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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>[DOCS-15223] [Server] Confusing Flow Control Description</title>
                <link>https://jira.mongodb.org/browse/DOCS-15223</link>
                <project id="10380" key="DOCS">Documentation</project>
                    <description>&lt;p&gt;In the documentation: &lt;a href=&quot;https://www.mongodb.com/docs/manual/tutorial/troubleshoot-replica-sets/#flow-control&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/tutorial/troubleshoot-replica-sets/#flow-control&lt;/a&gt;&lt;/p&gt;

&lt;p&gt;we can read:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;For flow control to engage, the replica set/sharded cluster must have:&#160;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/command/setFeatureCompatibilityVersion/#std-label-view-fcv&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;featureCompatibilityVersion (FCV)&lt;/a&gt;&#160;of&#160;&lt;tt&gt;4.2&lt;/tt&gt;&#160;and &lt;b&gt;read concern&lt;/b&gt;&#160;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/configuration-options/#mongodb-setting-replication.enableMajorityReadConcern&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;majority enabled&lt;/tt&gt;&lt;/a&gt;. That is, enabled flow control has no effect if FCV is not&#160;&lt;tt&gt;4.2&lt;/tt&gt;&#160;or if &lt;b&gt;read concern&lt;/b&gt; majority is disabled.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;However, flow control goal is to really throttle writes when there is certain replication lag.&#160;Also, in&#160;&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/parameters/#mongodb-parameter-param.flowControlTargetLagSeconds&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/reference/parameters/#mongodb-parameter-param.flowControlTargetLagSeconds&lt;/a&gt;&#160;we can read:&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;The target maximum&#160;&lt;a href=&quot;https://www.mongodb.com/docs/v4.2/reference/command/replSetGetStatus/#replSetGetStatus.optimes.lastCommittedOpTime&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;majority&#160;committed&lt;/tt&gt;&lt;/a&gt;&#160;lag when running with flow control. When flow control is enabled, the mechanism attempts to keep the&#160;&lt;a href=&quot;https://www.mongodb.com/docs/v4.2/reference/command/replSetGetStatus/#replSetGetStatus.optimes.lastCommittedOpTime&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;&lt;tt&gt;majority&#160;committed&lt;/tt&gt;&lt;/a&gt;&#160;lag under the specified seconds.&#160;&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;and in&#160;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/command/replSetGetStatus/#mongodb-data-replSetGetStatus.optimes.lastCommittedOpTime&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/reference/command/replSetGetStatus/#mongodb-data-replSetGetStatus.optimes.lastCommittedOpTime&lt;/a&gt;&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;Information, from the viewpoint of this member, regarding the most recent operation that has been &lt;b&gt;written to a majority of replica set members&lt;/b&gt;.&lt;/p&gt;&lt;/blockquote&gt;
&lt;p&gt;&#160;&lt;/p&gt;

&lt;p&gt;So I think it is clear that it is about &lt;b&gt;write concern&lt;/b&gt; and &lt;b&gt;not read concern?&lt;/b&gt;&lt;/p&gt;

&lt;p&gt;Besides, before version 4.4 it was even impossible to set read concern globally:&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://www.mongodb.com/docs/manual/reference/read-concern/&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://www.mongodb.com/docs/manual/reference/read-concern/&lt;/a&gt;&lt;/p&gt;
&lt;blockquote&gt;&lt;p&gt;Starting in MongoDB 4.4, replica sets and sharded clusters support setting a global default read concern.&lt;/p&gt;&lt;/blockquote&gt;</description>
                <environment></environment>
        <key id="2020871">DOCS-15223</key>
            <summary>[Server] Confusing Flow Control Description</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="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="-1">Unassigned</assignee>
                                    <reporter username="przemek.malkowski@gmail.com">Przemek Malkowski</reporter>
                        <labels>
                            <label>backlog</label>
                            <label>proactive</label>
                            <label>replication</label>
                            <label>sharding</label>
                    </labels>
                <created>Fri, 8 Apr 2022 22:28:33 +0000</created>
                <updated>Mon, 22 Jan 2024 16:20:51 +0000</updated>
                                                                            <component>manual</component>
                    <component>Server</component>
                        <due></due>
                            <votes>0</votes>
                                    <watches>3</watches>
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