Software Delivery in Small Batches

SLIs, SLOs, and SLAs create a hierarchy. They measure reliability and create an objective framework for prioritization and decision making.

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Adam Hawkins
Software Delivery Coach

What is Software Delivery in Small Batches?

Adam Hawkins presents the theory and practices behind software delivery excellence. Topics include DevOps, lean, software architecture, continuous delivery, and interviews with industry leaders.

Hey everybody! Adam here, welcome to another episode of Small Batches. This episode is directly inspired by my full-time job, where we are working on improving SLOs in our enginering team. SLOs and SLIs, SLAs, you've probably heard them before, perhaps you've not given them much thought or like you have not seen them work in practice, I figured I'd do an episode introduction to the topic, although the recent work of Google and SRE work book really puts a finger on what SLIs are and the SLOs is, what is for and how to maintain it with an air budget and these are fundamental practices to software delivery and site reliability engineering, they are critical to the success of software delivery at your organization and likely if you don't have them in place right now this is a great way to improve things at your team, so I hope you enjoy this episode on SLIs, SLOs and SLAs.

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 by the total HTTP requests. Express SLIs as ratios of the total number of good events 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 negatively 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 is ever broken and that’s just impossible. The real answer is that it just depends on the 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 the responsible party may commit to new work or eschew existing work to maintain it. On the other hand, this means the stakeholders' incorporate reliability into the dailywork 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 on 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 determine 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 manage risk, innovation, and reliability. This assumes though that all stakeholders 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 this?

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. Unfortunately, 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 ones. Or 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 over time. This is an act of continuous improvement — A.K.A the third way of DevOps.
Many topics in software delivery or 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. So refer to the SRE workbook and the SRE book for examples of determining, defining, and implementing everything discussed in this episode.

That completes these batches. Visit smallbatches.fm to subscribe to the show for free.