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    <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-32013] Ensure consistent getMore polling behaviour for sharded $changeStream</title>
                <link>https://jira.mongodb.org/browse/SERVER-32013</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;Sharded &lt;tt&gt;$changeStream&lt;/tt&gt; requires a regular stream of updates from each shard in order to return results to the client as soon as possible. To achieve this, we poll each shard with &lt;tt&gt;getMores&lt;/tt&gt; until it returns a batch of data. If we request a &lt;tt&gt;getMore&lt;/tt&gt; for a particular shard and it returns an empty batch after timing out, the ARM will automatically reschedule additional &lt;tt&gt;getMores&lt;/tt&gt; until a valid batch is received.&lt;/p&gt;

&lt;p&gt;However, there are two circumstances in which polling of shards &apos;stalls&apos;:&lt;/p&gt;
&lt;ul class=&quot;alternate&quot; type=&quot;square&quot;&gt;
	&lt;li&gt;If we are in &lt;tt&gt;kGetMoreWithAtLeastOneResultInBatch&lt;/tt&gt; context and we hit &lt;tt&gt;!_arm.ready()&lt;/tt&gt;, we return immediately without kicking off a new round of &lt;tt&gt;getMores&lt;/tt&gt; on the shard(s) whose buffers have been exhausted.&lt;/li&gt;
	&lt;li&gt;For &lt;tt&gt;kInitialFind&lt;/tt&gt; we always return an empty batch immediately regardless of &lt;tt&gt;maxTimeMS&lt;/tt&gt; or &lt;tt&gt;batchSize&lt;/tt&gt;, and do not schedule any shard &lt;tt&gt;getMores&lt;/tt&gt; until the following client &lt;tt&gt;getMore&lt;/tt&gt; is received.&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Previously we attempted to extend the ARM&apos;s existing auto-reschedule behaviour for these scenarios, but this required scheduling &lt;tt&gt;getMores&lt;/tt&gt; in the ARM constructor and in &lt;tt&gt;nextReady&lt;/tt&gt;, neither of which was appropriate. However, the same behaviour can be appropriately (and trivially) achieved in &lt;tt&gt;RouterStageMerge&lt;/tt&gt;, by simply scheduling and stashing an ARM event just before we return EOF from &lt;tt&gt;kInitialFind&lt;/tt&gt; or &lt;tt&gt;kGetMoreWithAtLeastOneResultInBatch&lt;/tt&gt; context. We already use this approach when we time out while waiting for results in &lt;tt&gt;kGetMoreNoResultsYet&lt;/tt&gt; context.&lt;/p&gt;</description>
                <environment></environment>
        <key id="460278">SERVER-32013</key>
            <summary>Ensure consistent getMore polling behaviour for sharded $changeStream</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>changestreams</label>
                    </labels>
                <created>Fri, 17 Nov 2017 13:58:57 +0000</created>
                <updated>Wed, 26 Jul 2023 22:09:07 +0000</updated>
                                                                            <component>Aggregation Framework</component>
                                        <votes>0</votes>
                                    <watches>5</watches>
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