Server Watchdog#

Monitor and auto-heal remote servers via SSH. Check services, databases, disk, memory — restart what's down, alert what's wrong.

Prerequisites#

• SSH access to target server (password or key-based)
• expect available locally (for password-based SSH)
• Target server runs PM2, systemd, or Docker for service management

Quick Reference#

Check PM2 services#

bash

ssh user@host "pm2 list"
ssh user@host "pm2 logs --lines 20 --nostream"

Check MongoDB#

bash

# Windows
ssh user@host "net start | findstr MongoDB"
ssh user@host "powershell -Command \"(Test-NetConnection -ComputerName 127.0.0.1 -Port 27017).TcpTestSucceeded\""

# Linux
ssh user@host "systemctl status mongod"
ssh user@host "mongosh --eval 'db.runCommand({ping:1})' --quiet"

Check disk & memory#

bash

# Linux
ssh user@host "df -h && free -h"

# Windows
ssh user@host "powershell -Command \"Get-PSDrive -PSProvider FileSystem | Select Root,Used,Free; \$os=Get-CimInstance Win32_OperatingSystem; Write-Output ('RAM: '+[math]::Round((\$os.TotalVisibleMemorySize-\$os.FreePhysicalMemory)/1MB,1)+'GB / '+[math]::Round(\$os.TotalVisibleMemorySize/1MB,1)+'GB')\""

Workflow#

1. Diagnose — SSH in, check service status, logs, disk, memory
2. Identify — Parse logs for errors, crashes, OOM, or unclean shutdowns
3. Fix — Restart crashed services ( pm2 restart, net start, systemctl restart)
4. Verify — Confirm service is back up and responding
5. Alert — Notify user via messaging with summary

Crash Analysis#

When a service is down, check these in order:

1. Service logs — pm2 logs, journalctl -u service, Windows Event Log
2. Application logs — Check log files at configured paths
3. System events — OOM killer, unexpected shutdowns, disk full
4. Database logs — MongoDB: check mongod.log for Fatal ( "s":"F") entries

MongoDB crash patterns#

text

"s":"F" — Fatal error (crash)
"Unhandled exception" — Internal bug (often FTDC related)
"Detected unclean shutdown" — Process killed without graceful shutdown
"WiredTiger error" — Storage engine corruption

Auto-Heal Recipes#

PM2 service restart#

bash

pm2 restart <service-name>
pm2 save  # persist across reboots

MongoDB (Windows)#

bash

net stop MongoDB
timeout /t 5
net start MongoDB

MongoDB (Linux)#

bash

sudo systemctl restart mongod

Deploy watchdog service#

For persistent monitoring, deploy the included watchdog script:

1. Copy scripts/mongodb-watchdog.js to target server
2. Install: npm init -y && npm install mongodb
3. Start: pm2 start mongodb-watchdog.js --name mongodb-watchdog
4. Save: pm2 save

SSH with password (via expect)#

When key-based auth isn't available:

bash

expect -c 'set timeout 20
spawn ssh -o StrictHostKeyChecking=no user@host "COMMAND"
expect {
    "password:" { send "PASSWORD\r"; exp_continue }
    eof
}
'

Alert Template#

text

🚨 Server Alert — [hostname]

⏰ Time: [timestamp]
❌ Issue: [service] is DOWN
📋 Cause: [crash reason from logs]
🔄 Action: Auto-restarted [service]
✅ Status: [service] is back online

📊 System Health:
• Memory: X GB / Y GB
• Disk: Z% used
• Services: N/N online