docs: update GRASP-02 proactive sync event sync approach

1 files changed, 91 insertions, 5 deletions

diff --git a/docs/explanation/monitoring.md b/docs/explanation/monitoring.md
index 3b1b1ac..9368bf4 100644
--- a/docs/explanation/monitoring.md
+++ b/docs/explanation/monitoring.md
@@ -90,10 +90,96 @@ For detailed per-repository investigation at scale, consider adding **Loki** (lo
 - Loki queries enable ad-hoc deep dives (e.g., find all transfers > 10MB)
 - Pairs with Prometheus for long-term trends
 
-## Future: Sync Metrics (GRASP-02)
+## Sync Metrics (GRASP-02)
 
-When GRASP-02 proactive sync is implemented, additional metrics will track:
+When GRASP-02 proactive sync is implemented, the following metrics will be added to track relay synchronization health. These metrics use in-memory tracking with Prometheus for operator visibility (no database persistence needed for <100 relays).
 
-- Events received from sync (live vs catchup)
-- Active outbound relay connections
-- Catchup gap (events found during catchup indicating sync failures)
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+### Sync Metrics Overview
+
+| Metric | Type | Labels | Description |
+|--------|------|--------|-------------|
+| `ngit_sync_relay_connected` | Gauge | relay | 1 if connected, 0 if not |
+| `ngit_sync_connection_attempts_total` | Counter | relay, result | Connection attempt outcomes |
+| `ngit_sync_relay_status` | Gauge | relay, status | 1 for current status, 0 otherwise |
+| `ngit_sync_relay_failures` | Gauge | relay | Current consecutive failure count |
+| `ngit_sync_events_total` | Counter | source | Events received by source type |
+| `ngit_sync_gap_events_total` | Counter | relay | Events found during catchup |
+| `ngit_sync_relays_tracked_total` | Gauge | - | Total relays discovered |
+| `ngit_sync_relays_connected_total` | Gauge | - | Currently connected relay count |
+| `ngit_sync_relays_dead_total` | Gauge | - | Relays marked as dead |
+
+### Event Sources
+
+The `source` label on `ngit_sync_events_total` tracks how events were received:
+
+- `direct` - Submitted directly to our relay by a user
+- `live_sync` - Received via live WebSocket subscription (expected path)
+- `catchup` - Found during negentropy catchup after reconnect
+- `daily_catchup` - Found during daily reconciliation
+
+**Catchup events indicate sync failures** - these should have been received via live sync. High catchup rates suggest connectivity issues or filter mismatches.
+
+### Relay Health States
+
+The `status` label on `ngit_sync_relay_status` tracks relay health:
+
+- `healthy` - Normal operation, connections working
+- `backoff` - Exponential backoff after failures (5s → 10s → ... → 1h)
+- `dead` - 24h of continuous failures, daily retry only
+
+### Example Grafana Queries
+
+```promql
+# Relay health overview - count by status
+sum by (status) (ngit_sync_relay_status == 1)
+
+# Connection success rate over last hour
+sum(rate(ngit_sync_connection_attempts_total{result="success"}[1h]))
+/ sum(rate(ngit_sync_connection_attempts_total[1h]))
+
+# Sync gap detection - events that should have been live synced
+sum(rate(ngit_sync_gap_events_total[1h])) by (relay)
+
+# Live sync effectiveness (lower is better - fewer gaps)
+sum(rate(ngit_sync_events_total{source=~"catchup|daily_catchup"}[1h]))
+/ sum(rate(ngit_sync_events_total[1h]))
+
+# Relays with high failure counts (potential issues)
+topk(10, ngit_sync_relay_failures)
+```
+
+### Example Alerts
+
+```yaml
+# Alert if relay stuck in dead state for > 1 day
+- alert: SyncRelayDead
+  expr: ngit_sync_relay_status{status="dead"} == 1
+  for: 1d
+  labels:
+    severity: warning
+  annotations:
+    summary: "Sync relay {{ $labels.relay }} is dead"
+
+# Alert if sync gap rate is high (>10% of events from catchup)
+- alert: SyncGapHigh
+  expr: >
+    sum(rate(ngit_sync_events_total{source=~"catchup|daily_catchup"}[1h]))
+    / sum(rate(ngit_sync_events_total[1h])) > 0.1
+  for: 30m
+  labels:
+    severity: warning
+  annotations:
+    summary: "High sync gap rate - {{ $value | humanizePercentage }} of events from catchup"
+```
+
+### Design Rationale
+
+**In-memory health tracking with Prometheus visibility** was chosen over database persistence because:
+
+1. **Scale**: <100 relays means per-relay labels have acceptable cardinality
+2. **Simplicity**: No database schema, migrations, or cleanup needed
+3. **Operator visibility**: Prometheus + Grafana provide better dashboards than custom queries
+4. **Restart behavior**: Conservative initial backoff (5s + jitter) avoids thundering herd on restart
+5. **Historical data**: Prometheus retains health history; in-memory state only needs current status
+
+See [GRASP-02 Proactive Sync](grasp-02-proactive-sync.md) for full architecture details.
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