Prometheus is a time series-based, open source monitoring system. It collects data by sending HTTP requests to hosts and services on metrics endpoints, which it then makes available for analysis and alerting using a powerful query language.
Even though Prometheus has graduated with the Cloud Native Computing Foundation (CNCF) by demonstrating stability, maturity, and solid governance, it is still evolving at a very rapid pace. At the time of writing, the current stable version of Prometheus is 2.9.2, and every component or feature that is going to be discussed will be based on this version. While there should be no major architectural changes within version 2, care should be taken when applying specific configuration that's been learned from this book to earlier or even later versions.