piter75 commented on issue #26329:
URL: https://github.com/apache/beam/issues/26329#issuecomment-1517629623

   Thanks for responding @kkdoon.
   
   Our use case for `BoundedSource` in streaming job stems from the fact that 
we want to have a hot start whenever we start/re-start the job.
   
   Part of our pipeline depends on the join between streams with different 
event frequency. On one side there is a stream that delivers thousands of 
messages per second and on the other is one that may not have matching messages 
for a day.
   We solved this frequency mismatch by loading the history of the "slow 
moving" stream from BigQuery and then making a union with a stream of messages 
that come straight from the PubSub topic.
   This way we have a union that is both complete from the start and still 
unbounded during the pipeline run.
   
   Unit testing this issue may be difficult because it is triggered when used 
on a specific runner in specific run mode and I wouldn't like to test the 
behaviour of the BigQuery source with the runner tests.
   
   I will try to come up with some reproduction example.
   
   What runner are you using by the way?


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