Arrangement
Arrangement stores the state in the dataflow's process. It stores the streams of update flows for further querying and updating.
The arrangement essentially stores key-value pairs with timestamps to mark their change time.
Internally, the arrangement receives tuples like
((Key Row, Value Row), timestamp, diff) and stores them in memory. One can query key-value pairs at a certain time using the get(now: Timestamp, key: Row) method.
The arrangement also assumes that everything older than a certain time (also known as the low watermark) has already been ingested to the sink tables and does not keep a history for them.
The arrangement allows for the removal of keys by setting the diff to -1 in incoming tuples. Moreover, if a row has been previously added to the arrangement and the same key is inserted with a different value, the original value is overwritten with the new value.