Stream-Static Join: How to refresh (unpersist/persist) static Dataframe periodically

Question

I am building a Spark Structured Streaming application where I am doing a batch-stream join. And the source for the batch data gets updated periodically.

So, I am planning to do a persist/unpersist of that batch data periodically.

Below is a sample code which I am using to persist and unpersist the batch data.

Flow:

• Read the batch data
• persist the batch data
• For every one hour, unpersist the data and read the batch data and persist it again.

But, I am not seeing the batch data getting refreshed for every hour.

Code:

var batchDF = handler.readBatchDF(sparkSession)
batchDF.persist(StorageLevel.MEMORY_AND_DISK)
var refreshedTime: Instant = Instant.now()

if (Duration.between(refreshedTime, Instant.now()).getSeconds > refreshTime) {
  refreshedTime = Instant.now()
  batchDF.unpersist(false)
  batchDF =  handler.readBatchDF(sparkSession)
    .persist(StorageLevel.MEMORY_AND_DISK)
}

Is there any better way to achieve this scenario in spark structured streaming jobs ?

Answer 1

You could do this by making use of the streaming scheduling capabilities that Structured Streaming provides.

You can trigger the refreshing (unpersist -> load -> persist) of a static Dataframe by creating an artificial "Rate" stream that refreshes the static Dataframe periodically. The idea is to:

1. Load the static Dataframe initially and keep as var
2. Define a method that refreshes the static Dataframe
3. Use a "Rate" Stream that gets triggered at the required interval (e.g. 1 hour)
4. Read actual streaming data and perform join operation with static Dataframe
5. Within that Rate Stream have a foreachBatch sink that calls refresher method created in step 2.

The following code runs fine with Spark 3.0.1, Scala 2.12.10 and Delta 0.7.0.

  // 1. Load the staticDataframe initially and keep as `var`
  var staticDf = spark.read.format("delta").load(deltaPath)
  staticDf.persist()

  //  2. Define a method that refreshes the static Dataframe
  def foreachBatchMethod[T](batchDf: Dataset[T], batchId: Long) = {
    staticDf.unpersist()
    staticDf = spark.read.format("delta").load(deltaPath)
    staticDf.persist()
    println(s"${Calendar.getInstance().getTime}: Refreshing static Dataframe from DeltaLake")
  }

  // 3. Use a "Rate" Stream that gets triggered at the required interval (e.g. 1 hour)
  val staticRefreshStream = spark.readStream
    .format("rate")
    .option("rowsPerSecond", 1)
    .option("numPartitions", 1)
    .load()
    .selectExpr("CAST(value as LONG) as trigger")
    .as[Long]

  // 4. Read actual streaming data and perform join operation with static Dataframe
  // As an example I used Kafka as a streaming source
  val streamingDf = spark.readStream
    .format("kafka")
    .option("kafka.bootstrap.servers", "localhost:9092")
    .option("subscribe", "test")
    .option("startingOffsets", "earliest")
    .option("failOnDataLoss", "false")
    .load()
    .selectExpr("CAST(value AS STRING) as id", "offset as streamingField")

  val joinDf = streamingDf.join(staticDf, "id")

  val query = joinDf.writeStream
    .format("console")
    .option("truncate", false)
    .option("checkpointLocation", "/path/to/sparkCheckpoint")
    .start()

  // 5. Within that Rate Stream have a `foreachBatch` sink that calls refresher method
  staticRefreshStream.writeStream
    .outputMode("append")
    .foreachBatch(foreachBatchMethod[Long] _)
    .queryName("RefreshStream")
    .trigger(Trigger.ProcessingTime("5 seconds")) // or e.g. 1 hour
    .start()

To have a full example, the delta table got created and updated with new values as below:

  val deltaPath = "file:///tmp/delta/table"

  import spark.implicits._
  val df = Seq(
    (1L, "static1"),
    (2L, "static2")
  ).toDF("id", "deltaField")

  df.write
    .mode(SaveMode.Overwrite)
    .format("delta")
    .save(deltaPath)