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From Abhijeet Kumar <>
Subject Why does join use rows that were sent after watermark of 20 seconds?
Date Mon, 10 Dec 2018 11:53:40 GMT

I’m using watermark to join two streams as you can see below:

val order_wm = order_details.withWatermark("tstamp_trans", "20 seconds")
val invoice_wm = invoice_details.withWatermark("tstamp_trans", "20 seconds")
val join_df = order_wm
  .join(invoice_wm, order_wm.col("s_order_id") === invoice_wm.col("order_id"))
My understanding with the above code, it will keep each of the stream for 20 secs. After it
comes but, when I’m giving one stream now and the another after 20secs then also both are
getting joined. It seems like even after watermark got finished Spark is holding the data
in memory. I even tried after 45 seconds and that was getting joined too.

I’m sending streams from two Kafka queues and tstamp_trans I’m creating with current timestamp
This is creating confusion in my mind regarding watermark.

Thank you,
Abhijeet Kumar
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