John,
Thanks for the nice explanation. When the repartitioning happens, does the
window get associated with the new partition i.e., now does a message with new
timestamp has to appear on the repartition topic for the window to expire ? It
is possible that there is new stream of messages coming in but post-map
operation, the partitions in the repartitioned topic does not see the same
thing.
Thanks
Mohan
On 6/24/19, 7:49 AM, "John Roesler" <j...@confluent.io> wrote:
Hey, this is a very apt question.
GroupByKey isn't a great example because it doesn't actually change
the key, so all the aggregation results are actually on records from
the same partition. But let's say you do a groupBy or a map (or any
operation that can change the key), followed by an aggregation. Now
it's possible that the aggregation would need to process records from
two different partitions. In such a case (key-changing operation
followed by a stateful operation), Streams actually round-trips the
data through an intermediate topic, called a repartition topic, before
the aggregation. This has the effect, similar to the "shuffle" phase
of map-reduce, of putting all the data into its *new* right partition,
so then the aggregation can still process each of its partitions
independently.
Regarding the latter statement, even though you only have one
instance, Streams _still_ processes each partition independently. The
"unit of work" responsible for processing a partition is called a
"task". So if you have 4 partitions, then your one instance actually
has 4 state stores, one for each task, where each task only gets
records from a single partition. The tasks can't see anything about
each other, not their state nor other metadata like their current
stream time. Otherwise, the results would depend on which tasks happen
to be co-located with which other tasks. So, having to send your
"purge" event to all partitions is a pain, but in the end, it buys you
a lot, as you can add another instance to your cluster at any time,
and Streams will scale up, and you'll know that the program is
executing exactly the same way the whole time.
-John
On Sat, Jun 22, 2019 at 4:37 PM Parthasarathy, Mohan <mpart...@hpe.com>
wrote:
>
> I can see the issue. But it raised other questions. Pardon my ignorance.
Even though partitions are processed independently, windows can be aggregating
state from records read from many partitions. Let us say there is a groupByKey
followed by aggregate. In this case how is the state reconciled across all the
application instances ? Is there a designated instance for a particular key ?
>
> In my case, there was only one instance processing records from all
partitions and it is kind of odd that windows did not expire even though I
understand why now.
>
> Thanks
> Mohan
>
>
> On 6/21/19, 2:25 PM, "John Roesler" <j...@confluent.io> wrote:
>
> No problem. It's definitely a subtlety. It occurs because each
> partition is processed completely independently of the others, so
> "stream time" is tracked per partition, and there's no way to look
> across at the other partitions to find out what stream time they have.
>
> In general, it's not a problem because you'd expect all partitions to
> receive updates over time, but if you're specifically trying to send
> events that cause stuff to get flushed from the buffers, it can mess
> with you. It's especially notable in tests. So, for most tests, I just
> configure the topics to have one partition.
>
> -John
>
> On Fri, Jun 21, 2019 at 3:56 PM Parthasarathy, Mohan
<mpart...@hpe.com> wrote:
> >
> > That change "In the same partition" must explain what we are
seeing. Unless you see one message per partition, all windows will not expire.
That is an interesting twist. Thanks for the correction ( I will go back and
confirm this.
> >
> > -mohan
> >
> >
> > On 6/21/19, 12:40 PM, "John Roesler" <j...@confluent.io> wrote:
> >
> > Sure, the record cache attempts to save downstream operators
from
> > unnecessary updates by also buffering for a short amount of time
> > before forwarding. It forwards results whenever the cache fills
up or
> > whenever there is a commit. If you're happy to wait at least
"commit
> > interval" amount of time for updates, then you don't need to do
> > anything, but if you're on the edge of your seat, waiting for
these
> > results, you can set cache.max.bytes.buffering to 0 to disable
the
> > record cache entirely. Note that this would hurt throughput in
> > general, though.
> >
> > Just a slight modification:
> > * a new record with new timestamp > (all the previous
timestamps +
> > grace period) will cause all the old windows *in the same
partition*
> > to close
> > * yes, expiry of the window depends only on the event time
> >
> > Hope this helps!
> > -John
> >
> > On Thu, Jun 20, 2019 at 11:42 AM Parthasarathy, Mohan
<mpart...@hpe.com> wrote:
> > >
> > > Could you tell me a little more about the delays about the
record caches and how I can disable it ?
> > >
> > > If I could summarize my problem:
> > >
> > > -A new record with a new timestamp > all records sent before,
I expect *all* of the old windows to close
> > > -Expiry of the windows depends only on the event time and not
on the key
> > >
> > > Are these two statements correct ?
