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    <title>MongoDB Jira</title>
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    <language>en-us</language>    <build-info>
        <version>9.7.1</version>
        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-39052] Reading and writing FCV is asymmetric in sharded clusters</title>
                <link>https://jira.mongodb.org/browse/SERVER-39052</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;&lt;a href=&quot;https://docs.mongodb.com/manual/reference/command/setFeatureCompatibilityVersion/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://docs.mongodb.com/manual/reference/command/setFeatureCompatibilityVersion/&lt;/a&gt; states that setting FCV is done on a mongos, but reading the FCV back on a mongos is not allowed.&lt;/p&gt;

&lt;p&gt;Writing:&lt;/p&gt;

&lt;p&gt;&amp;gt; For a sharded cluster, run the command on a mongos instance.&lt;/p&gt;

&lt;p&gt;Reading:&lt;/p&gt;

&lt;p&gt;&amp;gt; The operation is undefined on the mongos instances. For a sharded cluster that has access control enabled, to run the command against a member of the shard replica set, you must connect to the member as a shard local user.&lt;/p&gt;

&lt;p&gt;Why is mongos able to write FCV but not read it back?&lt;/p&gt;</description>
                <environment></environment>
        <key id="673526">SERVER-39052</key>
            <summary>Reading and writing FCV is asymmetric in sharded clusters</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</type>
                                            <priority id="4" iconUrl="https://jira.mongodb.org/images/icons/priorities/minor.svg">Minor - P4</priority>
                        <status id="10038" iconUrl="https://jira.mongodb.org/images/icons/subtask.gif" description="">Backlog</status>
                    <statusCategory id="2" key="new" colorName="default"/>
                                    <resolution id="-1">Unresolved</resolution>
                                        <assignee username="backlog-server-catalog-and-routing">Backlog - Catalog and Routing</assignee>
                                    <reporter username="oleg.pudeyev@mongodb.com">Oleg Pudeyev</reporter>
                        <labels>
                            <label>ShardingRoughEdges</label>
                            <label>oldshardingemea</label>
                            <label>sharding-common-backlog</label>
                            <label>shardingemea-qw</label>
                    </labels>
                <created>Wed, 16 Jan 2019 19:58:18 +0000</created>
                <updated>Thu, 26 Oct 2023 09:59:10 +0000</updated>
                                            <version>4.0.3</version>
                                                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="2120851" author="kaloian.manassiev" created="Fri, 18 Jan 2019 20:02:17 +0000"  >&lt;p&gt;You are right that this is a minor nuisance that contributes to the behavioural difference between sharded and unsharded systems. I am putting it on the sharding team&apos;s Backlog with a low priority and a proposed way to fix it.&lt;/p&gt;

&lt;p&gt;tl;dr&lt;br/&gt;
The feature compatibility version of a cluster is the FCV of its config server and there is no meaningful way to cache this information on the routers, because they don&apos;t synchronize with the config server. The &lt;a href=&quot;https://github.com/mongodb/mongo/blob/aad17aeba520f5c421716cb2d740057be96ce61a/src/mongo/db/commands/parameters.cpp#L231&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;getParameter&lt;/tt&gt;&lt;/a&gt; command just reads in-memory values from the parameter set and exposes no way to ask other nodes.&lt;/p&gt;

&lt;p&gt;The only way to fix this would be to specialise the &lt;tt&gt;getParameter&lt;/tt&gt; command on the routers and shards to ask the config server for the FCV parameter, if the nodes are sharding components.&lt;/p&gt;</comment>
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