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    <title>MongoDB Jira</title>
    <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>[SERVER-12312] getIndexes() on mongos reads from wrong shard if read preference is set</title>
                <link>https://jira.mongodb.org/browse/SERVER-12312</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;For a sharded collection, a call to &lt;tt&gt;.getIndexes()&lt;/tt&gt; or querying the &lt;tt&gt;system.indexes&lt;/tt&gt; collection with a namespace specified (those are equivalent) on a mongos will be directed to the first shard that has chunks of that collection, NOT to the primary shard. This is due to a deliberate hack, see:&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/blob/1b2af152b9806b0b3443bd4facddf87c8adecb0e/src/mongo/s/strategy.cpp#L95&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/blob/1b2af152b9806b0b3443bd4facddf87c8adecb0e/src/mongo/s/strategy.cpp#L95&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;However, this hack is being circumvented if a read preference is set on the query or if the query has an additional sort. (Other conditions may trigger the same behavior as well, those are the ones we have found so far). &lt;/p&gt;

&lt;p&gt;This seems to be a bug in the kernel, as the read preference has nothing to do with what shard is being selected. &lt;/p&gt;

&lt;p&gt;In particular, some drivers (Node.js for example) always set a default read preference of primary, therefore circumventing the &quot;hack&quot; and reading the indexes always from the primary shard, not from the first chunk-containing shard.&lt;/p&gt;

&lt;p&gt;This is especially problematic if an index is created through mongos where the collection has no chunks on the primary shard (due to tagging for example). The index is then not visible when queried with read preference.&lt;/p&gt;

&lt;p&gt;jstests to reproduce behavior (two variants) attached.&lt;/p&gt;</description>
                <environment>the second jstest (sh_index2.js) also triggers the bug on 2.5.4.</environment>
        <key id="105422">SERVER-12312</key>
            <summary>getIndexes() on mongos reads from wrong shard if read preference is set</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="9">Done</resolution>
                                        <assignee username="randolph@mongodb.com">Randolph Tan</assignee>
                                    <reporter username="thomas.rueckstiess@mongodb.com">Thomas Rueckstiess</reporter>
                        <labels>
                    </labels>
                <created>Fri, 10 Jan 2014 00:34:18 +0000</created>
                <updated>Mon, 11 Jul 2016 17:16:47 +0000</updated>
                            <resolved>Wed, 5 Feb 2014 18:58:30 +0000</resolved>
                                    <version>2.4.8</version>
                    <version>2.5.4</version>
                                    <fixVersion>2.6.0-rc0</fixVersion>
                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
                                                                                                                <comments>
                            <comment id="493953" author="xgen-internal-githook" created="Wed, 5 Feb 2014 18:57:46 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{u&apos;username&apos;: u&apos;renctan&apos;, u&apos;name&apos;: u&apos;Randolph Tan&apos;, u&apos;email&apos;: u&apos;randolph@10gen.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-12312&quot; title=&quot;getIndexes() on mongos reads from wrong shard if read preference is set&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-12312&quot;&gt;&lt;del&gt;SERVER-12312&lt;/del&gt;&lt;/a&gt; getIndexes() on mongos reads from wrong shard if read preference is set&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/86e948fdf8e900fa187f9b34d85841bca580729a&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/86e948fdf8e900fa187f9b34d85841bca580729a&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="479980" author="thomasr" created="Fri, 10 Jan 2014 01:03:07 +0000"  >&lt;p&gt;Attaching a second jstest &lt;span class=&quot;nobr&quot;&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/attachment/36114/36114_sh_index2.js&quot; title=&quot;sh_index2.js attached to SERVER-12312&quot;&gt;sh_index2.js&lt;sup&gt;&lt;img class=&quot;rendericon&quot; src=&quot;https://jira.mongodb.org/images/icons/link_attachment_7.gif&quot; height=&quot;7&quot; width=&quot;7&quot; align=&quot;absmiddle&quot; alt=&quot;&quot; border=&quot;0&quot;/&gt;&lt;/sup&gt;&lt;/a&gt;&lt;/span&gt; that shows a more realistic (and more severe) consequence of this bug. &lt;/p&gt;

&lt;ol&gt;
	&lt;li&gt;build index from mongos on a sharded collection that does not have chunks on the primary shard&lt;/li&gt;
	&lt;li&gt;read the indexes back, with a read preference&lt;/li&gt;
&lt;/ol&gt;


&lt;p&gt;The index appears to not exist, because it wasn&apos;t built on the primary shard (no chunks on it) but the indexes were read from the primary shard. &lt;/p&gt;

&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
		&lt;tbody &gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongos&amp;gt; db.system.indexes.find({ns: &quot;test.docs&quot;})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;_id&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;_id_&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;foo&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;foo_1&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongos&amp;gt; db.docs.ensureIndex({new_index: 1})&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongos&amp;gt; db.system.indexes.find({ns: &quot;test.docs&quot;}).readPref(&apos;primary&apos;)&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;_id&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;_id_&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;foo&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;foo_1&quot; }&lt;/span&gt;&lt;/pre&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;mongos&amp;gt; db.system.indexes.find({ns: &quot;test.docs&quot;})&lt;/span&gt;&lt;/pre&gt;
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						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;_id&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;_id_&quot; }&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;foo&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;foo_1&quot; }&lt;/span&gt;&lt;/pre&gt;
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					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;{ &quot;v&quot; : 1, &quot;key&quot; : { &quot;new_index&quot; : 1 }, &quot;ns&quot; : &quot;test.docs&quot;, &quot;name&quot; : &quot;new_index_1&quot; }&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
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			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;</comment>
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