NK node liveness monitoring across nodes. Down → alert all admins. All paid plans.
Heartbeat
Each node periodically POSTs /api/node/heartbeat to the central with node_id / lic_id / version / uptime / load / memory / disk / business metrics. Central writes el_node_heartbeat + updates el_node.last_heartbeat_at.
Down detection
Cron scans every minute: last_heartbeat_at > 120s → warn / > 600s → down → trigger alert.
SMTP notification
All admins + 10s interval (anti SMTP rate-limit) + try_lock prevents cron overlap + skip .local emails.
Multi-channel
el_alert_channel: email / telegram / webhook / sms. Priority + retry config; primary down falls back to secondary.