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From <bart.kasterm...@kpn.com>
Subject RE: Session windows - Evaluation of addition of element + window expires/gets discarded after some set time of inactivity
Date Fri, 21 Oct 2016 14:31:58 GMT
Here is a session trigger that I wrote (not quite the same rules around what a
session is, but should hopefully be a good start to work from).  I'd love to get
any feedback on how it could be improved.

- bart

import org.apache.flink.api.common.state.{ReducingState, ReducingStateDescriptor, ValueState,
import org.apache.flink.streaming.api.scala.function.util.ScalaReduceFunction
import org.apache.flink.streaming.api.windowing.time.Time
import org.apache.flink.streaming.api.windowing.triggers.Trigger.TriggerContext
import org.apache.flink.streaming.api.windowing.triggers.{Trigger, TriggerResult}
import org.apache.flink.streaming.api.windowing.windows.Window

import org.slf4j.LoggerFactory

  * @param maxSessionLength maximum number of pages that can be in a single session
  * @param sessionPauseMillis time duration in milliseconds indicating length of quiet after
which a session has ended
class sessionTrigger[E](val maxSessionLength: Int, val sessionPauseMillis: Long) extends Trigger[E,
Window] {

  val logger = LoggerFactory.getLogger(classOf[sessionTrigger[E]])
  val countState = new ReducingStateDescriptor[Long]("sessionCount", new ScalaReduceFunction[Long](_
+ _), classOf[Long])
  val timeState = new ValueStateDescriptor[Option[Long]]("sessionTimer", classOf[Option[Long]],

  override def onElement(t: E, l: Long, w: Window, triggerContext: TriggerContext): TriggerResult
= {
    logger.trace("received: " + t.toString)

    // remove old timer
    val time_state: ValueState[Option[Long]] = triggerContext.getPartitionedState(timeState)
    val time_set = time_state.value()
    if (time_set.isDefined) {

    // update count and check if over limit
    val state: ReducingState[Long] = triggerContext.getPartitionedState(countState)
    val ct = state.get()
    logger.trace("count: " + ct + " : " + t.toString)
    if (ct > maxSessionLength) {
      logger.trace("triggered by count")
    } else {
      // set new time and continue
      val new_time = triggerContext.getCurrentProcessingTime + 2000

  override def onProcessingTime(l: Long, w: Window, triggerContext: TriggerContext): TriggerResult
= {
    println("proc time trigger")

  override def onEventTime(l: Long, w: Window, triggerContext: TriggerContext): TriggerResult
= {
    println("even time trigger")

Van: Manu Zhang [owenzhang1990@gmail.com]
Verzonden: vrijdag 21 oktober 2016 10:52
Aan: user@flink.apache.org
Onderwerp: Re: Session windows - Evaluation of addition of element + window expires/gets discarded
after some set time of inactivity

Hi Anchit,

I think you need a customized EventTimeTrigger<https://github.com/apache/flink/blob/master/flink-streaming-java/src/main/java/org/apache/flink/streaming/api/windowing/triggers/EventTimeTrigger.java#L43>
which returns "TriggerResult.FIRE" both on new element and watermark.

Manu Zhang

On Fri, Oct 21, 2016 at 3:08 PM Anchit Jatana <development.anchit@gmail.com<mailto:development.anchit@gmail.com>>
Hi All,

I have a use case where in I'm supposed to work with Session Windows to maintain some values
for some sessionIDs/keys.

The use case is as follows:

I need to maintain a session window for the incoming data and discard the window after some
set gap/period of inactivity but what I want is that as soon as new element gets added to
the window, all the records that are currently in the window get processed using the window
transformation/function and the window does not get discarded.

The "Session windows implementation" as get processed only after the window is consider complete(based
on some gap time settings). But I wish to process the all the elements contained in the window
as soon as a new element gets added to the window(means addition of a new element triggers
the processing of all elements of the window) but the discarding of the window happens only
if there is a gap/inactivity for some set time. And when the window gets discarded/expires
I don't want it to be re-evaluated, since it's contents were processed when the last element
was added to the window.

Is this implementation possible? If yes, can someone please share some sample code to explain
the implementation.

Thank you!


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