# Alert Rules Example
+ **`PD_miss_peer_region_count`**
+ Alert rule:
  + **`(sum(pd_regions_status{type="miss_peer_region_count"}) by (instance) > 100) and (sum(etcd_server_is_leader) by (instance) > 0)`**
+ Description: 
  + The number of `Region` replicas is smaller than the value of **`max-replicas`**
+ Solution:
  + Find the cause of the issue by checking whether there is any `TiKV` machine that is down or being made offline
  + Watch the `Region` health panel and see whether **`miss_peer_region_count`** is continuously decreasing

| Severity Level  | Description                                                                                                                                                                        |
| :-------------- | :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Emergency-level | The highest severity level at which the service is unavailable. Emergency-level alerts are often caused by a service or node failure. Manual intervention is required immediately. |
| Critical-level  | Decreased service availability. For the critical-level alerts, a close watch on the abnormal metrics is required.                                                                  |
| Warning-level   | Warning-level alerts are a reminder for an issue or error.                                                                                                                         |


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According to the severity level, alert rules are divided into three categories (from high to low): `emergency-level`, `critical-level`, and `warning-level`. This division of severity levels applies to all alert items of each component below. 

Detail Alert description: `https://docs.pingcap.com/tidb/dev/alert-rules`