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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-33541] Add snapshot read support for aggregation</title>
                <link>https://jira.mongodb.org/browse/SERVER-33541</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;ol&gt;
	&lt;li&gt;aggregate
	&lt;ol&gt;
		&lt;li&gt;Including support for $lookup&lt;/li&gt;
		&lt;li&gt;The $out stage will be prohibited for use within pipelines executed with readConcern level snapshot due to use of metadata operations.&lt;/li&gt;
	&lt;/ol&gt;
	&lt;/li&gt;
&lt;/ol&gt;
</description>
                <environment></environment>
        <key id="503563">SERVER-33541</key>
            <summary>Add snapshot read support for aggregation</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="david.storch@mongodb.com">David Storch</assignee>
                                    <reporter username="milkie@mongodb.com">Eric Milkie</reporter>
                        <labels>
                    </labels>
                <created>Wed, 28 Feb 2018 16:10:55 +0000</created>
                <updated>Sun, 29 Oct 2023 22:34:22 +0000</updated>
                            <resolved>Fri, 9 Mar 2018 22:30:28 +0000</resolved>
                                                    <fixVersion>3.7.3</fixVersion>
                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="1829498" author="xgen-internal-githook" created="Fri, 9 Mar 2018 22:25:56 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;email&apos;: &apos;david.storch@10gen.com&apos;, &apos;name&apos;: &apos;David Storch&apos;, &apos;username&apos;: &apos;dstorch&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33541&quot; title=&quot;Add snapshot read support for aggregation&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33541&quot;&gt;&lt;del&gt;SERVER-33541&lt;/del&gt;&lt;/a&gt; Add readConcern level &apos;snapshot&apos; support for aggregation.&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/ed1e2b4d2a4987e3744484f9482fdc7a0e119e94&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/ed1e2b4d2a4987e3744484f9482fdc7a0e119e94&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="1823068" author="david.storch" created="Mon, 5 Mar 2018 15:14:23 +0000"  >&lt;blockquote&gt;
&lt;p&gt;In particular, I notice that the locking code will keep a list of resources to unlock when two-phase locking is enabled. Since $lookup normally acquires and drops locks repeatedly, this list could grow quite long. Is this a problem?&lt;/p&gt;

&lt;p&gt;This is something which we will probably fix. Dave will confirm that this issue exists and file a ticket as part of the local snapshot reads project.&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Filed &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-33610&quot; title=&quot;If using two-phase locking, lock manager&amp;#39;s list of resources to unlock can grow without bound&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-33610&quot;&gt;&lt;del&gt;SERVER-33610&lt;/del&gt;&lt;/a&gt; describing this issue.&lt;/p&gt;</comment>
                            <comment id="1820095" author="david.storch" created="Thu, 1 Mar 2018 17:57:21 +0000"  >&lt;p&gt;Recording answers to some of my open questions after discussing with &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=james.wahlin&quot; class=&quot;user-hover&quot; rel=&quot;james.wahlin&quot;&gt;james.wahlin&lt;/a&gt; and &lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=milkie&quot; class=&quot;user-hover&quot; rel=&quot;milkie&quot;&gt;milkie&lt;/a&gt;:&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;Should we make an effort to ban snapshot reads for aggregations that are actually reading metadata sources (as opposed to reading actual user data with DocumentSourceCursor, or DocumentSourceSampleFromRandomCursor)?&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Yes, we should do so, in addition to prohibiting the &lt;tt&gt;$out&lt;/tt&gt; stage. We also need to prohibit change streams, and probably &lt;tt&gt;$geoNear&lt;/tt&gt;.&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;I think we could end up taking a lot of locks, or taking the same lock recursively many times. In particular $lookup could pose a problem. Any issues we foresee here? I assume we should at least write a test case to make sure this doesn&apos;t completely blow up?&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;No specific problems other than the one below, though agree that we should have a targeted test case for &lt;tt&gt;$lookup&lt;/tt&gt;.&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;In particular, I notice that the locking code will keep a list of resources to unlock when two-phase locking is enabled. Since $lookup normally acquires and drops locks repeatedly, this list could grow quite long. Is this a problem?&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;This is something which we will probably fix. Dave will confirm that this issue exists and file a ticket as part of the local snapshot reads project.&lt;/p&gt;

&lt;blockquote&gt;
&lt;p&gt;Anything somehow related to transactions (but not snapshot reads) that I might be missing?&lt;/p&gt;&lt;/blockquote&gt;

&lt;p&gt;Not that we could think of. Eric points out that the lock acquisitions in the agg path need to be updated to have a timeout of zero, but that this work can happen in a later sweep.&lt;/p&gt;</comment>
                    </comments>
                <issuelinks>
                            <issuelinktype id="10320">
                    <name>Documented</name>
                                                                <inwardlinks description="is documented by">
                                        <issuelink>
            <issuekey id="486646">DOCS-11245</issuekey>
        </issuelink>
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                            <issuelinktype id="10010">
                    <name>Duplicate</name>
                                                                <inwardlinks description="is duplicated by">
                                        <issuelink>
            <issuekey id="530486">SERVER-34547</issuekey>
        </issuelink>
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                                        <issuelink>
            <issuekey id="482416">SERVER-84852</issuekey>
        </issuelink>
            <issuelink>
            <issuekey id="506070">SERVER-33683</issuekey>
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