Architecting for Scale, 2nd Edition

Chapter 8. Service-Level Agreements

Service-level agreements (SLAs) are all about expectations management. As discussed in Chapter 7, each service has different expectations around it. Many of these expectations are tied to the service tier of the service, but when we look deeper, the expectations are more specific than that.

SLAs as discussed in this book are not about legal or contractual agreements between a company and its customers; they’re agreements between teams and service owners. They provide a mechanism for communicating expectations between services.

SLA Versus SLO

In recent years, the term SLO, or service-level objective, has come into common usage. The distinction between SLA and SLO is that an SLA is used to describe a legal commitment to an external customer, while an SLO is used to describe the target for a service metric between teams. Using these definitions, agreements from one service to another such as those discussed in this chapter are more consistent with the term SLO. Technically, this is a valid distinction using this latest terminology.

However, I do not agree with this distinction. This is because, from my standpoint, this distinction waters down the importance of service-to-service commitments by using what seems like a less committed term (SLO). The term SLO appears to describe a weaker commitment than SLA describes. This is the heart of the problem. In my mind, the performance commitment that is made from one team for one service to another team ...