High availability, fault tolerance and disaster recovery

#

High availability (HA)

• HA aims to ensure an higher level of operational performance (uptime), during a higher than normal period.
• HA doesn't aim to stop failures - customers may face outages
• HA is not about user experience
• HA aims at maximizing a system's online time
• HA requires redundant servers/infrastructure to be in place ready to switch customers to, in the event of a disaster to minimise downtime

Key percentiles

• 99.9% = 8.77 hours per year downtime
• 99.999% = 5.26 minutes per year downtime

#

Fault tolerance (FT)

• FT is a property that enables a system to continue operating properly in the event of a failure of some of its components.
• FT aims at operating through failures
• Setting up an FT mechanism is expensive and takes longer time to implement.

#

Disaster Recovery (DR)

• DR is a set of policies or tools to enable the recovery or continuancy of vital technology infrastructure and systems following a natural or human-induced disaster.
• DR requires
  • pre-planning
  • backup premises
  • taking regular backups at standby locations (offsite)
  • copies of all processes