Graylog is a centralized logging solution that allows the user to aggregate and search through logs. It provides a powerful query language, a processing pipeline for data transformation, alerting abilities and much more. It is fully extensible through a REST API. Add-Ons can be downloaded from the Graylog Marketplace. Take a look at the minimal Graylog architecture to get the big picture of a Graylog setup. In essence, Graylog needs to talk to MongoDB to store configuration data as well as Elasticsearch to store the actual log data.