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SLI plugins

SLI plugins are small Go plugins (using Yaegi) that can be loaded on Sloth start.

These plugins can be referenced as an SLI on the SLO specs and will return a raw SLI type.

Common plugins

Sloth maintains a library with common SLI plugins that can be used on your SLOs or used as examples to develop your own ones.

  • Check plugins source code here.
  • Check the documentation here.

Developing plugins


Developing a prometheus/v1 SLI plugin is very easy, however you need to meet some requirements:

  • The plugin version used as a global called SLIPluginVersion.
  • A plugin ID global called SLIPluginID.
  • A Plugin logic function called SLIPlugin.
  • The plugin must be in a single file named plugin.go.
  • Plugins only can use the Go standard library (reflect and unsafe packages can't be used).
  • Plugin received options are a map[string]string to avoid interface{} problems on dynamic execution code, the conversion to specific types are responsibility of the plugin.
  • Plugins don't depend on go modules, GOPATH or similar (thanks to the previous requirements).

Sloth knows how to autodiscover plugins giving a path (--sli-plugins-path), and will load all the discovered ones.

A very simple example:

from plugins/x/y/plugin.go

package testplugin

import "context"

const (
  SLIPluginVersion = "prometheus/v1"
  SLIPluginID      = "test_plugin"

func SLIPlugin(ctx context.Context, meta, labels, options map[string]string) (string, error) {
  return "rate(my_raw_error_ratio_query{}[{{.window}}])", nil

Used in SLO spec:

version: "prometheus/v1"
service: "myservice"
  - name: "some-slo"
    objective: 99.9
        id: "test_plugin"
          opt1: "something"
          opt2: "something"

On spec load, Sloth will execute the referenced plugins with the options and use the result as a Raw SLI type, the one that returns the error ratio query.

Why should I use plugins?

By default you shouldn't unless you have scenarios where they can simplify, add security or improve the SLO adoption on the team/company. Some examples:

  • SLI custom validation (parameters, queries...).
  • Company wide precreated SLIs for common used libraries.
  • Complex Prometheus query SLIs.
  • Precreated SLIs for the team or company that normally everyones uses on the SLOs.
  • OSS SLI plugins that come with the apps, frameworks or libraries (e.g Kubernetes apiserver, http framework X...).
  • The company or the team could have a repository with all the shared plugins and everyone could use them and contribute with new ones.
  • Automation power when templates are not enough.
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