---
title: "Availability vs Reliability — Definition | FourSight Glossary"
url: https://foursight.cloud/glossary/availability-vs-reliability
description: "Availability measures whether a service responds (uptime %). Reliability measures whether it responds correctly and within acceptable latency. A service…"
lang: en
---

Back to glossary: https://foursight.cloud/glossary

Monitoring & Uptime

# Availability vs Reliability

Availability measures whether a service responds (uptime %). Reliability measures whether it responds correctly and within acceptable latency. A service can be available but unreliable if responses are slow or wrong.

Source

- Google SRE Book — Chapter 3: Embracing Risk (https://sre.google/sre-book/embracing-risk/)

## Related terms

Uptime

The percentage of time a service is reachable and responding correctly over a given window.
https://foursight.cloud/glossary/uptime

Downtime

Any period during which a service fails to respond, returns errors, or is so degraded it is unusable.
https://foursight.cloud/glossary/downtime

SLA (Service Level Agreement)

A formal contract between a service provider and a customer that defines minimum service levels and the con…
https://foursight.cloud/glossary/sla

SLO (Service Level Objective)

An internal reliability target — for example, "99.9% of API requests succeed within 500 ms over a rolling 3…
https://foursight.cloud/glossary/slo

SLI (Service Level Indicator)

The raw measurement that an SLO is calculated from — request success ratio, latency percentile, or freshnes…
https://foursight.cloud/glossary/sli

MTTR (Mean Time To Resolve / Recover)

Average time between an incident being detected and the service being fully restored.
https://foursight.cloud/glossary/mttr

## Start monitoring free — HTTP + Ping checks, 4 regions, no card

The Free plan includes 10 monitors with HTTP and Ping checks from all 4 probe regions, 5 minute intervals, email alerts, and 1 status page — $0, no credit card, commercial use allowed. All 8 monitor types unlock on Growth ($40/mo).

- HTTP + Ping checks
- 4 probe regions on every plan
- No credit card

Start monitoring free: https://foursight.cloud/auth?signup=true

## Structured data

```json
[
  {
    "@context": "https://schema.org",
    "@type": "DefinedTerm",
    "@id": "https://foursight.cloud/glossary/availability-vs-reliability",
    "name": "Availability vs Reliability",
    "description": "Availability measures whether a service responds (uptime %). Reliability measures whether it responds correctly and within acceptable latency. A service can be available but unreliable if responses are slow or wrong.",
    "url": "https://foursight.cloud/glossary/availability-vs-reliability",
    "termCode": "availability-vs-reliability",
    "inDefinedTermSet": {
      "@type": "DefinedTermSet",
      "name": "FourSight Monitoring & Reliability Glossary",
      "url": "https://foursight.cloud/glossary"
    },
    "sameAs": [
      "https://sre.google/sre-book/embracing-risk/"
    ]
  },
  {
    "@context": "https://schema.org",
    "@type": "BreadcrumbList",
    "itemListElement": [
      {
        "@type": "ListItem",
        "position": 1,
        "name": "Glossary",
        "item": "https://foursight.cloud/glossary"
      },
      {
        "@type": "ListItem",
        "position": 2,
        "name": "Availability vs Reliability",
        "item": "https://foursight.cloud/glossary/availability-vs-reliability"
      }
    ]
  }
]
```