Service-Level Objectives (SLO)

The Service-Level Objective (SLO) configuration specifies a target value or range of values for a service level that is measured by Service-Level Indicators (SLI).

Service-Level Objective

  • An SLO is defined per service.
  • An SLO consists of a filter that uniquely identifies a deployment of a service.
  • An SLO defines objectives for the service that depend on the selected comparison strategy.
  • An SLO returns a score represented by a value between 0 and 100.

Example of Service-Level Objective (SLO):

spec_version: '1.0'
filter:
  mz_id: "4711"
  svc_id: "a14b-cd87-0d51"
comparison:
  compare_with: "single_result"
  include_result_with_score: "pass"
  aggregate_function: avg
objectives:
  - sli: response_time_p95
    pass:
      - criteria:
          - "<=+10%"
          - "<1000"
    warning:
      - criteria:
          - "<=800"
total_score:
  pass: "90%"
  warning: "75%"

Filter

This property allows a list of key-value pairs that are used to uniquely identify a deployment of a service. This means that the key of a filter can be used as a placeholder in an SLI query. For example, the filter svc_id: "a14b-cd87-0d51" specifies a unique identifier of the deployment of a service. Consequently, the key of the filter (i.e., svc_id) can be referenced in an SLI query by $svc_id.

The filters project, stage, service, and deployment can be inferred from the Keptn configuration by using $PROJECT, $STAGE, $SERVICE, and $DEPLOYMENT in SLI queries respectively. These values can also be overwritten in the configuration. The default filters are:

  • project
  • stage
  • service
  • deployment

Example of an SLO with a list of filters:

spec_version: '1.0'
filter:
  mz_id: "4711"
  svc_id: "a14b-cd87-0d51"
comparison:
...

Example of an SLI with reference to the mz_id filter from the SLO:

spec_version: "1.0"
indicators:
  throughput: "builtin:service.requestCount.total:merge(0):count?scope=tag(keptn_service:$SERVICE),mzId($mz_id)"

Comparison

By default, Keptn compares with the previous values of the SLIs. To support more advanced comparion strategies, the following properties are available:

  • The compare_with config parameter controls how many previous results are compared: single_result or several_results.

  • The number_of_comparison_results config parameter configures the actual number of previous results if compare_with is set to several_results.

  • The include_result_with_score config parameter controls which of the previous results are included in the comparison: pass, pass_or_warn, or all (all is the default, also used if not specified).

Note: If you configure compare_with: "single_result" in combination with number_of_comparison_results, compare_with will negate the number_of_comparison_results.

1. Example:

comparison:
  compare_with: "single_result"
  include_result_with_score: "pass"
  aggregate_function: avg

This comparison configuration means that the current result is only compared to the last result that passed.

2. Example:

comparison:
  compare_with: "several_results"
  number_of_comparison_results: 3
  include_result_with_score: "pass_or_warn"
  aggregate_function: "avg"

This comparison configuration means that the current result is compared to the average of the three previous results that had pass or warning as a result.

Objectives

An objective consists of:

  • The SLI refers to an SLI from an SLI configuration.
  • The pass criteria represents the upper limit up to which an evaluation is successful.
  • The (optional) warning criteria describes the border where the result is not pass and not fail, and a manual approval might be needed to decide.
  • The (optional) weight criteria emphasizes the importance of one SLI over the others. By default, weight has a value of 1 for all SLIs and can be overwritten. The weight is important for calculating the total score.
  • The (optional) key_sli flag can be set to true meaning that the objective is not met if this SLI fails.

Configuring the criteria:

The pass and warning criteria allow a list of boolean expressions with a logical operator [<, <=, >, >=] and a absolute or relative value. While the absolute value is a numberical number, the realtive value requires a (+/-) at the beginning and a % sign at the end, e.g.: -10%.

  • All boolean expressions in the list are combined with a logical AND. According to the next example, the pass criteria is met when its measured absolute value is below 1000 and the increase of the relative value is lower/equals 10 percent.
pass:
  - criteria:
    - "<1000"
    - "<=+10%"
  • The criteria of a pass and warning can also be splitted. This means that the two lists are combined with a logical OR. According to the next example, the pass criteria is met when either the measured absolute value is below 1000 or the increase of the relative value is lower/equals 10 percent.
pass:
  - criteria:
    - "<1000"
  - criteria:
    - "<=+10%"

Example of an Objective:

objectives:
  - sli: response_time_p95
    pass:
      - criteria:
          - "<=+10%"
          - "<1000"
    warning:
      - criteria:
          - "<=800"
    weight: 2
    key_sli: true

Scoring

An evaluation for pass counts for one point, an evaluation for warning half a point, and an evaluation for fail zero points. The default weight of an SLI is 1 and can be overwritten. The maximum score is the sum of the weights of all SLIs.

The actual evaluation result is divided by the maximum score and gives the total_score in percent. For example, the maximum score is 92 and the evaluation result is 85 - the total_score is 92.39% (8592*100).

The pass and warning criteria for the total_score use the logical operator “>=” by default.

Add SLO configuration to a Service

Important: In the following command, the value of the resourceUri must be set to slo.yaml.

  keptn add-resource --project=sockshop --stage=staging --service=carts --resource=slo-quality-gates.yaml --resourceUri=slo.yaml