MINOR: Add note about topic IDs to upgrade doc
Reviewers: Jun Rao
The motivation is to unblock our future plan to enable the strongest message delivery guarantee by default.
Custom authorizer should consider providing a more efficient implementation that supports audit logging and any custom configs or access rules.
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+ <li>
+ IBP 2.8 introduces topic IDs to topics as a part of
+ <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-516%3A+Topic+Identifiers">KIP-516</a>.
+ When using ZooKeeper, this information is stored in the TopicZNode. If the cluster is downgraded to a previous IBP or version,
+ future topics will not get topic IDs and it is not guaranteed that topics will retain their topic IDs in ZooKeeper.
+ This means that upon upgrading again, some topics or all topics will be assigned new IDs.
+ </li>
<li>Kafka Streams introduce a type-safe <code>split()</code> operator as a substitution for deprecated <code>KStream#branch()</code> method
(cf. <a href="https://cwiki.apache.org/confluence/display/KAFKA/KIP-418%3A+A+method-chaining+way+to+branch+KStream">KIP-418</a>).
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