Provides slack alerts due to failures in critical services.

## Rationale¶

This service monitors data from auditing, if the error rate goes above a threshold an error will be emitted. If the alerting service at any points fail to contact Elasticsearch an error will be emitted.

At the moment this service is sending alerts to the #devalerts/#alerts channels of slack. The messages are sent to slack via their webhooks feature. This service is not specified only to work with slack, but can be hooked up to any chat/mail service that supports webhooks.

Elasticsearch cluster health

It is important that the Elasticsearch cluster always is at good health. Elasticsearch can be in three different states:

• Green: All good

• Yellow: Missing replicas of one or more indices

• Red: One or more primary shards have not been allocated

If the state changes to yellow and have not changed back to green within 5 minutes the service will send an alert. If the state however changes to red, it will alert after 1 minute. We can live without a replica but if we are missing a primary shard, then we have unavailable data.

Elasticsearch storage

Elasticsearch consists of multiple data nodes. Theses nodes are given an specific amount of storage to their disposal. If this storage is reaching pre-configured limits it will trigger and send a message. UCloud uses the following configuration:

• Send a information message clarifying which node has used 85% of its storage.

• Send a warning message clarifying which node has used 88% of its storage.

• Send a alert message clarifying which node has used 90% of its storage.

This gives time to either scale up or clean out in the elastic indices before the watermark limits are reached and further actions are required and limitation are enforced by elastic it self.

Number of Shards

Since Elasticsearch 7.0, a hard limit on number of shards per nodes have been introduced. If this limit is reached then no more shards, and therefore indices, cannot be allocated to the node.

This alert gives a heads up when running close to the cluster limit. This makes it possible to take actions before allocations are put on hold. If no configuration is given to the alert, then a default of 500 available shards are set as the threshold before an alert triggers.

To handle this situation either increase the number of shards allowed per node or reduce the number of shards/indices in the cluster. To help with the latter the elastic-management service can be used.

Since the number of shards do not change much during a day, this alert is only checking every 12 hours. Should an alert have fired, it checks every 30 min to be able to notify once the number of available shards are above the limit again.

Number of Documents

If a shard is containing to many documents (entries in Elasticsearch) then it might become to heavy to perform optimal. We have an alert that triggers if a shard exceeds 1.800.000.000 documents and again at 2.000.000.000. Should this trigger a reindex would be needed to split the original index into multiple indices or delete some data that might have been missed by the timed scan of logs.

5XX status codes

If request returns a 5XX code, then it could be a sign of something is wrong in either our code or our infrastructure.

This alert checks the audit trail for request responses that has a 5XX status code. If the number of request that returns a 5XX status code exceeds a pre-set percentage of all requests within the last 15 minutes, the alert triggers.

The check runs each 5 minutes and will also notify when the number of 5XX enters the acceptable range again.

Crash loop and pod failure detection (Kubernetes)

If any pods in the kubernetes cluster fails due to some error or if a pod keeps restarting and ends in a CrashLoppBackOff (multiple failing restarts within short time) this alert will trigger sending a message specifying which pod have failed.

Node monitoring (Kubernetes)

If a node changes status to a know error state, this alert will send a message declaring the node, state and problem. The follow state are monitored: