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        <build-number>970001</build-number>
        <build-date>13-04-2023</build-date>
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<item>
            <title>[SERVER-48526] Lengthy oplog scans may cause difficulty resuming change streams in a sharded cluster</title>
                <link>https://jira.mongodb.org/browse/SERVER-48526</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;If a change stream is resumed in a sharded cluster, all shards that have no oplog entries for the relevant namespace (either because they never had any data for the collection, or because they have not taken any writes recently) will scan through their oplogs until reaching EOF before returning their first &lt;tt&gt;getMore&lt;/tt&gt; batch to &lt;tt&gt;mongoS&lt;/tt&gt;. If the oplogs are large and the requested resume point is early, then this may take so long that the cursors on shards which &lt;b&gt;do&lt;/b&gt; have oplog entries time out, effectively making the stream unresumable. This problem could manifest even after &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-30784&quot; title=&quot;Allow sharded change streams to target just the shards that have chunks&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-30784&quot;&gt;SERVER-30784&lt;/a&gt; is complete, although the likelihood of such an occurrence will be significantly reduced.&lt;/p&gt;

&lt;p&gt;Addressing this may require changing the semantics of &lt;tt&gt;tailable&lt;/tt&gt;-&lt;tt&gt;awaitData&lt;/tt&gt; &lt;tt&gt;getMores&lt;/tt&gt; on &lt;tt&gt;mongoD&lt;/tt&gt;. Currently, if there are no matching entries, &lt;tt&gt;getMore&lt;/tt&gt; will scan to the end of the oplog regardless of how long it takes. We could instead have it proactively break and return after the specific &lt;tt&gt;awaitData&lt;/tt&gt; period expires, much like a non-throwing &lt;tt&gt;maxTimeMS&lt;/tt&gt; deadline. This would also be consistent with &lt;tt&gt;mongoS&apos;&lt;/tt&gt; current behaviour.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1368352">SERVER-48526</key>
            <summary>Lengthy oplog scans may cause difficulty resuming change streams in a sharded cluster</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="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-query-execution">Backlog - Query Execution</assignee>
                                    <reporter username="bernard.gorman@mongodb.com">Bernard Gorman</reporter>
                        <labels>
                            <label>change-streams-improvements</label>
                    </labels>
                <created>Mon, 1 Jun 2020 20:00:50 +0000</created>
                <updated>Tue, 6 Dec 2022 02:25:15 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>12</watches>
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