total`
**Type**: Counter
**Description**: Total number of duplicate requests detected via idempotency keys
**Labels**:
- `entry_type`: Type of journal entry

**Example**:
```promql
# Duplicate request rate (indicates retry behavior)
rate(ledger_duplicate_requests_total[5m])

# Percentage of duplicate requests
rate(ledger_duplicate_requests_total[5m]) / rate(ledger_journal_entries_total[5m]) * 100
```

---

### 7. gRPC Metrics (Built-in)

These are automatically provided by the gRPC framework:

#### `grpc_server_requests_total`
**Type**: Counter
**Labels**: `grpc_service`, `grpc_method`, `grpc_type`, `grpc_code`

#### `grpc_server_handling_seconds`
**Type**: Histogram
**Labels**: `grpc_service`, `grpc_method`, `grpc_type`, `grpc_code`

**Example**:
```promql
# gRPC error rate by method
rate(grpc_server_requests_total{grpc_code!="OK"}[5m])

# P95 latency for PostCredit RPC
histogram_quantile(0.95, rate(grpc_server_handling_seconds_bucket{grpc_method="PostCreditWithCharges"}[5m]))
```

---

## Common Queries

### Health & Availability

```promql
# Overall request rate
sum(rate(grpc_server_requests_total[5m]))

# Error rate (all operations)
sum(rate(ledger_journal_entry_errors_total[5m]))

# Success rate for journal entries
sum(rate(ledger_journal_entries_total{status="success"}[5m])) / sum(rate(ledger_journal_entries_total[5m]))
```

### Performance

```promql
# P99 latency for all journal entry types
histogram_quantile(0.99, sum(rate(ledger_journal_entry_duration_seconds_bucket[5m])) by (le, entry_type))

# Slowest operation types
topk(5, avg by (entry_type) (rate(ledger_journal_entry_duration_seconds_sum[5m]) / rate(ledger_journal_entry_duration_seconds_count[5m])))
```

### Business Insights

```promql
# Transaction volume by type
sum by (entry_type) (rate(ledger_journal_entries_total{status="success"}[1h]))

# Total money flow (sum of transaction amounts)
sum(rate(ledger_transaction_amount_sum[5m]))

# Most common error types
topk(10, sum by (error_type) (rate(ledger_journal_entry_errors_total[5m])))
```

---

## Grafana Dashboard

### Recommended Panels

1. **Request Rate** - `sum(rate(grpc_server_requests_total[5m]))`
2. **Error Rate** - `sum(rate(grpc_server_requests_total{grpc_code!="OK"}[5m]))`
3. **P95/P99 Latency** - Histogram quantiles
4. **Operations by Type** - Stacked graph of `ledger_journal_entries_total`
5. **Error Breakdown** - Pie chart of `ledger_journal_entry_errors_total` by `error_type`
6. **Transaction Volume** - Counter of successful entries
7. **Duplicate Requests** - `ledger_duplicate_requests_total` rate

---

## Alerting Rules

### Critical

```yaml
# High error rate
- alert: LedgerHighErrorRate
  expr: rate(ledger_journal_entry_errors_total[5m]) > 10
  for: 5m
  labels:
    severity: critical

# Service unavailable
- alert: LedgerServiceDown
  expr: up{job="ledger"} == 0
  for: 1m
  labels:
    severity: critical
```

### Warning

```yaml
# Slow operations
- alert: LedgerSlowOperations
  expr: histogram_quantile(0.95, rate(ledger_journal_entry_duration_seconds_bucket[5m])) > 1
  for: 10m
  labels:
    severity: warning

# High duplicate request rate (potential retry storm)
- alert: LedgerHighDuplicateRate
  expr: rate(ledger_duplicate_requests_total[5m]) / rate(ledger_journal_entries_total[5m]) > 0.2
  for: 5m
  labels:
    severity: warning
```

---

## Configuration

Metrics are configured in `config.yml`:

```yaml
metrics:
  address: ":9401"  # Metrics HTTP server address
```

## Dependencies

- Prometheus client library: `github.com/prometheus/client_golang`
- All metrics are registered globally and exposed via `/metrics` endpoint