Modern web applications are complex, have many layers and usually integrate many technologies. A well structured application log is invaluable for debuging your application in development and monitoring it in production. All this being said it’s amazing how many applications I encounter that either don’t have an application log or have a poor, inconsistent one which is little use to anyone! This session takes a brief look at the basics of logging (libraries and tools) and moves on to look at how to plan an effective logging strategy for your application. Questions like: What to log, logging levels, how much to log and how long to keep historical logs are all addressed. In the second half of the session some of the more interesting things you can do with logs are explained and demonstrated. Aggregating logs across multiple servers, aggregating logs and displaying the results on service dashboards with tools like statsd and Graphine.