Availability is the share of time in which a website or service responds correctly. An SLA (service level agreement) is a contract that promises a level of service, often an availability percentage, and states what happens if the provider misses it.
How are SLA, SLO and SLI related?
Google’s SRE book separates three terms:
- An SLI (service level indicator) is a carefully defined measure of some aspect of the service, for example availability or error rate.
- An SLO (service level objective) is a target value or range for an SLI.
- An SLA is a contract with users that includes consequences of meeting or missing the SLOs it contains.
The book gives a simple test. If no explicit consequence follows a missed target, the document is almost certainly an SLO. Consequences can be financial, such as refunds, or reputational. The Google Cloud blog adds that an SLA typically promises less than the internal objective, for example 99.9 percent against an internal 99.95 percent.
What does an availability figure mean in minutes?
For a 30-day month, 99.9 percent allows about 43 minutes of downtime and 99.95 percent about 22 minutes. Availability is measured by checks, which is why uptime monitoring supplies the SLI behind most availability statements.
Why does it matter for SEO teams?
SEO teams meet SLAs when they depend on hosting, CDN or CMS providers. A provider’s availability promise covers the infrastructure, but not whether a page carries the right robots tag or returns the right status code. Those errors fall outside most SLAs and need separate monitoring.
Example
In this made-up example, a hosting contract promises 99.9 percent monthly availability and a 10 percent credit if it is missed. The monitoring log shows 95 minutes of downtime in a 30-day month, which is 99.78 percent, so the credit applies.
Sources
- Service Level Objectives – Google SRE Booksre.google
- SRE fundamentals: SLIs, SLAs and SLOs – Google Cloud Blogcloud.google.com
- Website monitoring – Wikipedia (secondary)en.wikipedia.org
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