When specifying an allowed lateness greater than 0, the window along with its content is kept after the watermark passes the end of the window. In these cases, when a late but not dropped element arrives, it could trigger another firing for the window. These firings are called late firings, as they are triggered by late events and in contrast to the main firing which is the first firing of the window. In case of session windows, late firings can further lead to merging of windows, as they may “bridge” the gap between two pre-existing, unmerged windows.
Attention You should be aware that the elements emitted by a late firing should be treated as updated results of a previous computation, i.e., your data stream will contain multiple results for the same computation. Depending on your application, you need to take these duplicated results into account or deduplicate them.
