What does a watermark represent in data processing?

In data processing, a watermark primarily serves as a mechanism to track the progress of data consumption, particularly within streaming data applications. It helps to define boundaries in data flows, allowing systems to manage and process incoming data efficiently.

By establishing a watermark, systems can determine which data has been processed and which has not, thereby avoiding the processing of duplicates. This is especially crucial in environments where data flows continuously, as it ensures that each piece of data is handled only once.

For instance, when working with event streams, a watermark indicates that all events up to a certain point have been successfully processed. If a system were to restart or if there were any interruptions, the watermark provides the necessary checkpoint to resume processing without re-reading or duplicating data that has already been handled.

This ability to manage duplicates is essential for maintaining data integrity and ensuring that downstream consumers receive accurate and consistent data. Thus, it positions the watermark as a critical feature that supports efficient and effective data processing operations.