What is the role of watermarks in Apache Beam?

In Apache Beam, a watermark is a crucial concept for handling delayed and out-of-order data in data stream processing. Essentially, a watermark can be viewed as a timestamp indicating that the data processing system believes all data up to a certain point in time has been fully received.

Watermarks help data streaming systems process delayed and out-of-order data. By using watermarks, data streaming systems can determine the boundaries of processing data windows, thus identifying which data belongs to the current window and which data might be overwritten by subsequent data. Watermarks also assist the system in determining if certain operations can be performed, such as triggering window calculations or data aggregation.

Overall, watermarks play a crucial role in Apache Beam by helping the system handle out-of-order and late data, thus improving the accuracy and efficiency of data processing.