[KAFKA-7492] Explain `null` handling for reduce and aggregate

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.3.0
• Fix Version/s: 2.3.0
• Component/s: documentation, streams
• Labels: beginner docs newbie
• Assignee: Asutosh Pandya
• Reporter: Matthias J. Sax
• Created: 09/Oct/18 01:45
• Updated: 01/Mar/19 21:40
• Resolved: 22/Feb/19 17:24

Description

We currently don't explain how records with `null` value are handled in reduce and aggregate. In particular, what happens when the users' aggregation/reduce `apply()` implementation returns `null`.

We should update the JavaDocs accordingly and maybe also update the docs on the web page.

Cf. https://stackoverflow.com/questions/52692202/what-happens-if-the-aggregator-of-a-kgroupedstream-returns-null

Issue Links

• links to: GitHub Pull Request #6285