> > >
> > > Thanks
> > > Mohan
> > >
> > > On 6/20/19, 9:17 AM, "John Roesler" <j...@confluent.io> wrote:
> > >
> > > Hi!
> > >
> > > In addition to setting the grace period to zero (or some
small
> > > number), you should also consider the delays introduced
by record
> > > caches upstream of the suppression. If you're closely
watching the
> > > timing of records going into and coming out of the
topology, this
> > > might also spoil your expectations. You could always
disable the
> > > record cache to make the system more predictable
(although this would
> > > hurt throughput in production).
> > >
> > > Thanks,
> > > -John
> > >
> > > On Wed, Jun 19, 2019 at 3:01 PM Parthasarathy, Mohan
<mpart...@hpe.com> wrote:
> > > >
> > > > We do explicitly set the grace period to zero. I am
going to try the new version
> > > >
> > > > -mohan
> > > >
> > > >
> > > > On 6/19/19, 12:50 PM, "Parthasarathy, Mohan"
<mpart...@hpe.com> wrote:
> > > >
> > > > Thanks. We will give it a shot.
> > > >
> > > > On 6/19/19, 12:42 PM, "Bruno Cadonna"
<br...@confluent.io> wrote:
> > > >
> > > > Hi Mohan,
> > > >
> > > > I realized that my previous statement was not
clear. With a grace
> > > > period of 12 hour, suppress would wait for late
events until stream
> > > > time has advanced 12 hours before a result
would be emitted.
> > > >
> > > > Best,
> > > > Bruno
> > > >
> > > > On Wed, Jun 19, 2019 at 9:21 PM Bruno Cadonna
<br...@confluent.io> wrote:
> > > > >
> > > > > Hi Mohan,
> > > > >
> > > > > if you do not set a grace period, the grace
period defaults to 12
> > > > > hours. Hence, suppress would wait for an
event that occurs 12 hour
> > > > > later before it outputs a result. Try to
explicitly set the grace
> > > > > period to 0 and let us know if it worked.
> > > > >
> > > > > If it still does not work, upgrade to version
2.2.1 if it is possible
> > > > > for you. We had a couple of bugs in suppress
recently that are fixed
> > > > > in that version.
> > > > >
> > > > > Best,
> > > > > Bruno
> > > > >
> > > > > On Wed, Jun 19, 2019 at 8:37 PM
Parthasarathy, Mohan <mpart...@hpe.com> wrote:
> > > > > >
> > > > > > No, I have not set any grace period. Is
that mandatory ? Have you seen problems with suppress and windows expiring ?
> > > > > >
> > > > > > Thanks
> > > > > > Mohan
> > > > > >
> > > > > > On 6/19/19, 12:41 AM, "Bruno Cadonna"
<br...@confluent.io> wrote:
> > > > > >
> > > > > > Hi Mohan,
> > > > > >
> > > > > > Did you set a grace period on the
window?
> > > > > >
> > > > > > Best,
> > > > > > Bruno
> > > > > >
> > > > > > On Tue, Jun 18, 2019 at 2:04 AM
Parthasarathy, Mohan <mpart...@hpe.com> wrote:
> > > > > > >
> > > > > > > On further debugging, what we are
seeing is that windows are expiring rather randomly as new messages are being
processed. . We tested with new key for every new message. We waited for the
window time before replaying new messages. Sometimes a new message would come
in and create state. It takes several messages to make some of the old windows
to be closed (go past suppress to the next stage). We have also seen where one
of them never closed even but several other older ones expired. Then we
explicitly sent a message with the same old key and then it showed up. Also,
for every new message, only one of the previous window expires even though
there are several pending.
> > > > > > >
> > > > > > > If we don't use suppress, then there
is never an issue. With suppress, the behavior we are seeing is weird. We are
using 2.1.0 version in DSL mode. Any clues on what we could be missing ? Why
isn't there an order in the way windows are closed ? As event time progresses
by the new messages arriving, the older ones should expire. Is that right
understanding or not ?
> > > > > > >
> > > > > > > Thanks
> > > > > > > Mohan
> > > > > > >
> > > > > > > On 6/17/19, 3:43 PM, "Parthasarathy,
Mohan" <mpart...@hpe.com> wrote:
> > > > > > >
> > > > > > > Hi,
> > > > > > >
> > > > > > > We are using suppress in the
application. We see some state being created at some point in time. Now there
is no new data for a day or two. We send new data but the old window of data
(where we see the state being created) is not closing i.e not seeing it go
through suppress and on to the next stage. It is as though the state created
earlier was purged. Is this possible ?
> > > > > > >
> > > > > > > Thanks
> > > > > > > Mohan
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > >
> > > > > >
> > > >
> > > >
> > > >
> > > >
> > >
> > >
> >
> >
>
>
