How to handle delayed data in Flink?

Flink offers various ways to handle delayed data, here are some commonly used approaches:

1. Window delay processing: Delayed data can be handled by setting the allowed delay time for the window. When the window ends, Flink will wait for a period of time to receive delayed data into the window. The allowedLateness() method can be used to set the window’s allowed delay time.
2. Side output processing: when delayed data arrives, it can be sent to a special side output stream and then handled in another operation. OutputTag can be used to define the side output stream, and getSideOutput() method can be used to retrieve data from the side output stream.
3. Timer processing: The delay data can be handled using a timer. When data arrives, a timer can be set to process the data after a specified time trigger. TimerService can be used to register and trigger the timer.
4. Advancing Watermarks: Watermarks are used to identify the progress of event time. The timestamp of Watermark can be manually advanced to trigger the processing of delayed data sooner. WatermarkGenerator can be used to customize the generation logic of Watermark.

The above are some common methods for dealing with delayed data, and the specific choice depends on the application scenario and requirements.