At Booking.com, we’re big fans of monitoring. With the usual battery of system-level monitors, we keep an eye on the health of our servers. As you would expect from any commercial internet company, we also keep track of various business metrics. However, we also collect data specifically for monitoring the performance of our applications and subsystems. — We’re practitioners of frequent and liberal deployment (our twist on continuous deployment), and as such we need to quickly pinpoint any problem introduced by a specific code roll-out, or by the activation of a new feature — either plain run-time application errors, or noticeable performance problems. …