This is the second part of the stream processing blog post series. In the first part I presented aggregations in a fixed, non-overlapping windows. Now you will learn dynamic aggregations in data-driven windows, for which the size of each window depends on the input data instead of a predefined time based pattern. At the end, I also show the complex triggering policy to get speculative results for the latency sensitive pipelines.

User sessions

One of the most popular dynamic aggregations is based on a session window. The single session captures user activities within some period of time followed by the gap of inactivity.

As an example, the following stream of user activities is ingested from hypothetical mobile e-commerce application:

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"00:00:00" -> "open app"
"00:01:30" -> "show product"
"00:03:00" -> "add to cart"
"00:13:00" -> "checkout"
"00:13:10" -> "close app"

The activities should be aggregated into a single session if the maximum allowed gap between the activities is equal or greater than 10 minutes (the longest gap is between “add to cart” and “checkout” events):

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00:23:10 -> "open app", "show product", "add to cart", "checkout", "close app"

If the allowed gap between activities is shorter, e.g. for 5 minutes gap we will get two independent sessions:

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00:08:00 -> "open app", "show product", "add to cart"
00:18:10 -> "checkout", "close app"

This is an over-simplified aggregation example just for the blog academic purposes. Careful reader will notice that it doesn’t scale at all. All user activities during the session must fit into memory of the single machine. In real-world scenario the actions should be reduced into scalable algebraic data structures, e.g:

  • Various counters of the session like the number of unique visited products
  • Number of the deals
  • Click-through rate
  • Conversion rate

I’m going to follow the TDD technique again, so the implementation of the aggregation method will not be disclosed until the very end of the blog post. As for now, the following method signature should be enough to understand all scenarios:

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type User = String
type Activity = String

def activitiesInSessionWindow(
  userActions: SCollection[(User, Activity)],
  gapDuration: Duration,
): SCollection[(User, Iterable[Activity])]

No activities, no sessions

If the input stream of activities is empty the aggregation should not emit any session.

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"No activities" should "create empty session" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)].advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results should beEmpty
}

Nothing fancy, except one mystery method: advanceWatermarkToInfinity(). In a nutshell the method tells: all activities in the input stream have been already observed, calculate the sessions please. Unclear? If so, check the first part of the series.

Single session

The scenario in which all activities are aggregated into a single session.

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"Activities" should "be aggregated into single session" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:01:00", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:00") {
    containSingleValueAtTime("00:10:59.999", 
      ("joe", Iterable("open app", "close app")))
  }
}
  • The results window is defined as the half-open interval [00:00:00, 00:11:00)
  • End boundary gets 00:11:00 time, ten minutes (session gap duration) after the last observed activity event time
  • The result gets end-of-window time 00:10:59.999 as a new event-time
  • It means, that every session window in the pipeline introduces additional latency
  • Longer window requires more resources allocated by streaming runtime for keeping the state

Two simultaneous sessions

The scenario in which activities from two clients are aggregated into two simultaneous sessions.

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"Activities from two clients" should "be aggregated into two simultaneous sessions" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:00:00", ("ben", "open app"))
    .addElementsAtTime("00:01:00", ("joe", "close app"))
    .addElementsAtTime("00:01:30", ("ben", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:00") {
    containSingleValueAtTime("00:10:59.999", 
      ("joe", Iterable("open app", "close app")))
  }

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:30") {
    containSingleValueAtTime("00:11:29.999", 
      ("ben", Iterable("open app", "close app")))
  }
}
  • Again, all result elements get end-of-window time
  • End-of-window is calculated as the latest observed activity event time plus ten minutes gap
  • The activities from each user are aggregated into the independent, two simultaneous sessions

Single session with out-of-order activities

Out of order events are the everyday life in the distributed systems. The function activitiesInSessionWindow() should order the activities within the session by event time.

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"Out-of-order activities" should "be aggregated into single session" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:01:00", ("joe", "close app"))
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:00") {
    containSingleValueAtTime("00:10:59.999", ("joe", 
      Iterable("open app", "close app")))
  }
}
  • The activities in the session are ordered by activity event time
  • The result element gets end-of-window time, ten minutes after the oldest event time
  • The oldest event don’t have to be necessarily the latest one

Continues long-lasting session

Let’s check if the continuous session is preserved for activities that span for a longer period of time but still within the allowed gap.

