How alerting works

The Alerts feature displays a notification in the Fastly control panel and sends messages to your custom integrations (e.g., Slack channels and webhooks) when a metric:

Alerting when a metric exceeds a static threshold

The Alerts feature supports alerting above a threshold. When creating an alert definition, you can define an above threshold based condition by setting the Trigger an alert when the metric is menu to Above. This alert will fire when the observed metric goes above the specified threshold during an evaluation period at any one minute polling interval. The alert will resolve when there are zero one minute intervals in the evaluation period that exceed the threshold.

Above a static threshold alert example

Let's say you want to be notified in your #website-alerts Slack channel when the number of 5XX responses exceed 1,000 on a single service or all your services. When configuring the alert definition, you choose whether to report on a single service or an aggregate of all services. Then, you set:

  • the Data source menu to Historic Stats
  • the Metric menu to status_5xx
  • the Trigger an alert when the metric is: menu to Above
  • the Threshold field to 1000
  • the Evaluation period menu to 5 minutes
  • the Integration(s) menu to website-alerts

After saving the alert definition, the system starts evaluating the observed metric in 5 minute rolling time windows (the Evaluation period). During a polling minute in one of those evaluation periods, the observed metric goes above 1,000 (the Threshold):

Evaluation period minuteNumber of 5XX responsesThreshold exceeded
1988No
2765No
31021Yes
4843No
51000No

At the end of the fifth minute of that evaluation period, an alert is triggered because the observed metric went above the threshold during the third minute of the evaluation period. Notifications are also sent. Specifically, the Fastly application indicates that an alert has been triggered via the Alert bell icon and a message is sent to the #website-alerts Slack channel.

That alert then resolves at the end of this evaluation period because there are zero one minute intervals in the evaluation period that exceed the threshold:

Evaluation period minuteNumber of 5XX responsesThreshold exceeded
1576No
2674No
3516No
4577No
5732No

Alerting when a metric is below a static threshold

The Alerts feature supports alerting below a threshold. When creating an alert definition, you can define a below threshold based condition by setting the Trigger an alert when the metric is menu to Below. This alert will fire when the observed metric remains below the specified threshold for two consecutive minutes in an evaluation period. The alert will resolve when there aren't two consecutive minutes in an evaluation period that are below the threshold. We look for two consecutive minutes in an evaluation period because network latency occasionally delays metric aggregation across POPs, and we want to ensure accuracy and limit false positives.

Below a static threshold alert example

Let's say you want to be notified in your #website-alerts Slack channel when the number of 2XX responses is below 500 on a single service or all your services. When configuring the alert definition, you choose whether to report on a single service or an aggregate of all services. Then, you set:

  • the Data source menu to Historic Stats
  • the Metric menu to status_2xx
  • the Trigger an alert when the metric is: menu to Below
  • the Threshold field to 500
  • the Evaluation period menu to 5 minutes
  • the Integration(s) menu to website-alerts

After saving the alert definition, the system starts evaluating the observed metric in 5 minute rolling time windows (the Evaluation period). During two consecutive polling minutes in one of those evaluation periods, the observed metric is below 500 (the Threshold):

Evaluation period minuteNumber of 2XX responsesBelow the threshold
11098No
21254No
3372Yes
4415Yes
5500No

At the end of the fifth minute of that evaluation period, an alert is triggered because the observed metric was below the threshold during the third and forth minutes of the evaluation period. Notifications are also sent. Specifically, the Fastly application indicates that an alert has been triggered via the Alert bell icon and a message is sent to the #website-alerts Slack channel.

That alert then resolves at the end of this evaluation period because there aren't two consecutive minutes in the evaluation period that are below the threshold:

Evaluation period minuteNumber of 2XX responsesBelow the threshold
11108No
2285Yes
31284No
41065No
5396Yes

Alerting when a metric exceeds a rate-based threshold

The Alerts feature supports alerting above a rate of all requests. When creating an alert definition, you can define an above rate-based condition by selecting an error rate or success rate metric to observe and setting the Trigger an alert when the metric is menu to Above. This alert will fire when the observed metric goes above the specified percentage of overall requests during an evaluation period at any one minute polling interval. The alert will resolve when there are zero one minute intervals in the evaluation period that exceed the threshold.

Above a rate-based threshold alert example

Let's say you want to be notified in your #website-alerts Slack channel when the number of 4XX responses exceeds 5% of all responses sent by a single service or all your services. When configuring the alert definition, you choose whether to report on a single service or an aggregate of all services. Then, you set:

  • the Data source menu to Historic Stats
  • the Metric menu to 4XX Error Rate
  • the Trigger an alert when the metric is: menu to Above
  • the Threshold menu to 5%
  • the Evaluation period menu to 5 minutes
  • the Integration(s) menu to website-alerts

After saving the alert definition, the system starts evaluating the observed metric in 5 minute rolling time windows (the Evaluation period). During a polling minute in one of those evaluation periods, the observed metric goes above 5% (the Threshold):

Evaluation period minuteNumber of 4XX responsesTotal number of requestsPercentage of overall requestsThreshold exceeded
109880%No
2137651.67%No
36410216.27%Yes
4148431.66%No
5310000.3%No

At the end of the fifth minute of that evaluation period, an alert is triggered because the observed metric went above the threshold during the third minute of the evaluation period. Notifications are also sent. Specifically, the Fastly application indicates that an alert has been triggered via the Alert bell icon and a message is sent to the #website-alerts Slack channel.

That alert then resolves at the end of this evaluation period because there are zero one minute intervals in the evaluation period that exceed the threshold:

Evaluation period minuteNumber of 4XX responsesTotal number of requestsPercentage of overall requestsThreshold exceeded
1195763.30%No
2306744.45%No
3145162.71%No
405770%No
507320%No

Alerting when a metric is below a rate-based threshold

The Alerts feature supports alerting below a rate of all requests. When creating an alert definition, you can define a below rate-based condition by selecting an error rate or success rate metric to observe and setting the Trigger an alert when the metric is menu to Below. This alert will fire when the observed metric goes below the specified percentage of overall requests during an evaluation period at any one minute polling interval. The alert will resolve when there are zero one minute intervals in the evaluation period that exceed the threshold.

Below a rate-based threshold alert example

Let's say you want to be notified in your #website-alerts Slack channel when the Success Rate, that is, the percentage of responses with non-5XX codes, falls below 80% on a single service or all your services. When configuring the alert definition, you choose whether to report on a single service or an aggregate of all services. Then, you set:

  • the Data source menu to Historic Stats
  • the Metric menu to Success Rate
  • the Trigger an alert when the metric is: menu to Below
  • the Threshold menu to 80%
  • the Evaluation period menu to 5 minutes
  • the Integration(s) menu to website-alerts

After saving the alert definition, the system starts evaluating the observed metric in 5 minute rolling time windows (the Evaluation period). During a polling minute in one of those evaluation periods, the observed metric goes below 80% (the Threshold):

Evaluation period minuteSuccess RateBelow the threshold
199%No
279%Yes
375%Yes
495%No
590%No

At the end of the fifth minute of that evaluation period, an alert is triggered because the observed metric was below the threshold during two consecutive minutes in the evaluation period. Notifications are also sent. Specifically, the Fastly application indicates that an alert has been triggered via the Alert bell icon and a message is sent to the #website-alerts Slack channel.

That alert then resolves at the end of this evaluation period because there aren't two consecutive minutes in the evaluation period that are below the threshold:

Evaluation period minuteSuccess RateBelow the threshold
190%No
295%No
399%No
499%No
589%No
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