Uptime monitoring glossary
Plain definitions of the terms that come up around uptime, SLAs and incident response, with formulas and worked examples.
UptimeUptime is the share of a period during which a service worked as expected, usually given as a percentage such as 99.9%.DowntimeDowntime is the time during which a service is unavailable or does not work as expected.AvailabilityAvailability is the share of time, or of requests, for which a service can be used when someone needs it.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.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.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.Error budgetAn error budget is the amount of failure an SLO allows over its window: at 99.9% over a 30-day month, 43 minutes 12 seconds of downtime.MTTR (mean time to recovery)MTTR (mean time to recovery, or to repair) is the average time from the start of a failure until the service works again.MTBF (mean time between failures)MTBF (mean time between failures) is the average time a system works between one failure and the next.MTTA (mean time to acknowledge)MTTA (mean time to acknowledge) is the average time between an alert going out and a person acknowledging that they are working on it.Heartbeat monitoringHeartbeat monitoring checks that a job keeps running: the job sends a request to a unique URL each time it finishes, and an alert goes out when a request is late.Status pageA status page is a public page that shows whether a service and its parts are working, along with current incidents, planned maintenance and past uptime.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.False positive alertA false positive alert reports a failure that did not affect users, such as a single timed-out check caused by the network between the monitor and the server.Maintenance windowA maintenance window is a period set aside in advance for planned work, such as upgrades or migrations, during which the service may be unavailable.Nines of availability"Nines" is shorthand for an availability level by the number of 9s in it: two nines is 99%, three nines 99.9%, four nines 99.99% and five nines 99.999%.
Questions
What is the difference between SLA, SLO and SLI?
The SLI is what you measure, the SLO is the target you set for it, and the SLA is the promise in a contract. Each has its own page below.
Is uptime the same as availability?
They are often used as synonyms. When they are told apart, uptime says the system was running and availability says users could use it.
可用性监控术语表
围绕可用率、SLA 和事件响应的常见术语，用平实的语言解释，并附公式和示例。
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%。停机时间（Downtime）停机时间是指服务不可用、或无法按预期工作的那段时间。可用性（Availability）可用性是指在用户需要时，服务能够正常使用的时间（或请求）所占的比例。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。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。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。错误预算（Error budget）错误预算是 SLO 在其时间窗口内允许的失败量：以 30 天的月份计，99.9% 对应 43 分 12 秒的停机时间。MTTR（平均恢复时间）MTTR（平均恢复时间，也作平均修复时间）是从故障开始到服务重新正常工作的平均时长。MTBF（平均故障间隔时间）MTBF（平均故障间隔时间）是系统从一次故障恢复后，到下一次故障发生前平均正常运行的时间。MTTA（平均确认时间）MTTA（平均确认时间）是从告警发出，到有人确认正在处理之间的平均时长。心跳监控（Heartbeat monitoring）心跳监控用来确认任务一直在运行：任务每次完成时向一个专属 URL 发送请求，请求迟到时就发出告警。状态页（Status page）状态页是一个公开页面，展示服务及其各组成部分是否正常，以及当前事件、计划维护和历史可用率。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。误报（False positive alert）误报是指告警报告了一个并未影响用户的故障，例如监控与服务器之间的网络问题导致某一次检查超时。维护窗口（Maintenance window）维护窗口是提前预留的一段时间，用于升级、迁移等计划内工作，期间服务可能不可用。几个 9（Nines）“几个 9”是按百分比中 9 的个数来简称可用性级别：2 个 9 是 99%，3 个 9 是 99.9%，4 个 9 是 99.99%，5 个 9 是 99.999%。
常见问题
SLA、SLO 和 SLI 有什么区别？
SLI 是你测量的指标，SLO 是为它设定的目标，SLA 是写进合同的承诺。下面每个术语都有单独的页面。
Uptime 和可用性是一回事吗？
它们常被当作同义词。区分时，uptime 表示系统在运行，可用性表示用户能正常使用。