Read the SLA's fine print before building anything
Every real SLA has definitions buried in it that materially change what "compliant" means, and these vary contract to contract:
- Scheduled maintenance exclusions. Most SLAs exclude pre-announced maintenance windows from the uptime calculation entirely — monitoring that doesn't account for this will show "violations" that don't actually count against the agreement.
- What counts as an outage. Some SLAs define it purely as unreachability; others include a response-time threshold, where responses slower than a stated limit count as a violation even though the service technically responded.
- The measurement window. Monthly, quarterly, and annual SLA windows produce very different tolerances for a single bad day — a rough day is a much bigger dent in a monthly calculation than an annual one.
Monitoring built against a generic "uptime" definition rather than the specific SLA's actual text will produce numbers that don't match what the contract cares about — sometimes making things look worse than they contractually are, sometimes better.
Why independent measurement matters
A provider self-reporting its own uptime against its own SLA has an obvious incentive problem, even when done in good faith — internal monitoring that shares infrastructure with the service being measured can miss exactly the outages that take both down together. Third-party, externally run monitoring avoids this specific failure mode and gives both sides of an agreement a number neither one controls.
Credits are usually the whole point, and they need proof
Most SLAs tie a missed commitment to a concrete remedy — a service credit, a fee reduction. Claiming that remedy typically requires demonstrating the violation, which means the monitoring needs to produce something more durable than a live dashboard: timestamped historical records, with enough detail (when, how long, from where) to support a credit request months after the fact, not just a real-time status view.
The metric usually needs to be per-service, not per-server
Infrastructure-level uptime (is the server running) and service-level uptime (is the actual product working) frequently diverge — a server can be fully operational while the specific service or endpoint it's meant to provide is failing due to an application bug, a downstream dependency outage, or a full database. An SLA about a "service," not a "server," needs monitoring that checks the actual service behavior, not just whether the underlying machine responds to a ping.