Back to the Future: Omniscient Debugging

This article presents TOD (trace oriented debugger), a prototype scalable omniscient debugger for Java, which aims at making omniscient debugging practical, at last. Omniscient debuggers, also known as back-in-time or reversible debuggers, record the whole history, or execution trace, of a debugged program and let the user freely explore it. This approach combines the advantages of both log-based (past activity is never lost) and breakpoint based debugging (interactive navigation, step-by-step execution, and complete stack inspection). Omniscient debuggers simulate step-by-step execution both forward and backward, avoiding having to rerun the whole program many times to pinpoint the bug\´s root cause. More importantly, they make it possible to navigate through the history of a program by following causal links, so questions that would otherwise require a significant effort can be answered instantly for instance, "When was variable x assigned a null value?" or "What was the state of object o when it was passed as an argument to method foo?".