An SLI is a service level indicator—a carefully defined quantitative measure of some aspect of the level of service that is provided.

Most services consider request latency—how long it takes to return a response to a request—as a key SLI. Other common SLIs include the error rate, often expressed as a fraction of all requests received, and system throughput, typically measured in requests per second. The measurements are often aggregated: i.e., raw data is collected over a measurement window and then turned into a rate, average, or percentile.

Ideally, the SLI directly measures a service level of interest, but sometimes only a proxy is available because the desired measure may be hard to obtain or interpret. For example, client-side latency is often the more user-relevant metric, but it might only be possible to measure latency at the server.

Another kind of SLI important to SREs is availability, or the fraction of the time that a service is usable. It is often defined in terms of the fraction of well-formed requests that succeed, sometimes called yield. (Durability—the likelihood that data will be retained over a long period of time—is equally important for data storage systems.) Although 100% availability is impossible, near-100% availability is often readily achievable, and the industry commonly expresses high-availability values in terms of the number of “nines” in the availability percentage. For example, availabilities of 99% and 99.999% can be referred to as “2 nines” and “5 nines” availability, respectively, and the current published target for Google Compute Engine availability is “three and a half nines”—99.95% availability.