Requirements tell us what a system should do. Nonfunctional requirements tell us when the system is doing well. Architects should pay attention to nonfunctional requirements because the challenges they pose require similar solutions: billion user systems often share architectural patterns even if they do different things. Availability, performance, and scale are all forms of functional requirements, with scale typically reduced to maintain availability/performance goals even at scale.
When put in a production context and measured against the live system, common nomenclature for these types of requirements are service-level indicators and objectives (SLIs and SLOs). If used correctly, architects will find these to be invaluable for continuous design, adding confidence to tough decision making around architectural changes (akin to tests/refactoring and profiling/optimization). Alas, SLIs and SLOs are deceptively hard to use correctly. All the nines in the world won’t help if our indicators measure the wrong thing.
Yaniv Aknin details what’s useful to measure and explains why you should focus on the user. You’ll discover common technical and organizational impediments to doing just that, compare passive monitoring (aka real user monitoring) to active monitoring (aka probers), and learn the importance of segmenting your users to business-meaningful cohorts, as well as a few cohorts to pay attention to.
With the right measurements in place, you can set objectives (SLOs) to help interpret the data. Yaniv outlines behaviors that you’d want your SLOs to encourage, primarily “working” and “fast” but sometimes also “correct,” “complete,” and “durable.” Yaniv also covers good choices for objectives, explaining why neither “perfect” nor “no SLO” are useful for engineering decisions (even if management really wants either). He concludes by exploring the practicalities of interpreting latency measurements (and how latency != performance), along with choosing aggregation windows or reporting periods. You’ll leave able to design SLIs and SLOs to guide architectural decisions for a big, live system. Knuth’s adage still applies at scale: “Premature optimization is the root of all evil.”
Yaniv Aknin is the SRE tech lead for Google’s Cloud Services Group, covering products like App Engine, Kubernetes Engine, Cloud Functions, API infrastructure, and others. Yaniv is passionate about reliability metrics as a tool to keep SRE groups focused on lasting engineering projects and away from tactical operational overload. Outside of work, he enjoys travel, food, improv theater, and popsci, especially behavioral economics.
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