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"Continuous activities" should "be aggregated into single session" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:01:30", ("joe", "show product"))
    .addElementsAtTime("00:03:00", ("joe", "add to cart"))
    .addElementsAtTime("00:09:30", ("joe", "checkout"))
    .addElementsAtTime("00:13:10", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:23:10") {
    containSingleValueAtTime(
      "00:23:09.999",
      ("joe", Iterable("open app", "show product", "add to cart", "checkout", "close app")))
  }
}
  • There is no gap longer than 10 minutes in the activities stream
  • As expected, the single, continuous session is produced
  • Result element gets end-of-window time 00:23:09.999, ten minutes more than the oldest observed event time

Interrupted long-lasting session

What if the gap between events is longer than ten minutes?

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"Interrupted activities" should "be aggregated into two sessions" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app")
    .addElementsAtTime("00:01:30", ("joe", "show product"))
    .addElementsAtTime("00:03:00", ("joe", "add to cart"))
    .addElementsAtTime("00:13:00", ("joe", "checkout"))
    .addElementsAtTime("00:13:10", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:13:00") {
    containSingleValueAtTime("00:12:59.999",
      ("joe", Iterable("open app", "show product", "add to cart")))
  }

  results.withTimestamp should inOnTimePane("00:13:00", "00:23:10") {
    containSingleValueAtTime("00:23:09.999", 
      ("joe", Iterable("checkout", "close app")))
  }
}
  • The gap between “add to cart” and “checkout” is too long to produce the single session
  • Two independent sessions are produced
  • Result element always gets end-of-window time, ten minutes more than the oldest observed activity for the given session

Late activity without allowed lateness

We have slowly moved into the more interesting scenarios. What happens if the late activity will be observed in the stream and the allowed lateness isn’t explicitly defined?

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"Late activity" should "be silently discarded" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:01:30", ("joe", "show product"))
    .advanceWatermarkTo("00:13:00")
    .addElementsAtTime("00:03:00", ("joe", "add to cart")) // late activity
    .addElementsAtTime("00:09:30", ("joe", "checkout"))
    .addElementsAtTime("00:13:10", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(sc.testStream(activities), TenMinutesGap)

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:30") {
    containSingleValueAtTime("00:11:29.999", 
      ("joe", Iterable("open app", "show product")))
  }

  results.withTimestamp should inWindow("00:00:00", "00:13:00") {
    beEmpty
  }

  results.withTimestamp should inOnTimePane("00:09:30", "00:23:10") {
    containSingleValueAtTime("00:23:09.999", 
      ("joe", Iterable("checkout", "close app")))
  }
}
  • Late event is simply discarded, there is no result produced for window [00:00:00, 00:13:00)!
  • The activity “add to cart” at 00:03:00 is late event, watermark has been already advanced to 00:13:00
  • The first session is produces as expected, when watermark has passed allowed gap after the last observed event

Did you guess that the second session should have only the “close app” event? As you can see, you were wrong. The “checkout” activity at 00:09:30 has been counted as well. As long as watermark was hold on 00:13:00 all events after 00:03:00 would be aggregated into a new session window (watermark minus the allowed gap). Finally, the “checkout” activity creates a new session window, starting at “00:09:30”.

Late activity within allowed lateness (Fired Panes Discarded)

How late events will be handled if the late activity is observed within the allowed lateness?

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"Late activity within allowed lateness" should "be aggregated into late pane" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:01:30", ("joe", "show product"))
    .advanceWatermarkTo("00:13:00")
    .addElementsAtTime("00:03:00", ("joe", "add to cart")) // late activity within allowed lateness
    .addElementsAtTime("00:09:30", ("joe", "checkout"))
    .addElementsAtTime("00:13:10", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(
    sc.testStream(activities),
    TenMinutesGap,
    allowedLateness = Duration.standardMinutes(5))

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:30") {
    containSingleValueAtTime("00:11:29.999", 
      ("joe", Iterable("open app", "show product")))
  }

  results.withTimestamp should inLatePane("00:00:00", "00:13:00") {
    containSingleValueAtTime("00:12:59.999", 
      ("joe", Iterable("add to cart")))
  }

  results.withTimestamp should inOnTimePane("00:00:00", "00:23:10") {
    containSingleValueAtTime("00:23:09.999", 
      ("joe", Iterable("checkout", "close app")))
  }
}
  • The “add to cart” activity at 00:03:00 is late but within allowed lateness
  • Nothing special to the first session window
  • There is an additional session produced in the late pane, it contains only the late event
  • The session window for late pane starts at 00:00:00
  • Because the late event has closed the gap between the sessions, the second on-time session also starts at 00:00:00

Why are the activities from the first session not been preserved and counted into the second and third sessions?

