How to work with temporary tables in foreachBatch? - apache-spark

Question

We are building a streaming platform where it is essential to work with SQL's in batches.
val query = streamingDataSet.writeStream.option("checkpointLocation", checkPointLocation).foreachBatch { (df, batchId) => {
df.createOrReplaceTempView("events")
val df1 = ExecutionContext.getSparkSession.sql("select * from events")
df1.limit(5).show()
// More complex processing on dataframes
}}.trigger(trigger).outputMode(outputMode).start()
query.awaitTermination()
Error thrown is :
org.apache.spark.sql.streaming.StreamingQueryException: Table or view not found: events
Caused by: org.apache.spark.sql.catalyst.analysis.NoSuchTableException: Table or view 'events' not found in database 'default';
Streaming source is Kafka with watermarking and without using Spark-SQL we are able to execute dataframe transformations. Spark version is 2.4.0 and Scala is 2.11.7. Trigger is ProcessingTime every 1 minute and OutputMode is Append.
Is there any other approach to facilitate use of spark-sql within foreachBatch ? Would it work with upgraded version of Spark - in which case to version do we upgrade ?
Kindly help. Thank you.

Answer 1

tl;dr Replace ExecutionContext.getSparkSession with df.sparkSession.
The reason of the StreamingQueryException is that the streaming query tries to access the events temporary table in a SparkSession that knows nothing about it, i.e. ExecutionContext.getSparkSession.
The only SparkSession that has this events temporary table registered is exactly the SparkSession the df dataframe is created within, i.e. df.sparkSession.

Answer 2

Please check the code snippet below. Here, I have created two separate DataFrames, responseDF1 and responseDF2 from resultDF and shown the output in the console. responseDF2 is created using a temporary table. You can try the same.
resultDF.writeStream.foreachBatch {(batchDF: DataFrame, batchId: Long) =>
batchDF.persist()
val responseDF1 = batchDF.selectExpr("ResponseObj.type","ResponseObj.key", "ResponseObj.activity", "ResponseObj.price")
responseDF1.show()
responseDF1.createTempView("responseTbl1")
val responseDF2 = batchDF.sparkSession.sql("select activity, key from responseTbl1")
responseDF2.show()
batchDF.sparkSession.catalog.dropTempView("responseTbl1")
batchDF.unpersist()
()}.start().awaitTermination()
Code Snippet