In a compelling narrative shared by Uri Shamay, a Principal Lead System Software Engineer at Akamai, a legacy banking system faced significant challenges when transitioning its API call model from native threads to Non-Blocking I/O with Java NIO, aimed at reducing operation lead times from 30 days to just one. Despite the promising performance improvements, the new implementation encountered persistent deadlocks, exacerbated by the absence of source code and documentation, complicating efforts to diagnose and resolve the issues. The situation worsened with the tech lead's departure, leaving the team to tackle deadlocks with limited knowledge of the system's concurrency patterns. After numerous failed attempts to trace the problem using open-source tools and recreate it in a QA environment, the team opted for a temporary JMX-based auto-restart solution to manage the deadlocks, while planning a complete rewrite of the problematic libraries. This story underscores the complexities of maintaining and upgrading legacy systems, particularly when faced with undocumented and convoluted codebases.