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spark-sql-streaming-EventTimeWatermark.adoc

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EventTimeWatermark Unary Logical Operator

EventTimeWatermark is a unary logical operator (i.e. UnaryNode) that is created as the result of withWatermark operator.

val q = spark.
  readStream.
  format("rate").
  load.
  withWatermark(eventTime = "timestamp", delayThreshold = "30 seconds") // <-- creates EventTimeWatermark
scala> q.explain(extended = true)
== Parsed Logical Plan ==
'EventTimeWatermark 'timestamp, interval 30 seconds
+- StreamingRelation DataSource(org.apache.spark.sql.SparkSession@3d97b0a,rate,List(),None,List(),None,Map(),None), rate, [timestamp#10, value#11L]

== Analyzed Logical Plan ==
timestamp: timestamp, value: bigint
EventTimeWatermark timestamp#10: timestamp, interval 30 seconds
+- StreamingRelation DataSource(org.apache.spark.sql.SparkSession@3d97b0a,rate,List(),None,List(),None,Map(),None), rate, [timestamp#10, value#11L]

== Optimized Logical Plan ==
EventTimeWatermark timestamp#10: timestamp, interval 30 seconds
+- StreamingRelation DataSource(org.apache.spark.sql.SparkSession@3d97b0a,rate,List(),None,List(),None,Map(),None), rate, [timestamp#10, value#11L]

== Physical Plan ==
EventTimeWatermark timestamp#10: timestamp, interval 30 seconds
+- StreamingRelation rate, [timestamp#10, value#11L]

EventTimeWatermark uses spark.watermarkDelayMs key (in Metadata in output) to hold the event-time watermark delay.

Note
 The event-time watermark delay is used to calculate the difference between the event time of an event (that is modelled as a row in the Dataset for a streaming batch) and the time in the past.
Note

EliminateEventTimeWatermark logical optimization rule (i.e. Rule[LogicalPlan]) removes EventTimeWatermark logical operator from a logical plan if child logical operator is not streaming, i.e. when withWatermark operator is used on a batch query.

val logs = spark.
  read. // <-- batch non-streaming query that makes `EliminateEventTimeWatermark` rule applicable
  format("text").
  load("logs")

// logs is a batch Dataset
scala> logs.isStreaming
res0: Boolean = false

val q = logs.
  withWatermark(eventTime = "timestamp", delayThreshold = "30 seconds") // <-- creates EventTimeWatermark
scala> println(q.queryExecution.logical.numberedTreeString) // <-- no EventTimeWatermark as it was removed immediately
00 Relation[value#0] text
Note
 EventTimeWatermark is converted (aka planned) to EventTimeWatermarkExec physical operator in StatefulAggregationStrategy execution planning strategy.

output Property

output: Seq[Attribute]
Note
 output is a part of the QueryPlan Contract to describe the attributes of (the schema of) the output.

output finds eventTime column in the child's output schema and updates the Metadata of the column with spark.watermarkDelayMs key and the milliseconds for the delay.

output removes spark.watermarkDelayMs key from the other columns.

// See q created above
// FIXME How to access/show the eventTime column with the metadata updated to include spark.watermarkDelayMs?
import org.apache.spark.sql.catalyst.plans.logical.EventTimeWatermark
val etw = q.queryExecution.logical.asInstanceOf[EventTimeWatermark]
scala> etw.output.toStructType.printTreeString
root
 |-- timestamp: timestamp (nullable = true)
 |-- value: long (nullable = true)

Creating EventTimeWatermark Instance

EventTimeWatermark takes the following when created:

  • Event time column

  • Delay CalendarInterval

  • Child logical plan