Useful monitoring checks not only whether a server responds, but also whether the service that matters to users actually works. It combines website or API availability, process status, CPU and memory usage, disk capacity, application errors, database queues, certificate validity and the result of backup jobs. For each check, the normal range of values, the severity and the alert recipient are defined. An alert must carry context and lead to action; recurring false alarms are tuned, not ignored. Trend history matters too, because a filling disk or a gradual slowdown can be dealt with well before an outage. Monitoring is tested regularly and paired with a short response procedure, so that an alert does not remain just a graph with nobody responsible for it.
What to consider when deciding
The scope of monitoring follows from the impact of each specific service. A public website may need checking every minute from several locations, an internal batch job rather a confirmation that it completed, and an archive server tracking of its capacity trend. Installing a tool on its own is not enough; the value comes from the right thresholds, escalation and ongoing maintenance of the checks. At the first deployment, the critical signals are selected and further ones are added only as experience builds up. Backups are monitored separately, and their successful status is backed up by a practical restore test.