Add-on interface to StreamTask implementations to add code which will be run on
a specified time interval (via configuration). This can be used to implement direct time-based windowing or,
with a frequent window interval, windowing based on some other condition which is checked during the call to
window. The window method will be called even if no messages are received for a particular StreamTask.
Called by TaskRunner for each implementing task at the end of every specified window.
collector - Contains the means of sending message envelopes to the output stream. The collector must only
be used during the current call to the window method; you should not reuse the collector between invocations
of this method.
coordinator - Manages execution of tasks.
java.lang.Exception - Any exception types encountered during the execution of the processing task.