[System Design] Availability

Key Terms

• Availability

  The odds of a particular server or service being up and running at any point in time, usually measured in percentages. A server that has 99% availability will be operational 99% of the time (this would be described as having two nines of availability).

• High Availability

  Used to describe systems that have particularly high levels of availability, typically 5 nines or more; sometimes abbreviated "HA".

• Nines

  Typically refers to percentages of uptime. For example, 5 nines of availability means an uptime of 99.999% of the time. Below are the downtimes expected per year depending on those 9s:

  - 99% (two 9s): 87.7 hours
  - 99.9% (three 9s): 8.8 hours
  - 99.99%: 52.6 minutes
  - 99.999%: 5.3 minutes

• Redundancy

  The process of replicating parts of a system in an effort to make it more reliable.

• SLA

  Short for "service-level agreement", an SLA is a collection of guarantees given to a customer by a service provider. SLAs typically make guarantees on a system's availability, amongst other things. SLAs are made up of one or multiple SLOs.

  https://cloud.google.com/functions/sla

• SLO

  Short for "service-level objective", an SLO is a guarantee given to a customer by a service provider. SLOs typically make guarantees on a system's availability, amongst other things. SLOs constitute an SLA.