This fourth post in the microservices series looks at how we can sew together complex chains of services, efficiently, and accurately, using Apache Kafka’s Exactly-Once guarantees. Any service-based architecture is itself a distributed system, a field renowned for being difficult, particularly when things go wrong. We have thought experiments like The Two Generals Problem and proofs like FLP which highlight that these systems are difficult to work with. In practice we make compromises. The problem is that retries can result in duplicate processing—which can cause very real problems. Taking a payment, twice, from someone’s account will lead to an incorrect balance. Adding duplicate tweets to a user’s feed will lead to a poor user experience. The list goes on.
