Service Levels

• Service Level Agreement (SLA)

• An agreement between the service provider and the clients/users

• A legal contract that represents quality of the service. With explicit penalties and financial consequences
• E.g.,
  • Availability
  • Performance
  • Data durability
  • Time to respond to system failures

• SLAs are common for external paying users (always), free external users (sometimes) and internal users (occasionally)

• Service Level Objetives (SLO)

• Individual goals set for the system

• Each SLO represents a target value/range that the system needs to meet
• E.g.,
  • Availability of 3 nines
  • Response time of 100 ms for 90th percentile
• The quality attributes from the beginning of the design process will make their way to one or more SLOs

• If a system has an SLA, it will be detailed as multiple SLOs aggregated in a single legal document

• Service Level Indicator (SLI)

• Quantitative measure of the compliance with a SLO

• The actual numbers, metrics measured by monitoring a service
• SLOs rely on the SLIs to measure if the goal is achieved or not