A sliding window join operator for two streams processes elements within a defined window size, matching tuples from both streams that overlap in time. However, it is not deterministic because the relative arrival rates of the streams or their interleaving can lead to different results depending on how the window shifts. To guarantee determinism, an alternative design could involve buffering the streams into fixed-size chunks (or partitions), processing them in a way that ensures a consistent, ordered join regardless of input speed or interleaving.