How We Improved Payment System Throughput by 25% Using Apache Kafka at a Fortune 500 FinTech By Disha Sune — Java Backend Engineer | Spring Boot | Kafka | AWS | Fiserv At Fiserv, our payment processing platform handled millions of financial transactions daily for 600+ enterprise clients including McDonald's, Google, and Domino's. As transaction volumes grew, our legacy synchronous REST API archi
In August 2025, a user reported that Apache Kafka v3.9.0 dropped consumer throughput by 10x. Other users reproduced it. The culprit was a configuration called min.insync.replicas, and the fix was three lines of code. Sharad Garg opened a ticket titled "Consumer throughput drops by 10 times with Kafka v3.9.0 in ZK mode." Ritvik Gupta ran controlled tests and traced the issue to min.insync.replicas.
Some time ago, I was building a chat application using AWS Websocket API gateway. Things were going smoothly. I created a WebSocket API Gateway, added $connect, $disconnect, and sendMessage/addGroup routes. From the frontend (React) side, everything was fire-and-forget. You send a message, and the onMessageHandler takes care of it 💪🏼 But then a new requirement of uploading files using S3 signed