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    <link>https://jira.mongodb.org</link>
    <description>This file is an XML representation of an issue</description>
    <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>[DOCS-11387] Docs for SERVER-32635: Provide a framework to ensure a last stable binary version mongos crashes when attempting to connect to a latest featureCompatibilityVersion sharded cluster for all last stable binary versions &gt;= 3.8</title>
                <link>https://jira.mongodb.org/browse/DOCS-11387</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;h1&gt;&lt;a name=&quot;DocumentationRequestSummary%3A&quot;&gt;&lt;/a&gt;Documentation Request Summary: &lt;/h1&gt;

&lt;p&gt;This ticket makes the mongos crash when it communicates with a server that has been upgrade both a binary and feature compatibility version higher than itself. So a mongos v4.0 talking to a v4.2 mongod with feature compatibility version 4.2 &amp;#8211; a v4.2 mongod with FCV 4.0 will not cause a crash, to be clear.&lt;/p&gt;

&lt;p&gt;This kind of scenario can happen when a user forgets to upgrade the mongos binary and then calls setFCV(upgrade), leaving the still downgraded mongos unable to communicate. Rather than the mongos logging incompatible server version errors endlessly, we&apos;ve chosen to crash it.&lt;/p&gt;


&lt;h1&gt;&lt;a name=&quot;EngineeringTicketDescription%3A&quot;&gt;&lt;/a&gt;Engineering Ticket Description:&lt;/h1&gt;

&lt;p&gt;If a last-stable binary version mongos attempts to point to a latest featureCompatibilityVersion sharded cluster, the mongos will spin, repeatedly attempting to connect and &lt;a href=&quot;https://github.com/mongodb/mongo/blob/67500efdc1c677173c737ecde2b07b42015ae5fd/src/mongo/rpc/protocol.cpp#L210-L224&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;failing with IncompatibleServerVersion errors&lt;/a&gt;. Instead, the mongos should crash in this scenario. This framework should be available for all last-stable binary versions &amp;gt;= 3.8. This implementation should not cause a latest binary version mongos to crash upon attempting to connect to a last stable binary version node.&lt;br/&gt;
A proposed implementation for the framework follows:&lt;br/&gt;
The &lt;tt&gt;validateWireVersion&lt;/tt&gt; function in protocol.cpp can be modified to have a new error code that is returned when the client&apos;s maximum wireVersion is less than the server&apos;s minimum wireVersion. If the client is a mongos and it sees this error upon calling &lt;tt&gt;validateWireVersion&lt;/tt&gt;, it will crash. The client can use the &lt;tt&gt;&lt;a href=&quot;https://github.com/mongodb/mongo/blob/188f2fe21053be56cc325c03b2c5aebeef7ef377/src/mongo/s/is_mongos.cpp#L37-L39&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;isMongos()&lt;/a&gt;&lt;/tt&gt; function to determine whether it is a mongos.This low-level implementation is justifiable because a last-stable mongos should crash upon any attempts to connect to a latest featureCompatibilityVersion node in the cluster. This implementation assumes a mongos will only be a client to other members of the same sharded cluster and not to members of a different sharded cluster.&lt;/p&gt;</description>
                <environment></environment>
        <key id="504815">DOCS-11387</key>
            <summary>Docs for SERVER-32635: Provide a framework to ensure a last stable binary version mongos crashes when attempting to connect to a latest featureCompatibilityVersion sharded cluster for all last stable binary versions &gt;= 3.8</summary>
                <type id="3" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14718&amp;avatarType=issuetype">Task</type>
                                            <priority id="3" iconUrl="https://jira.mongodb.org/images/icons/priorities/major.svg">Major - P3</priority>
                        <status id="6" iconUrl="https://jira.mongodb.org/images/icons/statuses/closed.png" description="The issue is considered finished, the resolution is correct. Issues which are closed can be reopened.">Closed</status>
                    <statusCategory id="3" key="done" colorName="success"/>
                                    <resolution id="13201">Fixed</resolution>
                                        <assignee username="kay.kim@mongodb.com">Kay Kim</assignee>
                                    <reporter username="kay.kim@mongodb.com">Kay Kim</reporter>
                        <labels>
                    </labels>
                <created>Fri, 2 Mar 2018 17:57:18 +0000</created>
                <updated>Sun, 29 Oct 2023 13:58:17 +0000</updated>
                            <resolved>Fri, 29 Jun 2018 18:30:46 +0000</resolved>
                                                    <fixVersion>3.7.3</fixVersion>
                                        <due></due>
                            <votes>0</votes>
                                    <watches>1</watches>
                                                                                                                <comments>
                            <comment id="1935207" author="xgen-internal-githook" created="Fri, 29 Jun 2018 18:28:34 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;kay-kim&apos;, &apos;name&apos;: &apos;kay&apos;, &apos;email&apos;: &apos;kay.kim@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/DOCS-11387&quot; title=&quot;Docs for SERVER-32635: Provide a framework to ensure a last stable binary version mongos crashes when attempting to connect to a latest featureCompatibilityVersion sharded cluster for all last stable binary versions &amp;gt;= 3.8&quot; class=&quot;issue-link&quot; data-issue-key=&quot;DOCS-11387&quot;&gt;&lt;del&gt;DOCS-11387&lt;/del&gt;&lt;/a&gt;: mongos and fcv&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/docs/commit/57c8003ec9d02f36687c914133e48274dc272f38&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/docs/commit/57c8003ec9d02f36687c914133e48274dc272f38&lt;/a&gt;&lt;/p&gt;</comment>
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            <issuekey id="565706">DOCS-11840</issuekey>
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