Serializability (SER) and snapshot isolation (SI) are widely used transactional isolation levels in database systems. The isolation checking problem asks whether a given execution history of a database system satisfies a specified isolation level. However, existing SER and SI checkers, whether traditional black-box checkers or recent timestamp-based white-box ones, operate offline and require the entire history to be available to construct a dependency graph, making them unsuitable for continuous and ever-growing histories. This paper addresses online isolation checking by extending the timestamp-based isolation checking approach to online settings. Specifically, we design Chronos, an efficient timestamp-based offline SI checker. Chronos is incremental and avoids constructing a start-ordered serialization graph for the entire history, making it well-suited for online scenarios. We further extend Chronos into an online SI checker, Aion, addressing several key challenges unique to online settings. Additionally, we develop Aion-SER for online SER checking. Experiments highlight that Chronos processes offline histories with up to one million transactions in seconds, greatly outperforming existing SI checkers. Furthermore, Aion and Aion-ser sustain a throughput of approximately 12K transactions per second, demonstrating their practicality for online isolation checking.

MongoDB is one of the first commercial distributed databases that support causal consistency. Its implementation of causal consistency combines several research ideas for achieving scalability, fault tolerance, and security. Given its inherent complexity, a natural question arises: Has MongoDB correctly implemented causal consistency as it claimed?