Timestamp synchronization is not perfect, and as a matter of fact, we
fixed a few gaps in 3.0.0 release. We actually hope, that we closed the
last gaps in 3.0.0... *fingers-crossed* :)
We are using a timestamp extractor that returns 0.
You can do this, and it effectively "disables" timestamp synchronization
as records on the KTable side don't have a timeline any longer. As a
side effect it also allows you to "bootstrap" the table, as records with
timestamp zero will always be processed first (as they are smaller). Of
course, you also don't have time synchronization for "future" data and
your program becomes non-deterministic if you reprocess old data.
his seemed to be the only
way to bootstrap enough records at startup to avoid the missed join.
Using 3.0.0 and enabling timestamp synchronization via
`max.task.idle.ms` config, should allow you to get the correct behavior
without the zero-extractor (of course, your KTable data must have
smaller timestamps that your KStream data).
If I use "timestamp synchronization" do I have to remove the zero
timestamp extractor? If I remove the zero timestamp extractor will
timestamp synchronization take care of the missed join issue on startup?
To be more precise: timestamp synchronization is _always_ on. The
question is just how strict it is applied. By default, we do the weakest
from which is only best effort.
I'm guessing the issue here is that occasionally the poll request is not
returning the matching record for the KTable side of the join before the
task goes off and starts processing records.
Yes, because of default best effort approach. That is why you should
increase `max.task.idle.ms` to detect this case and "skip" processing
and let KS do another poll() to get KTable data.
2.8 and earlier:
max.task.idle.ms=0 -> best effort (no poll() retry)
max.task.idle.ms>0 -> try to do another poll() until data is there or
idle time passed
Note: >0 might still "fail" even if there is data, because consumer
fetch behavior is not predictable.
3.0:
max.task.idle.ms=-1 -> best effort (no poll() retry)
max.task.idle.ms=0 -> if there is data broker side, repeat to poll()
until you get the data
max.task.idle.ms>0 -> even if there is not data broker side, wait until
data becomes available or the idle time passed
Hope this helps.
-Matthias
On 11/1/21 4:29 PM, Guozhang Wang wrote:
Hello Chad,
From your earlier comment, you mentioned "In my scenario the records were
written to the KTable topic before the record was written to the KStream
topic." So I think Matthias and others have excluded this possibility while
trying to help investigate.
If only the matching records from KStream are returned via a single a
consumer poll call but not the other records from KTable, then you would
miss this matched join result.
Guozhang
On Sun, Oct 31, 2021 at 7:28 AM Chad Preisler <chad.preis...@gmail.com>
wrote:
Thank you for your response and the links to the presentations.
*However, this seems tobe orthogonal to your issue?*
Yes. From what I see in the code it looks like you have a single consumer
subscribed to multiple topics. Please correct me if I'm wrong.
*By default, timestamp synchronization is disabled. Maybeenabling it would
help?*
We are using a timestamp extractor that returns 0. We did that because we
were almost always missing joins on startup, and this seemed to be the only
way to bootstrap enough records at startup to avoid the missed join. We
found a post that said doing that would make the KTable act like the
GlobalKTable at startup. So far this works great, we never miss a join on a
startup. If I use "timestamp synchronization" do I have to remove the zero
timestamp extractor? If I remove the zero timestamp extractor will
timestamp synchronization take care of the missed join issue on startup?
I'm guessing the issue here is that occasionally the poll request is not
returning the matching record for the KTable side of the join before the
task goes off and starts processing records. Later when we put the same
record on the topic and the KTable has had a chance to load more records
the join works and everything is good to go. Because of the way our system
works no new status records have been written and so the new record joins
against the correct status.
Do you agree that the poll request is returning the KStream record but not
returning the KTable record and therefore the join is getting missed? If
you don't agree, what do you think is going on? Is there a way to prove
this out?
Thanks,
Chad
On Sat, Oct 30, 2021 at 2:09 PM Matthias J. Sax <mj...@apache.org> wrote:
Yes, a StreamThread has one consumer. The number of StreamThreads per
instance is configurable via `num.stream.threads`. Partitions are
assigned to threads similar to consumer is a plain consumer group.
