SLI (service level indicator)

An SLI (service level indicator) is a measurement of service behaviour, usually the ratio of good events to all events, such as successful checks divided by all checks.

Common SLIs are availability (the share of successful requests or checks), latency (the share of responses faster than a threshold) and correctness (the share of responses with the right content).

Where you measure changes the result. Server logs miss requests that never arrived; checks from outside, ideally from several regions, see what users see, including DNS, TLS and network problems.

At a glance

SLI = good events ÷ valid events × 100%

In OpenUptime

Each OpenUptime check is recorded with its result and response time, so the share of successful checks and the share of fast ones can serve as SLIs. Slow-response alerts fire after a set number of checks in a row exceed a time limit.

Questions

Is response time an SLI?

Usually as a ratio: the share of responses faster than, say, 500 ms. Averages hide slow outliers, so percentiles or threshold ratios are preferred.

What is the difference between an SLI and a metric?

Any number is a metric. An SLI is the metric you have agreed reflects what users experience and that an SLO is set on.

SLO (service level objective)An SLO (service level objective) is a target a team sets for a measured indicator of reliability, such as 99.95% of checks succeeding over 30 days.SLA (service level agreement)An SLA (service level agreement) is a promise to customers about the level of service, such as 99.9% monthly uptime, and what they receive if it is not met.Synthetic monitoringSynthetic monitoring checks a service by sending it requests on a schedule from outside, as a user would, rather than waiting for real traffic.UptimeUptime is the share of a period during which a service worked as expected, usually given as a percentage such as 99.9%.

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术语表

SLI（服务等级指标）

SLI（服务等级指标）是对服务表现的测量，通常是“好的事件”占全部事件的比例，例如成功检查次数除以总检查次数。

常见的 SLI 有：可用性（成功请求或检查的比例）、延迟（快于某个阈值的响应所占比例）和正确性（内容正确的响应所占比例）。

在哪里测量会影响结果。服务器日志看不到根本没到达的请求；从外部检查，最好从多个区域检查，看到的才是用户所见，包括 DNS、TLS 和网络问题。

速览

SLI = 好的事件数 ÷ 有效事件总数 × 100%

在 OpenUptime 中

OpenUptime 记录每次检查的结果和响应时间，所以成功检查的比例和快速响应的比例都可以作为 SLI。连续若干次检查超过设定的响应时间时，会触发慢响应告警。

常见问题

响应时间是 SLI 吗？

通常以比例的形式出现：例如快于 500 毫秒的响应所占比例。平均值会掩盖慢的极端情况，所以更推荐用百分位数或阈值比例。

SLI 和普通指标有什么区别？

任何数字都可以是指标。SLI 是大家一致认为能反映用户体验、并以此设定 SLO 的那个指标。

SLO（服务等级目标）SLO（服务等级目标）是团队为某个可靠性指标设定的目标，例如 30 天内 99.95% 的检查成功。SLA（服务等级协议）SLA（服务等级协议）是对客户的服务水平承诺，例如每月 99.9% 的可用率，以及未达成时客户能得到什么补偿。主动监控（Synthetic monitoring）主动监控（synthetic monitoring）是按计划从外部向服务发送请求来检查它，就像用户在访问一样，而不是等真实流量来发现问题。可用时间（Uptime）Uptime 指在一段时间内，服务按预期正常工作的时间所占的比例，通常用百分比表示，例如 99.9%。

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从一个或多个区域检查，告警发到你常用的工具，还有面向客户的状态页。免费开始使用，以 MIT 许可证开源。

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