[jira] [Created] (KAFKA-5581) Streams can be smarter in deciding when to create changelog topics for state stores

["Guozhang Wang (JIRA)" <ji...@apache.org>]

Guozhang Wang created KAFKA-5581:
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             Summary: Streams can be smarter in deciding when to create changelog topics for state stores
                 Key: KAFKA-5581
                 URL: https://issues.apache.org/jira/browse/KAFKA-5581
             Project: Kafka
          Issue Type: Bug
          Components: streams
            Reporter: Guozhang Wang


Today Streams make all state stores to be backed by a changelog topic by default unless users overrides it by {{disableLogging}} when creating the state store / materializing the KTable. However there are a few cases where a separate changelog topic would not be required as we can re-use an existing topic for that. A few examples:

There are a few places where the materialized store do not need a separate changelog topic, for example:

1) If a KTable is read directly from a source topic, and is materialized i.e. 

{code}
table1 = builder.table("topic1", "store1")`.
{code}

In this case {{table1}}'s changelog topic can just be {{topic1}}, and we do not need to create a separate {{table1-changelog}} topic.

2) if a KTable is materialized and then sent directly into a sink topic with the same key, e.g.

{code}
table1 = stream.groupBy(...).aggregate("state1").to("topic2");
{code}

In this case {{state1}}'s changelog topic can just be {{topic2}}, and we do not need to create a separate {{state1-changelog}} topic anymore;

3) if a KStream is materialized for joins where the streams are directly from a topic, e.g.:

{code}
stream1 = builder.stream("topic1");
stream2 = builder.stream("topic2");
stream3 = stream1.join(stream2, windows);  // stream1 and stream2 are materialized with a changelog topic
{code}

Since stream materialization is append-only we do not need a changelog for the state store as well but can just use the source {{topic1}} and {{topic2}}.

4) When you have some simple transformation operations or even join operations that generated new KTables, and which needs to be materialized with a state store, you can use the changelog topic of the previous KTable and applies the transformation logic upon restoration instead of creating a new changelog topic. For example:

{code}
table1 = builder.table("topic1");
table2 = table1.filter(..).join(table3); // table2 needs to be materialized for joining
{code}

We can set the {{getter}} function of table2's materialized store, say {{state2}} to be reading from {{topic1}} and then apply the filter operator, instead of creating a new {{state2-changelog}} topic in this case.

5) more use cases ...

We can come up with a general internal impl optimizations to determine when / how to set the changelog topic for those materialized stores at the runtime startup when generating the topology.



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