Hi Jan, It's a good question. Your observations are correct. A KTable is sometimes materialised to a changelog topic lazily, depending on whether the result in the KTable is needed for subsequent operations or not.
We are working to improving the documentation of the materialisation rules and should have something shortly to share on the wiki https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Streams+Discussions. Thanks Eno > On 16 Jun 2016, at 12:45, Jan Ehrhardt <jan.ehrha...@gmail.com> wrote: > > Hi, > > I am curious about the relationship of KTables and state store in Kafka > Streams. When I create a table with the `table` method, it uses the > original topic as a changelog, thus persisting a table changelog does not > make much sense. When I create invoke `countByKey` on a KStream, the result > is a KTable and I find a changelog topic for it created. Thus Kafka Streams > can recreate the state store. When I join two tables, this will also result > in a new KTable, but this time, there is no changelog topic created. > > This is a little bit confusing. Does the `table` method read the whole > original topic as an input again after a restart, or how does it load the > whole table again? Can someone explain the rules to persist or restore a > KTable to or from a changelog? > > Best regards > Jan
