cmccabe commented on code in PR #14160:
URL: https://github.com/apache/kafka/pull/14160#discussion_r1406786099
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docs/ops.html:
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@@ -3603,39 +3603,45 @@ <h4 class="anchor-heading"><a id="kraft_missing"
class="anchor-link"></a><a href
<ul>
<li>Supporting JBOD configurations with multiple storage directories</li>
<li>Modifying certain dynamic configurations on the standalone KRaft
controller</li>
- <li>Delegation tokens</li>
</ul>
<h4 class="anchor-heading"><a id="kraft_zk_migration"
class="anchor-link"></a><a href="#kraft_zk_migration">ZooKeeper to KRaft
Migration</a></h4>
<p>
<b>ZooKeeper to KRaft migration is considered an Early Access feature and
is not recommended for production clusters.</b>
+ Please report issues with ZooKeeper to KRaft migration using the
+ <a href="https://issues.apache.org/jira/projects/KAFKA";
target="_blank">project JIRA</a> and the "kraft" component.
</p>
- <p>The following features are not yet supported for ZK to KRaft
migrations:</p>
-
+ <h3>Terminology</h3>
<ul>
- <li>Downgrading to ZooKeeper mode during or after the migration</li>
- <li>Other features <a href="#kraft_missing">not yet supported in
KRaft</a></li>
+ <li>Brokers that are in <b>ZK mode</b> store their metadata in Apache
ZooKepeer. This is the old mode of handling metadata.</li>
+ <li>Brokers that are in <b>KRaft mode</b> store their metadata in a KRaft
quorum. This is the new and improve mode of handling metadata.</li>
+ <li><b>Migration</b> is the process of moving cluster metadata from
ZooKeeper into a KRaft quorum.</li>
</ul>
- <p>
- Please report issues with ZooKeeper to KRaft migration using the
- <a href="https://issues.apache.org/jira/projects/KAFKA";
target="_blank">project JIRA</a> and the "kraft" component.
- </p>
+ <h3>Migration Phases</h3>
+ In general, the migration process passes through several phases.
- <h3>Terminology</h3>
- <p>
- We use the term "migration" here to refer to the process of changing a
Kafka cluster's metadata
- system from ZooKeeper to KRaft and migrating existing metadata. An
"upgrade" refers to installing a newer version of Kafka. It is not recommended
to
- upgrade the software at the same time as performing a metadata migration.
- </p>
+ <ul>
+ <li>In the <b>initial phase</b>, all the brokers are in ZK mode, and there
is a ZK-based controller.</li>
+ <li>During the <b>initial metadata load</b>, a KRaft quorum loads the
metadata from ZooKeeper,</li>
+ <li>In <b>hybrid phase</b>, some brokers are in ZK mode, but there is a
KRaft controller.</li>
+ <li>In <b>dual-write phase</b>, all brokers are KRaft, but the KRaft
controller is continuing to write to ZK.</li>
+ <li>When the migration has been <b>finalized</b>, we no longer write
metadata to ZooKeeper.</li>
+ </ul>
Review Comment:
I think distinguishing "hybrid phase" from "dual-write phase" is useful,
even though they both map to the same `MigrationState` enum.
"migration phases" aren't "migration states" (and actually, we don't discuss
migration states in this doc)
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