Hello Lei, As Matthias mentioned, the key question here is that because of the late arrivals of records which may indicate a shorter session gap interval, some session windows may be "mistakenly" merged and hence need to be undone the merge, i.e. to split them again.
Back to my example, you are right that the processing result of (10, 10), (19, 5), (15, 3) .. should be the same as the processing result of (10, 10), (15, 3), (19, 5) .. Note that the second value is NOT the window end time, but the extracted window gap interval, as you suggested in the KIP this value can be dynamically changed a. If you take a look at the second ordering, when we receive (10, 10) it means a window starting at 10 is created, and its gap interval is 10, which means that if by the timestamp of 20 we do not receive any new data, then the window should be closed, i.e. the window [10, 20). b. When later we received (15, 3), it means that this record ** changed ** the window gap interval from 10 to 3, and hence we received a new record at 15, with the new window gap of 3, it means that by timestamp 18 (15 + 3) if we have not received any new data, the window should be closed, i.e. the window is now [10, 18) which includes two records at 10 and 15. c. The third record is received at 19, which is after the window close time 18, it means that we should now start a new window starting at 19, i.e. the window is [19, 24), BUT, because of the out of ordering, we did not receive (15, 3) in time, but received (19, 5), it will cause us to mistakenly merge the window of [10, 20) with [19, 24) to [10, 24), and only when later we received (15, 3) we realized that the previous window should have been ended at 18. Does that make sense to you? Guozhang On Thu, Sep 6, 2018 at 9:50 PM, Matthias J. Sax <matth...@confluent.io> wrote: > I cannot follow the example: > > >> (10, 10), (15, 3), (19, 5) ... > > First, [10,10] is created, second the window is extended to [10,15], and > third [19,19] is created. Why would there be a [15,15]? And why would > (19,5) be merged into [15,15] -- the gap was set to 3 via (15,3) and > thus [19,19] should be its own window? > > > Take a look at another example, > > (13, 3), (19, 5), (15, 3) ... > > > > in this case when (15, 3) is received, [13,13] should be retrieved and > > merged to a new window [13, 15], then [19,19] should be updated to [13, > > 19]. Correct? > > This example makes sense. However, Guozhang's example was different. The > late even, _reduces_ the gap and this can lead to a window split. > Guozhang's example was > > >>> (10, 10), (19, 5), (15, 3) ... > > First [10,10] is created, second [10,19] is create (gap is 10, so 10 and > 19 merge). Last, (15,3) reduced the gap from 10 to 3, thus [10,15] and > [19,19] must be two windows, ie, original window [10,19] must be split. > > > Or maybe you have different semantic about gaps are dynamically modified > in mind? It's a little unclear for the KIP itself what semantics dynamic > sessions windows should have. > > > What is also unclear to me atm is, what use cases you have in mind? The > KIP only says > > > the statistical aggregation result, liquidity of the records, > > > I am not sure what this means. Can you elaborate? > > > > -Matthias > > > > On 8/30/18 3:32 PM, Lei Chen wrote: > > Hi Guozhang, > > > > Thanks for reviewing the proposal. I didn't think of out of order events > > and glad that you brought it up. > > > > In the example you gave, > > > > (10, 10), (19, 5), (15, 3) ... > > > > my understanding is that the correct result window should be the same as > in > > order events > > > > (10, 10), (15, 3), (19, 5) ... > > > > when (15, 3) is received, [15,15] is creatd > > when (19, 5) is received, [15, 15] and [19, 19] are merged and [15, 19] > is > > created, meanwhile [15,15] is removed > > > > back to out of order case, > > > > when (19 ,5) is received, [19, 19] is created > > when (15, 3) is received, in order to generate the same result, > > 1. if late event is later than retention period, it will be dropped > > 2. otherwise, adjacent session windows within gap should be retrieved and > > merged accordingly, in this case [19, 19], and create a new session [15, > 19] > > I'm little confused when you said "the window [15, 15] SHOULD actually be > > expired at 18 and hence the next record (19, 5) should be for a new > session > > already.". If i understand it correctly, the expiration of the window is > > only checked when next event (19,5) comes and then it should be merged to > > it. [15, 15] will then be closed. Is that also what you meant? > > I cannot think of a case where a window will be split by a late event, > > because if event A and C fall into the same session window, a late event > B > > in middle will definitely fall into C's gap as well. IOW, late event will > > only cause window extension, not split. > > > > Take a look at another example, > > (13, 3), (19, 5), (15, 3) ... > > > > in this case when (15, 3) is received, [13,13] should be retrieved and > > merged to a new window [13, 15], then [19,19] should be updated to [13, > > 19]. Correct? > > > > To be able to achieve that, like you said, the gap needs to be stored for > > sessions. We don't need to save the gap with each event, but only for > each > > session window. To avoid upgrading existing session window, how about > > create a new Window type extended from SessionWindow along with a new > > KeySchema? > > > > What do you think? > > > > Lei > > > > > > On Fri, Aug 24, 2018 at 9:42 AM Guozhang Wang <wangg...@gmail.com> > wrote: > > > >> Hello Lei, > >> > >> Thanks for the proposal. I've just made a quick pass over it and there > is a > >> question I have: > >> > >> The session windows are defined per key, i.e. does that mean that each > >> incoming record of the key can dynamically change the gap of the window? > >> For example, say you have the following record for the same key coming > in > >> order, where the first time is the timestamp of the record, and the > second > >> value is the extracted gap value: > >> > >> (10, 10), (19, 5), ... > >> > >> > >> When we receive the first record at time 10, the gap is extracted as 10, > >> and hence the window will be expired at 20 if no other record is > received. > >> When we receive the second record at time 19, the gap is modified to 5, > and > >> hence the window will be expired at 24 if no other record is received. > >> > >> > >> If that's the case, I'm wondering how out-of-order data can be handled > >> then, consider this stream: > >> > >> (10, 10), (19, 5), (15, 3) ... > >> > >> I.e. you received a late record indicating at timestamp 15, which > shorten > >> the gap to 3. It means that the window SHOULD actually be expired at 18, > >> and hence the next record (19, 5) should be for a new session already. > >> Today Streams session window implementation does not do "window split", > so > >> have you thought about how this can be extended? > >> > >> Also since in your proposal each session window's gap value would be > >> different, we need to store this value along with each record then, how > >> would we store it, and what would be the upgrade path if it is not a > >> compatible change on disk storage etc? > >> > >> > >> > >> Guozhang > >> > >> > >> > >> On Wed, Aug 22, 2018 at 10:05 AM, Lei Chen <ley...@gmail.com> wrote: > >> > >>> Hi All, > >>> > >>> I created a KIP to add dynamic gap session window support to Kafka > >> Streams > >>> DSL. > >>> > >>> https://cwiki.apache.org/confluence/display/KAFKA/KIP- > >>> 362%3A+Support+dynamic+gap+session+window > >>> > >>> Please take a look, > >>> > >>> Thanks, > >>> Lei > >>> > >> > >> > >> > >> -- > >> -- Guozhang > >> > > > > -- -- Guozhang
