High Availability Concepts
High availability concepts
What you will learn
- Explain the difference between high availability (HA) and disaster recovery (DR) in scope.
- Recall the "five nines" (99.999%) concept and what downtime allowance it implies.
- Distinguish active-active from active-passive setups.
- Identify a single point of failure (SPOF) in a simplified scenario and propose a fix.
High availability (HA) differs from disaster recovery (DR) in scope, not goal: HA handles a single component failing within the same site (a switch, a server, a link) and keeps the service running immediately, invisibly to the user, while DR handles losing the entire site.
Core concepts
| Five 9s (99.999%) | A common availability standard allowing under 5.26 minutes of downtime per full year. |
|---|---|
| SPOF | Single Point of Failure — one component that takes down the whole service if it fails, because no redundant backup exists for it. |
| Redundancy | An extra redundant component (server, link, power supply) that takes over from any failed one without noticeable interruption. |
| Failover | The automatic process of switching over to the backup component when the primary one fails. |
Active-Active vs. Active-Passive
| Active-Active | Active-Passive | |
|---|---|---|
| Component state | All components actively serve traffic at the same time | Only one component is active; the other stands by until needed |
| Resource usage | Full utilization of available capacity at all times | The standby component sits idle most of the time |
| Setup complexity | Higher — needs load balancing and state sync between both sides | Simpler — one clear switchover on failure |
Common confusion: HA vs. DR
"Two servers working together in the same data center to tolerate one failing" is high availability (HA), while "a full backup site in another city to tolerate losing the entire data center" is disaster recovery (DR) — the difference is the scale of the expected incident, not just the words used.
Quick check
Answer in your head first, then reveal.
About how much yearly downtime does "five nines" allow?Reveal the answer
Under 5.26 minutes per full year.
What's the key difference between active-active and active-passive?Reveal the answer
In active-active, all components work together; in active-passive, only one is active while the other stands by.
What's the scope difference between HA and DR?Reveal the answer
HA handles a single component failing within the same site; DR handles losing the entire site.
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Sources
Used to verify the facts. The writing is original to Passuit.