ijuma commented on code in PR #18960:
URL: https://github.com/apache/kafka/pull/18960#discussion_r1961774094
##########
docs/upgrade.html:
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@@ -29,25 +29,39 @@ <h5><a id="upgrade_410_notable"
href="#upgrade_410_notable">Notable changes in 4
</ul>
</li>
</ul>
-<h4><a id="upgrade_4_0_0" href="#upgrade_4_0_0">Upgrading to 4.0.0 from any
version 0.8.x through 3.9.x</a></h4>
+<h4><a id="upgrade_4_0_0" href="#upgrade_4_0_0">Upgrading to 4.0.0 from any
version 3.3.x through 3.9.x</a></h4>
+
+<p><b>For a rolling upgrade:</b></p>
+
+<ol>
+ <li>Upgrade the brokers one at a time: shut down the broker, update the
code, and restart it. Once you have done so, the
+ brokers will be running the latest version and you can verify that the
cluster's behavior and performance meets expectations.
+ </li>
+ <li>Once the cluster's behavior and performance has been verified, bump
the metadata.version by running
+ <code>
+ bin/kafka-features.sh upgrade --metadata 4.0
+ </code>
+ </li>
+ <li>Note that cluster metadata downgrade is not supported in this version
since it has metadata changes.
+ Every <a
href="https://github.com/apache/kafka/blob/trunk/server-common/src/main/java/org/apache/kafka/server/common/MetadataVersion.java";>MetadataVersion</a>
+ has a boolean parameter that indicates if there are metadata changes
(i.e. <code>IBP_4_0_IV1(23, "4.0", "IV1", true)</code> means this version has
metadata changes).
+ Given your current and target versions, a downgrade is only possible
if there are no metadata changes in the versions between.</li>
+</ol>
+
<h5><a id="upgrade_400_notable" href="#upgrade_400_notable">Notable
changes in 4.0.0</a></h5>
<ul>
<li>
- Since Apache Kafka 4.0.0, we have added a system property
("org.apache.kafka.sasl.oauthbearer.allowed.urls") to
- set the allowed URLs as SASL OAUTHBEARER token or jwks endpoints.
By default the value is an empty list.
- Users should explicitly set the allowed list if necessary.
- </li>
Review Comment:
Moved this lower in the page since this is less important than the items
below it.
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