High availability (HA) is a design approach that aims to keep a service operating continuously, with very little downtime, by using redundancy, health monitoring and automatic failover.
Why it matters
Availability is usually expressed as a percentage. For example, 99.9% allows roughly 8.8 hours of downtime per year, while 99.99% allows about 53 minutes. Each extra nine is much harder and costlier to achieve.
HA requires every layer to be considered: load balancers, application servers, databases, networking and power. A chain is only as available as its weakest link.
Example
A site runs on three web servers across two availability zones behind a managed load balancer, with a database primary and replica. Health checks remove failed nodes automatically, so users rarely notice a failure.
Common mistakes
- Making the web tier redundant but leaving a single database or DNS provider.
- Promising a high percentage without measuring it from outside.
- Forgetting maintenance, which can cause more downtime than hardware failures.
Related
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यह पेज सामान्य जानकारी है, जिसे हमारी संपादकीय टीम ने लिखा है और दस्तावेज़ीकरण बदलने पर अपडेट किया जाता है। सर्वर और प्रदाता अलग-अलग होते हैं, इसलिए बदलाव पहले सुरक्षित तरीके से जाँचें। देखें: संपादकीय नीति और अस्वीकरण.