Database metrics alerts for CPU memory disk and connections

Setup Database Metrics Alerts (CPU, Memory, Disk, Connections)

An alert is a notification before users notice the problem. "Disk 95% full" is a disaster. "Disk 75% full, growing 2 GB/day" is 10 days to solve it. Correct thresholds and correct delivery channels are half the work.

Monitoring Stack

Standard stack for databases:

PostgreSQL/MySQL/MongoDB
        │
   Exporter (postgres_exporter / mysqld_exporter / mongodb_exporter)
        │  metrics in Prometheus format
   Prometheus (collection and storage of metrics)
        │
   Alertmanager (alert routing)
        │
   Telegram / Slack / PagerDuty
        │
   Grafana (visualization)

Installing Exporters

# PostgreSQL
docker run -d \
  --name postgres_exporter \
  -e DATA_SOURCE_NAME="postgresql://monitoring:password@localhost:5432/postgres?sslmode=disable" \
  -p 9187:9187 \
  quay.io/prometheuscommunity/postgres-exporter:latest

# MySQL
docker run -d \
  --name mysqld_exporter \
  -e DATA_SOURCE_NAME="monitoring:password@(localhost:3306)/" \
  -p 9104:9104 \
  prom/mysqld-exporter:latest

# Node Exporter for system metrics (CPU, RAM, disk)
docker run -d \
  --name node_exporter \
  --pid="host" \
  -v /:/host:ro,rslave \
  -p 9100:9100 \
  quay.io/prometheus/node-exporter:latest \
  --path.rootfs=/host

Monitoring user for PostgreSQL (minimum privileges):

CREATE USER monitoring WITH PASSWORD 'monitoring_password';
GRANT pg_monitor TO monitoring;  -- system role PG 10+
-- or manually:
GRANT SELECT ON pg_stat_database TO monitoring;
GRANT SELECT ON pg_stat_replication TO monitoring;
GRANT EXECUTE ON FUNCTION pg_current_wal_lsn() TO monitoring;

Prometheus Alert Rules

# /etc/prometheus/rules/database.yml
groups:

  - name: postgresql_critical
    rules:

      # Database unavailable
      - alert: PostgreSQLDown
        expr: pg_up == 0
        for: 30s
        labels:
          severity: critical
        annotations:
          summary: "PostgreSQL unavailable on {{ $labels.instance }}"
          description: "Database not responding for over 30 seconds"

      # Disk > 85% full
      - alert: DiskSpaceHigh
        expr: |
          (node_filesystem_size_bytes{mountpoint="/var/lib/postgresql"} -
           node_filesystem_free_bytes{mountpoint="/var/lib/postgresql"}) /
           node_filesystem_size_bytes{mountpoint="/var/lib/postgresql"} * 100 > 85
        for: 5m
        labels:
          severity: warning
        annotations:
          summary: "PostgreSQL disk {{ $value | printf \"%.1f\" }}% full"

      # Critical disk fill
      - alert: DiskSpaceCritical
        expr: |
          (node_filesystem_size_bytes{mountpoint="/var/lib/postgresql"} -
           node_filesystem_free_bytes{mountpoint="/var/lib/postgresql"}) /
           node_filesystem_size_bytes{mountpoint="/var/lib/postgresql"} * 100 > 95
        for: 1m
        labels:
          severity: critical
        annotations:
          summary: "CRITICAL: PostgreSQL disk {{ $value | printf \"%.1f\" }}% full"

      # Too many connections
      - alert: PostgreSQLTooManyConnections
        expr: |
          pg_stat_activity_count / pg_settings_max_connections * 100 > 80
        for: 2m
        labels:
          severity: warning
        annotations:
          summary: "{{ $value | printf \"%.0f\" }}% of PostgreSQL connections used"
          description: "Consider PgBouncer or increase max_connections"

      # Long running transaction (> 10 minutes)
      - alert: PostgreSQLLongRunningTransaction
        expr: |
          pg_stat_activity_max_tx_duration{state="active"} > 600
        for: 1m
        labels:
          severity: warning
        annotations:
          summary: "Transaction running for {{ $value | printf \"%.0f\" }} seconds"

      # Replication lag > 60 seconds
      - alert: PostgreSQLReplicationLag
        expr: pg_replication_lag > 60
        for: 2m
        labels:
          severity: critical
        annotations:
          summary: "Replica is {{ $value | printf \"%.0f\" }} seconds behind"

  - name: postgresql_warning
    rules:

      # Cache hit rate < 99%
      - alert: PostgreSQLLowCacheHitRate
        expr: |
          (sum(pg_stat_database_blks_hit) /
          (sum(pg_stat_database_blks_hit) + sum(pg_stat_database_blks_read))) * 100 < 99
        for: 10m
        labels:
          severity: warning
        annotations:
          summary: "Cache hit rate: {{ $value | printf \"%.1f\" }}% (normal > 99%)"

      # CPU > 80%
      - alert: HighCPUUsage
        expr: |
          100 - (avg by(instance)(rate(node_cpu_seconds_total{mode="idle"}[5m])) * 100) > 80
        for: 5m
        labels:
          severity: warning
        annotations:
          summary: "CPU {{ $labels.instance }}: {{ $value | printf \"%.0f\" }}%"

      # Free RAM < 10%
      - alert: LowFreeMemory
        expr: |
          node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes * 100 < 10
        for: 5m
        labels:
          severity: warning
        annotations:
          summary: "Free memory: {{ $value | printf \"%.1f\" }}%"

Alertmanager: Telegram Routing

# /etc/alertmanager/alertmanager.yml
global:
  resolve_timeout: 5m

route:
  group_by: ['alertname', 'instance']
  group_wait:      30s
  group_interval:  5m
  repeat_interval: 4h
  receiver: telegram-critical

  routes:
    - match:
        severity: critical
      receiver: telegram-critical
      repeat_interval: 30m  # repeat critical every 30 minutes

    - match:
        severity: warning
      receiver: telegram-warning
      repeat_interval: 4h

receivers:
  - name: telegram-critical
    telegram_configs:
      - api_url: "https://api.telegram.org"
        bot_token: "BOT_TOKEN"
        chat_id: -1001234567890  # group/channel ID