Github user florianschmidt1994 commented on the issue:
https://github.com/apache/flink/pull/5342
@bowenli86 the document should now be be public for everyone to comment on.
Yes, it caches data on either side, and for each incoming element it looks
up eligible records from the other side, and joins and emits those if they
fulfil the user criteria. Entries get removed from the cache whenever they are
too old to be joined, which is determined by a combination of the current
watermark and the time boundary defined by the user.
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