A first good source of information that tells you what is happening inside a Prometheus server are its various web-based status pages. We'll take a look at the most important ones here that provide us with:

• Scrape status information for all active targets (/targets),
• Service discovery status information (/service-discovery),
• TSDB status information (/tsdb-status).

Getting insight into these areas already goes a long way towards identifying and understanding potential issues with a Prometheus server.

There are also a few more status pages which we will not look at specifically, but which could be interesting to you as well:

• Run-time and build information (/status),
• Command-line flags and their values (/flags),
• The currently loaded configuration (/config),
• Rule evaluation states (/rules).