It seems you run with the default of one thread per instance. As you
spin up 12 instances, it results in 12 threads for the application. As
you have 12 partitions, using more threads won't be useful as no
partitions are left for them to process.
For a stream-table joins, there will be one task per "partition pair"
that computes the join for those partitions. So you get 12 tasks, and
each thread processes one task in your setup. Ie, a thread consumer is
reading data for both input topics.
Pausing happens on a per-partition bases: for joins there is two buffers
per task (one for each input topic partition). It's possible that one
partition is paused while the other is processed. However, this seems to
be orthogonal to your issue?
For a GlobalKTable, you get an additional GlobalThread that only reads
the data from the "global topic" to update the GlobalKTable. Semantics
of KStream-KTable and KStream-GlobalKTable joins are different: Cf
https://www.confluent.io/events/kafka-summit-europe-2021/temporal-joins-in-kafka-streams-and-ksqldb/
For the timestamp synchronization, you may checkout `max.task.idle.ms`
config. By default, timestamp synchronization is disabled. Maybe
enabling it would help?
You may also check out slides 34-38:
https://www.confluent.io/kafka-summit-san-francisco-2019/whats-the-time-and-why/
There is one corner case: if two records with the same timestamp come
it, it's not defined which one will be processed first.
Hope this helps.
-Matthias
On 10/30/21 6:45 AM, Chad Preisler wrote:
Yes, this helped. I have some additional questions.
Does StreamThread have one consumer? (Looks like it, but just want to
confirm)
Is there a separate StreamThread for each topic including the KTable?
If a KTable is a StreamThread and there is a StreamTask for that
KTable,
could my buffer be getting filled up, and the mainConsumer for the
KTable
be getting paused? I see this code in StreamTask#addRecords.
// if after adding these records, its partition queue's buffered size
has
been
// increased beyond the threshold, we can then pause the
consumption for this partition
if (newQueueSize > maxBufferedSize) {
mainConsumer.pause(singleton(partition));
}
Is there any specific logging that I can set to debug or trace that
would
help me troubleshoot? I'd prefer not to turn debug and/or trace on for
every single class.
Thanks,
Chad
On Sat, Oct 30, 2021 at 5:20 AM Luke Chen <show...@gmail.com> wrote:
Hi Chad,
I'm wondering if someone can point me to the Kafka streams internal
code
that reads records for the join?
--> You can check StreamThread#pollPhase, where stream thread (main
consumer) periodically poll records. And then, it'll process each
topology
node with these polled records in stream tasks (StreamTask#process).
Hope that helps.
Thanks.
Luke
On Sat, Oct 30, 2021 at 5:42 PM Gilles Philippart
<gilles.philipp...@fundingcircle.com.invalid> wrote:
Hi Chad, this talk around 24:00 clearly explains what you’re seeing
https://www.confluent.io/events/kafka-summit-europe-2021/failing-to-cross-the-streams-lessons-learned-the-hard-way/
<
https://www.confluent.io/events/kafka-summit-europe-2021/failing-to-cross-the-streams-lessons-learned-the-hard-way/
Gilles
On 30 Oct 2021, at 04:02, Chad Preisler <chad.preis...@gmail.com>
wrote:
Hello,
I have a stream application that does a KStream to KTable left join.
We
seem to be occasionally missing joins (KTable side is null).
I'm wondering if someone can point me to the Kafka streams internal
code
that reads records for the join? I've poked around the Kafka code
base,
but
there is a lot there. I imagine there is some consumer poll for each
side
of the join, and possibly a background thread for reading the KTable
topic.
I figure there are several possible causes of this issue, and since
nothing
is really jumping out in my code, I was going to start looking at
the
Kafka
code to see if there is something I can do to fix this.
Thanks,
Chad
--
Funding Circle Limited is authorised and regulated by the Financial
Conduct Authority under firm registration number 722513. Funding
Circle
is
not covered by the Financial Services Compensation Scheme. Registered
in
England (Co. No. 06968588) with registered office at 71 Queen
Victoria
Street, London EC4V 4AY.
