What does "highly available" actually mean, and how do you achieve it?
What they are really testing: Whether you reduce HA to its core mechanism, eliminating single points of failure through redundancy and automatic failover, rather than saying "lots of servers".
A real interview question
What does "highly available" actually mean, and how do you achieve it?
What most people say
drag me
“High availability means running lots of servers so the site stays up.”
More servers behind a single load balancer or one database is still a single point of failure. HA is specifically about eliminating SPOFs with redundancy across failure domains plus automatic failover, not raw server count.
The follow-ups they ask next
What is "shared fate" and why does it undermine redundancy?
Redundant copies that depend on one common thing (same zone, same config push, same DB) fail together when it fails, so the redundancy is an illusion. True HA requires the copies to be genuinely independent across failure domains.
Why is redundancy without health-checked failover not enough?
If nothing detects the failure and reroutes, traffic keeps hitting the dead instance. You need health checks plus automatic failover so the spare actually takes over quickly.
What the interviewer is listening for
- Defines HA as removing SPOFs
- Redundancy across failure domains + automatic failover
- Mentions shared fate / measured in nines
What sinks the answer
- "Just run more servers"
- No notion of failover or failure domains
- Misses shared fate
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“HA means [the system survives component failures by eliminating single points of failure]. Achieve it with [redundancy of each tier across failure domains (AZs)], [health checks + automatic failover so a spare takes over fast], and [avoiding shared fate, copies that secretly share a zone/config/DB]. Measured in [nines].”
Keep going with reliability
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