What does SLI and SLO stand for?
SLI is an initialism for Service Level Indicator. SLO is an initialism for Service Level Objective.
A Service Level Indicator is a metric of service such as query-per-second, megabytes per second, transactions per minute, HTTP requests per minute etc. These are quantifiable measurements of some I.T. service. SLIs, among other things, are the metrics of a monitoring system.
A Service Level Objective is a goal to facilitate a Service Level Indicator at a specified level. SLOs are closely related to the alerting thresholds in monitoring systems. The responding team will need time to react to an outage or near outage. An SLO usually connotes internal to the I.T. service provider. A managed service provider hosting a website or database would have an SLO with its staff that would be greater than the minimum associated with a Service Level Agreement.
A Service Level Agreement is an agreed upon (either contractually or through informal communications) that an I.T. company or group will maintain a minimum SLO. Should the provider fail to maintain a minimum SLO, the recourse would be a partial or complete refund of the money it received in consideration for accepting the contract.
To learn more about SLIs, SLOs, and SLAs, see these links:
To learn about tail latency, see this posting.