[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.

 

About HA

When design a system, it is also important to ask which part of system require HA?

For example, Stripe, payment service, should be HA, because it is core fundamention of the product. But dashbaord for monitoring is not;

 

How to get HA

Avoid single point of failure - redundency