Gateways based on the programmable P4 language are becoming a key component in data center traffic management, offering cost-effective solutions for high-throughput environments. However, traditional monitoring techniques like sFlow and NetFlow lack the needed precision to meet the demands of large-scale data centers. In this paper, we presentFlowLog, the first sketch-based and end-to-end flow monitoring system capable of accurate flow size estimation in 400 Gbps production environments. FlowLog integrates the novelByteSketchalgorithm, a transmission subsystem, and a high-speed analysis subsystem, achieving high accuracy even in demanding data center scenarios. Deployed for over six months in ByteDance’s data center with peak bandwidths exceeding 400 Gbps, FlowLog outperforms existing solutions such as Bytehunter sFlow and state-of-the-art sketches in both accuracy and efficiency. Additionally, through real-world deployment, we gained valuable insights that guided improvements in system compatibility, integration ease, and traffic detection. These lessons resulted in a more adaptable system, better handling complex traffic patterns and ensuring minimal overhead during monitoring.