val df = Seq(("1","One"),("2","two")).toDF("key","value")
df.printSchema()
df.show(false)
df.selectExpr("CAST(key AS STRING)", "CAST(value AS STRING)")
  .write
  .format("kafka")
  .option("kafka.bootstrap.servers", "127.0.0.1:9092")
  .option("topic", "testtopic")
  .save()