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How does it work

Sloth uses Prometheus rules to generate SLOs. Based on the generated recording and alert rules it creates a reliable and uniform SLO implementation:

direction LR

input: 1 Sloth spec
sloth: Sloth
output: N prometheus rules

input --> sloth
sloth --> output

state sloth {
    direction LR
    load: Load SLO Spec
    metadata: Gen Metadata rules
    slis: Gen SLI rules
    alerts: Gen Alert rules
    save: Out format

    load --> metadata
    load --> slis
    load --> alerts
    metadata --> save
    slis --> save
    alerts --> save

The Prometheus rules that Sloth generates, fall in 3 categories:

  • SLIs: These rules are the base, they use the queries provided by the user to get a value used to show what is the error service level (availability). It creates multiple rules for different time windows, these different results will be used for the alerts.
  • Metadata: These are used as informative metrics, like the remaining error budget, the SLO objective percent... These are very handy for SLO visualization, e.g Grafana dashboard.
  • Alerts: These are the multiwindow-multiburn alerts that are based on the SLI rules.

Sloth will take the service level spec and for each SLO in the spec will create 3 rule groups with the above categories.


The Sloth generated rules will share the same metric name across SLOs. However the labels are the key to identify the different services, SLO... This is how we obtain a uniform way of describing all the SLOs across different teams and services.

To get all the available metric names created by Sloth, use this query:

count({sloth_id!=""}) by (__name__)


Sloth SLO alerts use multiwindow-multiburn method. It will generate 2 types of alerts:

  • Critical/page: Pay attention right now.
  • Warning/ticket: Take into account, however is not urgent.

Using 4 different alerts across different time window periods:

  • Critical/Page in short time window: Very very high rate of burning budget.
  • Critical/Page in medium time window: High rate rate of burning budget.
  • Warning/Ticket in medium time window: Constant normal rate of burning budget.
  • Warning/Ticket in long time window: Constant low rate of burning budget.

Sloth doesn't trigger/generate the alerts itself, Prometheus will by using Sloth generated alerting rules. Normally something connected with Prometheus (like alertmanager) will trigger these alerts notifications (e.g Slack, Pagerduty...).

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