Late activity within allowed lateness (Fired Panes Accumulated)

To accumulate activities from already fired panes the accumulation mode has to be defined as ACCUMULATING_FIRED_PANES.

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"Late activity within allowed lateness" should "be aggregated and accumulated into late pane" in runWithContext { sc =>
  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "open app"))
    .addElementsAtTime("00:01:30", ("joe", "show product"))
    .advanceWatermarkTo("00:13:00")
    .addElementsAtTime("00:03:00", ("joe", "add to cart")) // late event within allowed lateness
    .addElementsAtTime("00:09:30", ("joe", "checkout"))
    .addElementsAtTime("00:13:10", ("joe", "close app"))
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(
    sc.testStream(activities),
    TenMinutesGap,
    allowedLateness = Duration.standardMinutes(5),
    accumulationMode = AccumulationMode.ACCUMULATING_FIRED_PANES)

  results.withTimestamp should inOnTimePane("00:00:00", "00:11:30") {
    containSingleValueAtTime("00:11:29.999", 
      ("joe", Iterable("open app", "show product")))
  }

  results.withTimestamp should inLatePane("00:00:00", "00:13:00") {
    containSingleValueAtTime("00:12:59.999", 
      ("joe", Iterable("open app", "show product", "add to cart")))
  }

  results.withTimestamp should inOnTimePane("00:00:00", "00:23:10") {
    containSingleValueAtTime("00:23:09.999", 
      ("joe", Iterable("open app", "show product", "add to cart", "checkout", "close app")))
  }
}
  • The accumulationMode parameter is set to ACCUMULATING_FIRED_PANES
  • The first session is produced as before
  • The session in the late pane contains all activities from already fired, on-time pane
  • Now, it’s much easier to understand why late pane window starts at 00:00:00
  • There is also second on-time pane with all the events, the late event has closed the gap and the session with all observed activities is produced effectively

The aggregation in ACCUMULATING_FIRED_PANES mode could effectively produce some duplicates. All downstream processing steps must be aware of that fact. In the above scenario they could use the latest observed session as the most complete one. The previously emitted, partially overlapped session for user “joe” might be discarded.

Speculative results

It’s time for the last, the most comprehensive example in the blog post.

If the user plays with our e-commerce site for a very long time, the session would be produced at the very end of his/her journey. It could take hours, what if we need more up-to-date results, even if the user session has not been finished yet?

We should define the more comprehensive trigger for the session window. The following trigger defines that the aggregation will produce early, speculative results every minute after the first element has been seen. Then the trigger produces an on-time result when the watermark passes the end of the window (a default behaviour). Finally, the late results will be produced as well, on every observed late activity.

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AfterWatermark
  .pastEndOfWindow()
  .withEarlyFirings(
    AfterProcessingTime.pastFirstElementInPane().plusDelayOf(OneMinute))
  .withLateFirings(
    AfterPane.elementCountAtLeast(1))

Instead of e-commerce action names I decided to introduce simple indices: 0, 1, 2 … It should make the reasoning about processing time a little easier for the readers :)

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"Activities" should "be aggregated speculatively on every minute, on-time, and finally on every late activity" in runWithContext { sc =>
  val OneMinute = Duration.standardMinutes(1L)

  val activities = testStreamOf[(User, Activity)]
    .addElementsAtTime("00:00:00", ("joe", "0")).advanceProcessingTime(OneMinute)
    .addElementsAtTime("00:01:00", ("joe", "1")).advanceProcessingTime(OneMinute)
    .addElementsAtTime("00:02:00", ("joe", "2")).advanceProcessingTime(OneMinute)
    .addElementsAtTime("00:03:00", ("joe", "3")).advanceProcessingTime(OneMinute)
    .addElementsAtTime("00:04:00", ("joe", "4")).advanceProcessingTime(OneMinute)
    .addElementsAtTime("00:05:00", ("joe", "5")).advanceProcessingTime(OneMinute)
    .advanceWatermarkTo("00:20:00") // more than 00:05:00 + 10 minutes of gap
    .addElementsAtTime("00:06:00", ("joe", "6")) // late event within allowed lateness
    .addElementsAtTime("00:07:00", ("joe", "7")) // late event within allowed lateness
    .advanceWatermarkToInfinity()

  val results = activitiesInSessionWindow(
    sc.testStream(activities),
    TenMinutesGap,
    accumulationMode = AccumulationMode.ACCUMULATING_FIRED_PANES,
    allowedLateness = Duration.standardMinutes(10),
    trigger = AfterWatermark
      .pastEndOfWindow()
      .withEarlyFirings(
        AfterProcessingTime.pastFirstElementInPane().plusDelayOf(OneMinute))
      .withLateFirings(
        AfterPane.elementCountAtLeast(1))
  )
  (...)
}
  • In addition to the watermark, the processing time is advanced as well to fire early, speculative results
  • To make a speculative results more meaningful, the ACCUMULATING_FIRED_PANES mode is configured

