SLO

Service Level Objectives

An SLO is a measurable reliability target for a service.

SLOs give teams a shared reliability target. They also show when reliability is degrading before users are heavily impacted.

SLO = SLI measured over a time window must meet a target percentage

Example:

99.9% of HTTP requests to the payment service must succeed over a 28-day rolling window.

Related pages:

• Create SLO

• SLO Dashboard

Key concepts

#. SLI

An SLI is the metric that measures service behaviour.

In this implementation, the SLI is a ratio:

SLI = (Good Events / Total Events) × 100%

Use these terms when defining an SLI:

• Good Events: requests or events that met the quality bar

• Total Events: all requests or events attempted

• SLI Ratio: the percentage of good events

Typical example:

• Good Events: HTTP 2xx responses

• Total Events: all HTTP requests

• SLI Ratio: 99.95%

The Ascent platform supports query mode:

• Ratio mode: select a success metric and a total metric separately

#. SLO

An SLO is the target for the SLI. It is measured over a rolling time window.

Default values:

• Target percentage: 99.9%

• Time window: 28d

Supported examples:

2h, 24h, 7d, 28d, 30d, 90d

An SLO is met when:

SLI ≥ Target Percentage over the entire time window

#. Error budget

The error budget is the allowed amount of unreliability.

It is calculated as:

Error Budget = 100% − Target Percentage

Examples:

• 99.9% target → 0.1% error budget

• 99.5% target → 0.5% error budget

• 99.0% target → 1.0% error budget

• 95.0% target → 5.0% error budget

For a 28d window:

• 99.9% target allows about 40 minutes of downtime

• 99.5% target allows about 3.4 hours of downtime

• 99.0% target allows about 6.7 hours of downtime

The error budget is shown live in the target step. It updates automatically when the target percentage changes.

#. Error budget remaining

Error budget remaining shows how much budget is still available in the current SLO window.

The progress bar uses these states:

• More than 50% remaining: healthy

• 10% to 50% remaining: warning

• Less than 10% remaining: critical

When the remaining budget reaches 0%, the SLO is violated.

#. Burn rate

Burn rate shows how fast the service is consuming its error budget.

Burn Rate = Current Error Rate / Error Budget Rate

Examples:

• 1×: using budget at the expected pace

• 2×: using budget twice as fast

• 14×: using budget fast enough to exhaust it in about 2 days for a 28d window

Burn rate matters because a service can still look acceptable at a glance while burning through its budget too quickly.