Structured Streaming doesn't write checkpoint log when I use coalesce

[WangXiaolong]

Hi,


   Lately, I encountered a problem, when I was writing as structured streaming 
job to write things into opentsdb.
  The write-stream part looks something like 


      outputDs
          .coalesce(14)
          .writeStream
          .outputMode("append")
          .trigger(Trigger.ProcessingTime(s"$triggerSeconds seconds"))
          .option("checkpointLocation",s"$checkpointDir/$appName/tsdb")
          .foreach {
            TsdbWriter(
              tsdbUrl,
              MongoProp(mongoUrl, mongoPort, mongoUser, mongoPassword, 
mongoDatabase, mongoCollection,mongoAuthenticationDatabase)
            )(createMetricBuilder(tsdbMetricPrefix))
          }
          .start()


    And when I check the checkpoint dir, I discover that the 
"/checkpoint/state" dir  is empty. I looked into the executor's log and found 
that the HDFSBackedStateStoreProvider didn't write anything on the checkpoint 
dir.


   Strange thing is, when I replace the "coalesce" function into "repartition" 
function, the problem solved. Is there a difference between these two functions 
when using structured streaming?


  Looking forward to you help, thanks.