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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