<p dir="auto">See <a href="https://github.com/elastic/elasticsearch/blob/master/core/src/main/java/org/elasticsearch/cluster/metadata/MetaData.java#L1150">https://github.com/elastic/elasticsearch/blob/master/core/src/main/java/org/elasticsearch/cluster/metadata/MetaData.java#L1150</a>. If an element is not recognized in a cluster state, it is simply ignored.</p>
<p dir="auto">If a plugin adds custom cluster state parts and the plugin isn't installed on all nodes then it could cause custom metadata to be removed by a node just by reading it.</p>
<p dir="auto"><a href="https://github.com/elastic/elasticsearch/pull/21243#discussion_r87375313" data-hovercard-type="pull_request" data-hovercard-url="/elastic/elasticsearch/pull/21243/hovercard">Brought up</a> while reviewing <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="186571273" data-permission-text="Title is private" data-url="https://github.com/elastic/elasticsearch/issues/21243" data-hovercard-type="pull_request" data-hovercard-url="/elastic/elasticsearch/pull/21243/hovercard" href="https://github.com/elastic/elasticsearch/pull/21243">#21243</a>.</p>