*********** High Availability*** ***Fault Tolerance ******* DR ************
HA - Not to remove failure but the ability to recover from it. Correcting the fault and get back to work or ability of your system to recover from fault. It doesn't mean never have any fault but he ability to recover from it.
e.g Auto-scale if instance failed a new instance would be created.
FT - Situation where we can not repair or recover from failure but an emergency landing is required.
e.g. LB or caching instances
in FT - System assumes failure and work through failure - multiple things run in parallel in order to assume failure hence more expensive.
DR - previous situations where recovery from failure or operating through failure but DR is about Protecting critical system data so that you can use that to create new working system incase of DR.
***************** **********RTO & RPO **********************
RPO : Recovery point objective -
Time between when a disaster occurs and the last recoverable copy of key business data was created.
e.g. if last back done at 11PM Monday and disaster occur at 3AM Tuesday then RPO is 4 Hours.
Minimize the length of this time period via regular backups, snapshots and transaction logs to avoid business disruptive data loss.
Organization like financial , hospital ,eCommerce where data loss not acceptable RPO is going to be very less, that can be done regular snapshot or point in time backups regularly
RTO : Recovery Time Objective -
The time between when a disaster occurs and when the system can be restored to an operational state and handed over to the business for testing. lest see above e.g.
e.g. if last back done at 11PM Monday and disaster occur at 3AM Tuesday and systems came back active and operational at 10AM Tuesday then - RTO is 7 hours and RPO is 4.
RTO depends on several things not only on technical aspects e.g. locate where data is, staff available, knowledge available, hardware available and also how quickly system can be created or spin etc etc.
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