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Service level indicators, service level objectives, and service level agreements create a hierarchy. Service level indicators define a number. Service level objectives set a target for the number. Service level agreements define rules and expectations for when the target is hit or missed.

These are powerful tools for aligning stakeholders and determining priorities. Each builds upon the other, so let's work bottom up starting with service level indicators.

SLIs, or service level indicators, quantify aspects of the provided service. Examples include latency, uptime, and error rate. Use this template for defining SLIs: blank as measured by blank. Here's an example: success rate as measured by the number of successful HTTP requests divided the total HTTP requests. Express SLIs as ratios of the total number of good divided by total number of events. This provides a sliding scale between zero and one hundred.

There are infinite choices for SLIs, so they must be chosen carefully to measure value of the provided service. Ask this question as a gut check when considering an SLI: "will the service loose users if this indicator is negative impacted?"

So what's the right number? This number is the SLO, or service level objective. Clearly 0 is the wrong number because that means everything is broken. On the other hand, 100% is not the right answer because that means nothing every breaks. That's impossible. The real answer is that it just depends on business.

The SLO sits at the intersection of product and engineering. Stakeholders must agree that if the SLO drops below a threshold then it's worth reprioritizing work to reach the SLO. The means that responsible party may commit to new work or eschew existing work to maintain it. On the other hand, this means that work in ways to maintain reliability so the SLO does not drop below the threshold in the first place.

Once the SLO is defined, then the stakeholders can form an agreement for how to maintain the SLO and what to do when it's missed. This is the service level agreement or SLA. They typically take a form of financial penalties or other negative consequences written into a contract. These agreements are made between the consumer and provider.

However I'm focused on the agreement between those responsible for maintaining the SLO. The site reliability engineering literature recommends using an error budget for this.

Here's an example. Say the SLO is is 99% per quarter. That provides an error budget of 1%. So, if a problem causes a failure rate of .5% then 50% of the error budget is used.

The error budget is an objective way to manage risk in relation to the SLO. The team may decide to undertake a risky database migration when the error budget is full. On the other hand the team may be more conservative when the budget is low by focusing on low risk releases. The error budget enables teams to balance risk, innovation, and reliability. This assumes all stakeholder accept the error budget as means of planning and maintaining the SLO. In other words, if the error budget is exhausted then no releases may be possible. Are the stakeholders OK with that?

That right there friends is the hard problem. Measuring something is the easy problem. Giving the SLO power to change priorities is the hard problem. That's why it's so important, and I cannot overstate this enough, that the SLO directly relates to business success. It needs teeth!

Many teams do not solve the hard problem. As a result the SLOs are kabuki theater or relegated to some KPI spreadsheet. That's largely been my experience. However I have had positive experiences with well designed SLOs and an agreement to commit to them.

Lastly, I want to circle back to software delivery and the three ways of DevOps.

SLIs & SLOs provide feedback to the software delivery process. Feedback is the second way of DevOps. Teams cannot asses their progress without feedback from production. SLOs provide feedback to all stakeholders, be it engineers, business executives or product managers. Feedback is the second half of the continuous delivery cycle. Fast flow--A.K.A. continuous delivery--only happens with fast feedback.

SLOs are not static either. They will change as the business changes. Early stage businesses may accept more risk than established businesses. Over time, systems may change to a point where maintaining the SLOs has negative business impact. Maintaining SLOs is a never ending balancing act that requires feedback from stakeholders and the flexibility to improve them over time. This is an act of continuous improvement--A.K.A the third way of DevOps.

Many topics in software delivery, release engineering, or site reliability engineering connect back to SLOs, so don't forgo them. They're mandatory for laddering up the SRE hierarchy of needs. Refer to the SRE workbook and the SRE book for examples of determining, defining, and implementing everything discussed in this episode.