In a project that I used to work on we had a system that would receive messages from a device and make decisions on whether that information would be passed to the user. There were multiple decision levels and one of the problems we always found ourselves asking was if a message was being lost on it’s way through the system.
Before we moved to Spring, it was nearly impossible to tell the answer to that question. Attempts were made to use logging, but the sheer volume of messages that decisions were made on made it tedious at best. Other attempts were made using debuggers but a combination of the volume and the timing changes led to only intermittent success.