Most components of a web application produce operational log files. Some logs are written by each application in a unique format. Other components generate out-of-the-box logs. Monitoring system logs is an essential activity for anyone charged with taking decisions. System administrators need to monitor logs to look out for unusual activity, to troubleshoot applications and websites that are under their control. By scanning logs, extracting and correlating data, system administrators can investigate and resolve problems, carry out capacity planning, help to detect vulnerabilities, ensure the smooth running of services and balancing capacity, and establish who has used services and when.