A lot of people use HTTP APIs to communicate between their services (let's have another article about whether it's a good solution or not), and in a majority of cases, it works fine. However, we can get into situations where performance really matters, and especially for libraries writers, JFR can be a very useful tool to see I/O works in their code.
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This article is not an introduction to Flight Recorder, it's rather more practical. If you want to learn more about this technology, you can start with JEP 328: Flight Recorder.