KTable-KTable Foreign-Key Join Semantics

3
votes

The documentation from Confluent website mentions the following:

The left KTable can have multiple records which map to the same key on the right KTable. An update to a single left KTable entry may result in a single output event, provided the corresponding key exists in the right KTable. Consequently, a single update to a right KTable entry will result in an update for each record in the left KTable that has the same foreign key.

Looking at the example below the explanation:

KTable-KTable Foreign-Key Join

According to explanation, in the case of inner join (10, baz) on the right side should trigger two records on the left side that will be added to resulting join table. But in the explanation it's not in the table. Is this a typo? Or I'm missing something.

1
apache-kafkaapache-kafka-streams
From the documentation I agree that the INNER JOIN should also yield updates (q,10,baz),(r,10,baz) after the foreign key is created. But maybe there's a issue somewhere when the foreign key 10 does not exist at the moment that (q,10) and (r,10) are processed, the inner join considers them dead. – GeertPt
Can you please share a code snippet where you are making a foreign key join between two ktables? I want to do the same but I haven't been able to implement it on my own . – Ishani Vij

1 Answers

2
votes

Yes, it's an error in the docs. -- Thanks for calling it out. We will fix it.

Btw: feel free to email to [email protected] if you encounter errors in the docs so we can fix them!