Hello Mangat, With at least once, although some records maybe processed multiple times their process ordering should not be violated, so what you observed is not expected. What caught my eyes are this section in your output changelogs (high-lighted):
Key1, V1 Key1, null Key1, V1 Key1, null (processed again) Key1, V2 Key1, null *Key1, V1Key1,V2* Key1, V2+V1 (I guess we didn't process V2 tombstone yet but reprocessed V1 again due to reassignment) They seem to be the result of first receiving a tombstone which removes V1 and then a new record that adds V2. However, since caching is disabled you should get *Key1,V1* *Key1,null* *Key1,V2* instead; without the actual code snippet I cannot tell more what's happening here. If you can look into the logs you can record each time when partition migrates, how many records from the changelog was replayed to restore the store, and from which offset on the input topic does Streams resume processing. You may also consider upgrading to 2.6.x or higher version and see if this issue goes away. Guozhang On Tue, Apr 6, 2021 at 8:38 AM mangat rai <mangatm...@gmail.com> wrote: > Hey, > > We have the following setup in our infrastructure. > > 1. Kafka - 2.5.1 > 2. Apps use kafka streams `org.apache.kafka` version 2.5.1 library > 3. Low level processor API is used with *atleast-once* semantics > 4. State stores are *in-memory* with *caching disabled* and *changelog > enabled* > > > Is it possible that during state replication and partition reassignment, > the input data is not always applied to the state store? > > 1. Let's say the input topic is having records like following > > ``` > Key1, V1 > Key1, null (tombstone) > Key1, V2 > Key1, null > Key1, V3 > Key1, V4 > ``` > 2. The app has an aggregation function which takes these record and update > the state store so that changelog shall be > > ``` > Key1, V1 > Key1, null (tombstone) > Key1, V2 > Key1, null > Key1, V3 > Key1, V3 + V4 > ``` > Let's say the partition responsible for processing the above key was > several times reallocated to different threads due to some infra issues we > are having(in Kubernetes where we run the app, not the Kafka cluster). > > I see the following record in the changelogs > > ``` > Key1, V1 > Key1, null > Key1, V1 > Key1, null (processed again) > Key1, V2 > Key1, null > Key1, V1 > Key1,V2 > Key1, V2+V1 (I guess we didn't process V2 tombstone yet but reprocessed V1 > again due to reassignment) > Key1,V1 (V2 is gone as there was a tombstone, but then V1 tombstone should > have been applied also!!) > Key1, V2+V1 (it is back!!!) > Key1,V1 > Key1, V1 + V2 + V3 (This is the final state)! > ``` > > If you see this means several things > 1. The state is always correctly applied locally (in developer laptop), > where there were no reassignments. > 2. The records are processed multiple times, which is understandable as we > have at least symantics here. > 3. As long as we re-apply the same events in the same orders we are golden > but looks like some records are skipped, but here it looks as if we have > multiple consumers reading and update the same topics, leading to race > conditions. > > Is there any way, Kafka streams' state replication could lead to such a > race condition? > > Regards, > Mangat > -- -- Guozhang
