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https://issues.apache.org/jira/browse/KAFKA-15297?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17751821#comment-17751821
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Guozhang Wang commented on KAFKA-15297:
---------------------------------------
I think this is indeed a general issue, that state stores are initialized in
the order of the topology which is essentially the "processor node order", as
in ``InternalTopologyBuilder#build``. This works when a state store is only
associated with one processors, or when a store is associated with multiple
processors but they are built as part of a built-in operator (like a join in
DSL) in which case we carefully make sure that state stores order is adherent
with the processors order; but in a PAPI scenario like Bruno reported in this
one, all bets are off.
I think a general fix would be, in the above ``build`` function, we only build
the processors in the first loop pass without initializing the state stores,
and then based on the built processors order the state stores to be
initialized, and do that in another pass.
> Cache flush order might not be topological order
> -------------------------------------------------
>
> Key: KAFKA-15297
> URL: https://issues.apache.org/jira/browse/KAFKA-15297
> Project: Kafka
> Issue Type: Bug
> Components: streams
> Affects Versions: 3.4.0
> Reporter: Bruno Cadonna
> Priority: Major
> Attachments: minimal_example.png
>
>
> The flush order of the state store caches in Kafka Streams might not
> correspond to the topological order of the state stores in the topology. The
> order depends on how the processors and state stores are added to the
> topology.
> In some cases downstream state stores might be flushed before upstream state
> stores. That means, that during a commit records in upstream caches might end
> up in downstream caches that have already been flushed during the same
> commit. If a crash happens at that point, those records in the downstream
> caches are lost. Those records are lost for two reasons:
> 1. Records in caches are only changelogged after they are flushed from the
> cache. However, the downstream caches have already been flushed and they will
> not be flushed again during the same commit.
> 2. The offsets of the input records that caused the records that now are
> blocked in the downstream caches are committed during the same commit and so
> they will not be re-processed after the crash.
> An example for a topology where the flush order of the caches is wrong is the
> following:
> {code:java}
> final String inputTopic1 = "inputTopic1";
> final String inputTopic2 = "inputTopic2";
> final String outputTopic1 = "outputTopic1";
> final String processorName = "processor1";
> final String stateStoreA = "stateStoreA";
> final String stateStoreB = "stateStoreB";
> final String stateStoreC = "stateStoreC";
> streamsBuilder.stream(inputTopic2, Consumed.with(Serdes.String(),
> Serdes.String()))
> .process(
> () -> new Processor<String, String, String, String>() {
> private ProcessorContext<String, String> context;
> @Override
> public void init(ProcessorContext<String, String> context) {
> this.context = context;
> }
> @Override
> public void process(Record<String, String> record) {
> context.forward(record);
> }
> @Override
> public void close() {}
> },
> Named.as("processor1")
> )
> .to(outputTopic1, Produced.with(Serdes.String(), Serdes.String()));
> streamsBuilder.stream(inputTopic1, Consumed.with(Serdes.String(),
> Serdes.String()))
> .toTable(Materialized.<String, String, KeyValueStore<Bytes,
> byte[]>>as(stateStoreA).withKeySerde(Serdes.String()).withValueSerde(Serdes.String()))
> .mapValues(value -> value, Materialized.<String, String,
> KeyValueStore<Bytes,
> byte[]>>as(stateStoreB).withKeySerde(Serdes.String()).withValueSerde(Serdes.String()))
> .mapValues(value -> value, Materialized.<String, String,
> KeyValueStore<Bytes,
> byte[]>>as(stateStoreC).withKeySerde(Serdes.String()).withValueSerde(Serdes.String()))
> .toStream()
> .to(outputTopic1, Produced.with(Serdes.String(), Serdes.String()));
> final Topology topology = streamsBuilder.build(streamsConfiguration);
> topology.connectProcessorAndStateStores(processorName, stateStoreC);
> {code}
> This code results in the attached topology.
> In the topology {{processor1}} is connected to {{stateStoreC}}. If
> {{processor1}} is added to the topology before the other processors, i.e., if
> the right branch of the topology is added before the left branch as in the
> code above, the cache of {{stateStoreC}} is flushed before the caches of
> {{stateStoreA}} and {{stateStoreB}}.
> You can observe the flush order by feeding some records into the input topics
> of the topology, waiting for a commit, and looking for the following log
> message:
> https://github.com/apache/kafka/blob/2e1947d240607d53f071f61c875cfffc3fec47fe/streams/src/main/java/org/apache/kafka/streams/processor/internals/ProcessorStateManager.java#L513
>
> I changed the log message from trace to debug to avoid too much noise.
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