The document discusses LINE's use of Prometheus and Grafana to monitor its Hadoop and Fluentd clusters. It introduces Promgen, a tool LINE created to manage server lists and configuration for Prometheus since it operates in an on-premises environment without service discovery. It then describes how LINE exports metrics from its Hadoop components like HDFS, YARN and Hive using custom exporters as well as from Fluentd using a Prometheus plugin. These metrics are collected by Prometheus and visualized in Grafana for cluster monitoring and alerting.