Reputation: 71
Watermarking enables automatic dropping of old state data in Apache Spark Structured Streaming. In structured-streaming-programming-guide.md, word count example demonstrates how watermarking can easily drop the records or events that arrive late in the system. ( https://github.com/apache/spark/blob/master/docs/structured-streaming-programming-guide.md )
words.withWatermark("timestamp", "10 minutes")
Is there a way to save the records that are dropped or discarded by watermarking on a disk or in a table?
Upvotes: 3
Views: 1075
Reputation: 821
With paying some perf overhead, should be doable using query listener + broadcast variable and filter func. Some thing like:
class WaterMark extends Serializable {
var ws: Long = 0L;
def set(value: Long) : Unit = {
ws = value
}
def get(): Long = { ws}
}
var currentWs = spark.sparkContext.broadcast[WaterMark](new WaterMark)
df.filter(row => {
if(row.get("timestamp") < currentWs.value.ws){
//this will be filtered by watermark. we can persist it using custom method}
.........................
//Not filtering the row as that would be done by watermarking
true
})
..............................
class QueryListener (currentWs: Broadcast[WaterMark]) extends StreamingQueryListener {
import java.util.Locale
val format = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'")
format.setTimeZone(TimeZone.getTimeZone("UTC"))
...........................
override def onQueryProgress(event: StreamingQueryListener.QueryProgressEvent): Unit = {
//un-persist the broadcast var so that it can be updated with next batch watermark
currentWs.unpersist(true)
currentWs.value.set(format.parse(event.progress.eventTime.get("watermark")).getTime)
println("Listener: " + currentWs.value.ws)
}
......................
}
NOTE: I have not tried it end-to-end myself and it does not handle the case when we restart the query due to failure OR code change (checkpoint_dir/commit folder to rescue??)
Upvotes: 0
Reputation: 62
Yes,spark doesn't have the function to trace these records.But flink does it !
Upvotes: 2