Early, speculative results:

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results.withTimestamp should inEarlyPane("00:00:00", "00:11:00") {
  containSingleValueAtTime("00:10:59.999", 
    ("joe", Iterable("0", "1")))
}
results.withTimestamp should inEarlyPane("00:00:00", "00:13:00") {
  containSingleValueAtTime("00:12:59.999", 
    ("joe", Iterable("0", "1", "2", "3")))
}
results.withTimestamp should inEarlyPane("00:00:00", "00:15:00") {
  containSingleValueAtTime("00:14:59.999", 
    ("joe", Iterable("0", "1", "2", "3", "4", "5")))
}
  • The first speculative result is produced one minute after “0” activity, so it contains “0” and “1” activities
  • The first pane emission resets the trigger, so the second speculative result is again produced 1 minute after “2” activity and contains activities from “0” to “3”
  • There is also a third early pane, with all already observed activities
  • The event time of the results is always the oldest observed event time plus the configured gap of ten minutes
  • The downstream steps in the pipeline get the speculative results even if the session has not been completed yet

On-time result:

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results.withTimestamp should inOnTimePane("00:00:00", "00:15:00") {
  containSingleValueAtTime("00:14:59.999", 
    ("joe", Iterable("0", "1", "2", "3", "4", "5")))
}
  • The on-time pane is produced when watermark has passed
  • Window and elements times are exactly like for the latest early-pane, because there has not been any new activity observed since the last early-pane was emitted
  • When on-time pane is emitted, the downstream steps in the pipeline could decide to invalidate the speculative results and use the complete session instead

Late results:

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results.withTimestamp should inLatePane("00:00:00", "00:16:00") {
  containSingleValueAtTime("00:15:59.999", ("joe", Iterable("0", "1", "2", "3", "4", "5", "6")))
}
results.withTimestamp should inLatePane("00:00:00", "00:17:00") {
  containSingleValueAtTime("00:16:59.999", ("joe", Iterable("0", "1", "2", "3", "4", "5", "6", "7")))
}
  • The late panes are triggered on every late element and the session contains all already observed actions (ACCUMULATING_FIRED_PANES mode)
  • You should already know why all the panes get the 00:00:00 as a start of the window, if not - study the previous examples again :)
  • The downstream steps in the pipeline could decide to take into account late results or not

Summary

Feel free to inspect source code to get the whole picture of the presented examples. Below you will find the actual activitiesInSessionWindow implementation:

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type User = String
type Activity = String

def activitiesInSessionWindow(
  activities: SCollection[(User, Activity)],
  gapDuration: Duration,
  allowedLateness: Duration = Duration.ZERO,
  accumulationMode: AccumulationMode = AccumulationMode.DISCARDING_FIRED_PANES,
  trigger: Trigger = DefaultTrigger.of()
): SCollection[(User, Iterable[Activity])] = {
  val windowOptions = WindowOptions(
    allowedLateness = allowedLateness,
    accumulationMode = accumulationMode,
    trigger = trigger,
  )
  
  activities
    .withTimestamp
    .map { case ((user, action), timestamp) => (user, (action, timestamp)) }
    .withSessionWindows(gapDuration, windowOptions)
    .groupByKey
    .mapValues(sortByTimestamp)
    .mapValues(withoutTimestamp)
}

private def sortByTimestamp(activities: Iterable[(Activity, Instant)]): Iterable[(Activity, Instant)] = {
  val ordering: Ordering[(Activity, Instant)] = Ordering.by { case (_, instant) => instant.getMillis }
  activities.toSeq.sorted(ordering)
}

private def withoutTimestamp(activities: Iterable[(Activity, Instant)]): Iterable[Activity] =
  activities.map(_._1)

Key takeaways:

  • Data-driven windows like session window greatly widen the streaming processing design patterns
  • Session window is much more complex for reasoning and test than fixed window
  • Late event under allowed lateness closes the gap of already produced sessions, the results could be surprising
  • Session window could never end, but to get the speculative, early results you can define the special trigger
  • Early results are always triggered when the processing time advances, there is no correlation with the event time

Let me know what you think as a comment below.

Updated:

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