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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>
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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-39410] Re-enable batching in DSCursor for change stream cursors</title>
                <link>https://jira.mongodb.org/browse/SERVER-39410</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Previously, for &lt;tt&gt;needsMerge:false&lt;/tt&gt; streams on a single replica set, we permitted our results to be batched at the &lt;tt&gt;DocumentSourceCursor&lt;/tt&gt; level; in other words, we would suck up to 4MB of data into the cursor before starting to pull those oplog events through the pipeline. We could do this because we did not need to track the oplog timestamp for single-replica streams. By contrast, for &lt;tt&gt;needsMerge:true&lt;/tt&gt; streams that were producing data to be merged on mongoS, we had to include the &lt;tt&gt;$_internalLatestOplogTimestamp&lt;/tt&gt; field. We therefore could not allow any batching, because pulling in 4MB of oplog events before starting to process the first of them would cause the latest oplog timestamp to jump 4MB ahead of the event that was actually being processed. Instead, we force the oplog scan to yield after every document, so the batch size is effectively 1 and the latest oplog timestamp stays in sync with the event being processed.&lt;/p&gt;

&lt;p&gt;But for the change stream high water mark project, streams on a single replica set must &lt;b&gt;also&lt;/b&gt; track the latest oplog timestamp; we need it in order to generate a high water mark token. And so &lt;a href=&quot;https://github.com/mongodb/mongo/commit/fc5bc0947ceedee3b61b2d922cabd3e5df7ec07c#diff-78915c57f2ef81dae40822dc10c4a5e5&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;this change within the SERVER-38408 commit&lt;/a&gt; extends the &quot;no batching&quot; rule to single replica streams. The upshot of this is that the stream is slower to return results (because we&apos;re yielding after every document) and more variable (because we are no longer guaranteed to return all available results up to 4MB on each &lt;tt&gt;getMore&lt;/tt&gt;). This is also the underlying cause of &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-38942&quot; title=&quot;Improve robustness of postBatchResumeToken integration tests&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-38942&quot;&gt;&lt;del&gt;SERVER-38942&lt;/del&gt;&lt;/a&gt;.&lt;/p&gt;

&lt;p&gt;We should allow both sharded and unsharded change stream cursors to batch their results, in order to improve the latency and consistency with which change stream events are provided to the client.&lt;/p&gt;</description>
                <environment></environment>
        <key id="685281">SERVER-39410</key>
            <summary>Re-enable batching in DSCursor for change stream cursors</summary>
                <type id="4" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14710&amp;avatarType=issuetype">Improvement</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="bernard.gorman@mongodb.com">Bernard Gorman</assignee>
                                    <reporter username="bernard.gorman@mongodb.com">Bernard Gorman</reporter>
                        <labels>
                    </labels>
                <created>Thu, 7 Feb 2019 14:26:16 +0000</created>
                <updated>Sun, 29 Oct 2023 22:24:23 +0000</updated>
                            <resolved>Thu, 21 Feb 2019 02:26:22 +0000</resolved>
                                                    <fixVersion>4.0.7</fixVersion>
                    <fixVersion>4.1.9</fixVersion>
                                    <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>2</watches>
                                                                                                                <comments>
                            <comment id="2167363" author="xgen-internal-githook" created="Fri, 1 Mar 2019 14:56:47 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Bernard Gorman&apos;, &apos;email&apos;: &apos;bernard.gorman@gmail.com&apos;, &apos;username&apos;: &apos;gormanb&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-39410&quot; title=&quot;Re-enable batching in DSCursor for change stream cursors&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-39410&quot;&gt;&lt;del&gt;SERVER-39410&lt;/del&gt;&lt;/a&gt; Re-enable batching in DSCursor for change stream cursors&lt;/p&gt;

&lt;p&gt;(cherry picked from commit 04882fa7f5210cfb14918ecddbbc5acbd88e86b6)&lt;br/&gt;
Branch: v4.0&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/204d63a92d588b9891277caf70a257b42f82ac32&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/204d63a92d588b9891277caf70a257b42f82ac32&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2157965" author="xgen-internal-githook" created="Thu, 21 Feb 2019 02:24:45 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;name&apos;: &apos;Bernard Gorman&apos;, &apos;username&apos;: &apos;gormanb&apos;, &apos;email&apos;: &apos;bernard.gorman@gmail.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-39410&quot; title=&quot;Re-enable batching in DSCursor for change stream cursors&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-39410&quot;&gt;&lt;del&gt;SERVER-39410&lt;/del&gt;&lt;/a&gt; Re-enable batching in DSCursor for change stream cursors&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/04882fa7f5210cfb14918ecddbbc5acbd88e86b6&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/04882fa7f5210cfb14918ecddbbc5acbd88e86b6&lt;/a&gt;&lt;/p&gt;</comment>
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