Shadow testing using traffic mirroring is something we did extensively at Twitter when we built Diffy but many problems remained unanswered about this approach

Shadow testing using traffic mirroring is something we did extensively at Twitter when we built Diffy but many problems remained unanswered about this approach to test automation and observability. e.g. How do we safely replay write traffic? How do we deal with downstream dependencies in general? Can we document our functional interfaces automatically? Can we observe the data patterns flowing through our code in production? Isotope answers these problems and introduces the most advanced form of shadowing built so far.