SLO vs SLA: A Best Practices Guide

Service-level objectives (SLOs) and service-level agreements (SLAs) are closely related measurements designed to achieve service and business success, but they serve distinctly different audiences and purposes. In general terms, executive leadership focuses on contractual commitments within SLAs, while engineers concentrate on SLOs that reflect system performance and reliability. This dual perspective requires both parties to understand the critical differences between these metrics. 

In this article, we will explore best practices and practical examples that showcase the key distinctions between SLAs and SLOs, also examining how modern observability platforms can streamline SLO management and enable closely tied SLAs.

Summary of key SLO and SLA best practices

Perform discovery to understand user expectations

SLOs and SLAs have a common unifying goal of measuring, improving, and protecting the customer experience. This makes it critically important that teams understand how the services they provide and their components influence the overall user experience.

The discovery phase maps out service components, dependencies, user interactions, and system behavior. Take our example online store and integrated chatbot service, which could look like the map in the diagram below.

Example: mapping architecture components and dependencies for user journey discovery

Teams should conduct a detailed service discovery and analysis in this phase to ensure that they can proceed to develop meaningful and accurate SLOs. These will eventually be used to formulate SLAs that satisfy customer expectations. Those SLAs must be set within the limits of the team’s proven engineering and support capabilities. 

This analysis should capture key user journeys which, in our example, it could look like this:

Example: identifying technical dependencies in specific user journeys

Analyzing further, teams can determine the technical dependencies involved in these user journeys, for example:

Example: identifying and mapping specific user journeys

Operate, monitor, and right-size SLOs

Having designed and implemented their SLOs, teams enter the operating phase, configuring accompanying monitoring, alerting, and error budget policies. This is an iterative process where the SLOs are regularly fine-tuned to ensure that they first lead to the definition of realistic SLAs and then continue to defend those SLAs once in place.

SLOs are the key guardrail and alerting mechanism to prevent SLA breach. A key part of the alerting mechanism is to ensure that you are enforcing error budget policies. The policies define exactly what action should be taken when your acceptable level of risk, or error rate, is exceeded. Error budget alerting plays a key role in maintaining SLO and, hence, SLA compliance.

Nobl9's centralized alerting center is a good example of error budget management and alerting. It offers the ability to simplify the configuration of custom alerts, notification channels, and intelligent alerting logic through a guided wizard or in YAML code, as shown below.

Guided alert creation:

Example: Nobl9’s guided alert creation interface

Alert policies in code:

apiVersion: n9/v1alpha
kind: AlertPolicy
metadata:
 name: fast-burn
 project: default
spec:
 alertMethods: []
 conditions:
 - alertingWindow: 5m
   measurement: averageBurnRate
   value: 20
   op: gte
 coolDown: 5m
 description: "Policy that triggers when the average burn rate based based on the last 5 minutes is greater than or equal to 20x"
 severity: High

Example: Nobl9 Alert Policy defined in code

Tuning SLOs in day-to-day operations can help teams strike the right balance between dedicating time to operations and new features. Alerting policies can be refined to ensure sufficient reaction time for corrective action, e.g., altering the balance between development and operations.

Define specific SLAs based on specific SLO data analysis

Once SLOs are embedded and operational, they can be used to define SLAs. SLOs can incorporate current and historical reliability data, giving a detailed quantitative and proven picture of actual reliability. Features such as Nobl9’s Replay also allow historical monitoring data to be used to extend the look-back period for new SLOs. This can be used to account for past SLA breach events.

Operational SLOs provide organizations with rich data and evidence to formulate an accurate SLA for their service. The SLA should be less challenging and demanding than the related SLO. 

To illustrate, the table below shows how elements of an operational SLO can translate into specific SLAs:

Monitoring related to the SLO provides an early warning of a threat to the SLA, so the SLO should leave sufficient “headroom” by comparison. This is also why teams have different objectives as part of the same SLO, e.g., realistic and aspirational dimensions.

Ultimately, detailed SLO data gives technical teams demonstrable evidence to enable business teams to set competitive and achievable SLAs.

Conduct regular reviews

SLOs and SLAs should be reviewed regularly to ensure that they are current and reflect the latest information. There are key differences in the frequency of these review cycles, however:

For SLOs, this continual refinement process should also contain a feedback loop incorporating analysis from any recent incidents that have caused a breach of an SLA. This can establish a root-cause analysis and further improve the SLO. Likewise, an SLO's review and revision process can be integrated into an organisation’s retrospective and post-mortem processes to strengthen the continuous improvement benefits.

Similarly, the latest SLO data should inform accurate and complementary SLA revisions before SLA renewal. To ensure that all SLO data is incorporated into the review process, dashboards can be consulted, and reports can be produced for the previous SLA period.

This regular review and continuous improvement approach is illustrated in this Periodic Review Checklist Example from SLODLC, which gives teams a template for structuring and documenting their reviews.