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https://issues.apache.org/jira/browse/SPARK-20065?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15937904#comment-15937904
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Genmao Yu commented on SPARK-20065:
-----------------------------------
Make sense, I will give a fast update.
> Empty output files created for aggregation query in append mode
> ---------------------------------------------------------------
>
> Key: SPARK-20065
> URL: https://issues.apache.org/jira/browse/SPARK-20065
> Project: Spark
> Issue Type: Bug
> Components: Structured Streaming
> Affects Versions: 2.1.0
> Reporter: Silvio Fiorito
>
> I've got a Kafka topic which I'm querying, running a windowed aggregation,
> with a 30 second watermark, 10 second trigger, writing out to Parquet with
> append output mode.
> Every 10 second trigger generates a file, regardless of whether there was any
> data for that trigger, or whether any records were actually finalized by the
> watermark.
> Is this expected behavior or should it not write out these empty files?
> {code}
> val df = spark.readStream.format("kafka")....
> val query = df
> .withWatermark("timestamp", "30 seconds")
> .groupBy(window($"timestamp", "10 seconds"))
> .count()
> .select(date_format($"window.start", "HH:mm:ss").as("time"), $"count")
> query
> .writeStream
> .format("parquet")
> .option("checkpointLocation", aggChk)
> .trigger(ProcessingTime("10 seconds"))
> .outputMode("append")
> .start(aggPath)
> {code}
> As the query executes, do a file listing on "aggPath" and you'll see 339 byte
> files at a minimum until we arrive at the first watermark and the initial
> batch is finalized. Even after that though, as there are empty batches it'll
> keep generating empty files every trigger.
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