Glossary
Uptime & SLA
Uptime is the percentage of time a service is available; an SLA is the provider’s contractual guarantee of that level, with credits if missed.
Uptime is the proportion of time a hosting service is online and reachable, usually expressed as a percentage. An SLA (Service Level Agreement) is the provider's contractual promise of a minimum uptime, with remedies (often account credits) if they fall short.
What the percentages mean
Small differences hide large amounts of downtime per year:
- 99.9% ("three nines") — about 8.8 hours of downtime per year.
- 99.99% — about 53 minutes per year.
- 99.999% ("five nines") — about 5 minutes per year.
What to read in the fine print
- Whether scheduled maintenance is excluded.
- How downtime is measured and how you claim credits.
- Whether the SLA covers the network only or the application too.
Why it matters for hosting
An SLA sets expectations and your recourse. High availability is achieved through redundancy — a load-balancer, cloud-hosting, and ddos protection all reduce the outages an SLA must cover.
See also
Cloud Hosting
Cloud hosting runs your site across a pool of virtual servers that scale on demand, billed by usage for elasticity and high availability.
Load Balancer
A load balancer distributes incoming traffic across multiple servers to improve capacity, reliability, and uptime for a website or application.
DDoS (Distributed Denial of Service)
A DDoS attack floods a server with traffic from many sources to overwhelm it and knock a website offline; mitigation absorbs or filters the flood.
Datacenter
A datacenter is a secure facility housing servers, storage, and networking with redundant power and cooling, where your website is physically hosted.