Prometheus is an open source monitoring and alerting toolkit originally built at SoundCloud. It collects and stores time-series metrics as key/value pairs, using a powerful query language (PromQL) to slice, aggregate, and alert on those metrics in real time.
Prometheus follows a pull model — it scrapes targets at configured intervals rather than waiting for agents to push data. This makes it straightforward to monitor microservices, Kubernetes workloads, and infrastructure components. It is a founding project of the Cloud Native Computing Foundation (CNCF).