In a world where modern applications demand robust logging for observability and debugging, developers often face the choice of using ThreadContext.put() or MDC.put() when working with Log4j. Both methods play a crucial role in setting contextual data for logs, but their differences and nuances can significantly impact your application's performance, behavior, and maintainability. To make the best choice for your logging needs, it’s essential to dive deeper into these methods, how they function, and their real-world implications.