What is the difference between availability and reliability?
Availability is whether the system is up and responding when asked. Reliability is whether it keeps giving correct results over time without failing. A system can be available yet wrong, so it is available but not reliable.
Explain it simply
Picture an auto-rickshaw. Availability is "is one standing at the stop when I need it?" Reliability is "does it actually take me to the right place without breaking down halfway?"
A service that always answers but sometimes returns wrong balances is available and unreliable. A service that is correct every time but is down every night for maintenance is reliable in what it does but has poor availability.
Related terms help in interviews:
- MTBF: mean time between failures (how often it breaks).
- MTTR: mean time to repair (how fast you recover).
- Availability is roughly MTBF / (MTBF + MTTR), so you can raise it by failing less or by recovering faster.
Example
A server fails once every 1000 hours and takes 1 hour to restore.
Availability = 1000 / (1000 + 1) = 0.999 = 99.9%
Halve the repair time to 0.5 hour:
Availability = 1000 / 1000.5 = 0.9995 = 99.95%Faster recovery improved availability without making the server fail less often.
What to say in the interview
"Availability is the share of time the system responds. Reliability is the probability it behaves correctly for a given period. Availability equals MTBF over MTBF plus MTTR, so besides preventing failures I invest in fast detection and automatic recovery to shrink MTTR."