Add upgrade/downgrade notes for 2.1.x
We seemed to be missing the usual rolling upgrade instructions so I've added them and emphasized the impact for downgrades after bumping the inter-broker protocol version.
Author: Jason Gustafson
Reviewers: Dong Lin Ismael Juma
Closes from hachikuji/KAFKA-7481
<script id="upgrade-template" type="text/x-handlebars-template">
<h4><a id="upgrade_2_1_0" href="#upgrade_2_1_0">Upgrading from 0.8.x, 0.9.x, 0.10.0.x, 0.10.1.x, 0.10.2.x, 0.11.0.x, 1.0.x, 1.1.x, or 2.0.0 to 2.1.0</a></h4>
+
+<p><b>Note that 2.1.x contains a change to the internal schema used to store consumer offsets. Once the upgrade is
+ complete, it will not be possible to downgrade to previous versions. See the rolling upgrade notes below for more detail.</b></p>
+
+<p><b>For a rolling upgrade:</b></p>
+
+<ol>
+ <li> Update server.properties on all brokers and add the following properties. CURRENT_KAFKA_VERSION refers to the version you
+ are upgrading from. CURRENT_MESSAGE_FORMAT_VERSION refers to the message format version currently in use. If you have previously
+ overridden the message format version, you should keep its current value. Alternatively, if you are upgrading from a version prior
+ to 0.11.0.x, then CURRENT_MESSAGE_FORMAT_VERSION should be set to match CURRENT_KAFKA_VERSION.
+ <ul>
+ <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (e.g. 0.8.2, 0.9.0, 0.10.0, 0.10.1, 0.10.2, 0.11.0, 1.0, 1.1).</li>
+ <li>log.message.format.version=CURRENT_MESSAGE_FORMAT_VERSION (See <a href="#upgrade_10_performance_impact">potential performance impact
+ following the upgrade</a> for the details on what this configuration does.)</li>
+ </ul>
+ If you are upgrading from 0.11.0.x, 1.0.x, 1.1.x, or 2.0.x and you have not overridden the message format, then you only need to override
+ the inter-broker protocol version.
+ <ul>
+ <li>inter.broker.protocol.version=CURRENT_KAFKA_VERSION (0.11.0, 1.0, 1.1, 2.0).</li>
+ </ul>
+ </li>
+ <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.
+ It is still possible to downgrade at this point if there are any problems.
+ </li>
+ <li> Once the cluster's behavior and performance has been verified, bump the protocol version by editing
+ <code>inter.broker.protocol.version</code> and setting it to 2.1.
+ </li>
+ <li> Restart the brokers one by one for the new protocol version to take effect. Once the brokers begin using the latest
+ protocol version, it will no longer be possible to downgrade the cluster to an older version.
+ </li>
+ <li> If you have overridden the message format version as instructed above, then you need to do one more rolling restart to
+ upgrade it to its latest version. Once all (or most) consumers have been upgraded to 0.11.0 or later,
+ change log.message.format.version to 2.1 on each broker and restart them one by one. Note that the older Scala clients,
+ which are no longer maintained, do not support the message format introduced in 0.11, so to avoid conversion costs
+ (or to take advantage of <a href="#upgrade_11_exactly_once_semantics">exactly once semantics</a>),
+ the newer Java clients must be used.
+ </li>
+</ol>
+
<p><b>Additional Upgrade Notes:</b></p>
<ol>
