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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-46603] disallow empty collection index build optimization on secondaries</title>
                <link>https://jira.mongodb.org/browse/SERVER-46603</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;summary: shouldBuildIndexesOnEmptyCollectionSinglePhased() is racy and can make index builds to hang forever and can block DDL operations.&lt;/p&gt;

&lt;p&gt;The &lt;a href=&quot;https://github.com/mongodb/mongo/blob/4628f264e60fd69cd09388e6fca0d3dd1b82f14c/src/mongo/db/index_builds_coordinator.cpp#L104&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;empty collection&lt;/a&gt; check is racy.&lt;/p&gt;

&lt;p&gt;[Acquires collection lock in X mode.&lt;br/&gt;
Register index build.&lt;br/&gt;
Performs collection empty check by opening cursor and see if record exists or not.&lt;br/&gt;
Releases the collection lock.]&lt;/p&gt;

&lt;p&gt;&lt;font color=&quot;red&quot;&gt;======&amp;gt;&amp;gt;&amp;gt;&amp;gt;Documents can get deleted here&lt;/font&gt;&lt;/p&gt;

&lt;p&gt;[Index build thread pool scheduling&lt;br/&gt;
Collection scan phase&lt;br/&gt;
Drain phase&lt;br/&gt;
Commit phase]&lt;/p&gt;</description>
                <environment></environment>
        <key id="1220181">SERVER-46603</key>
            <summary>disallow empty collection index build optimization on secondaries</summary>
                <type id="1" iconUrl="https://jira.mongodb.org/secure/viewavatar?size=xsmall&amp;avatarId=14703&amp;avatarType=issuetype">Bug</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="benety.goh@mongodb.com">Benety Goh</assignee>
                                    <reporter username="suganthi.mani@mongodb.com">Suganthi Mani</reporter>
                        <labels>
                    </labels>
                <created>Wed, 4 Mar 2020 07:22:31 +0000</created>
                <updated>Sun, 29 Oct 2023 22:11:18 +0000</updated>
                            <resolved>Wed, 11 Mar 2020 16:13:49 +0000</resolved>
                                                    <fixVersion>4.4.0-rc0</fixVersion>
                    <fixVersion>4.7.0</fixVersion>
                                    <component>Storage</component>
                                        <votes>0</votes>
                                    <watches>3</watches>
                                                                                                                <comments>
                            <comment id="2972118" author="xgen-internal-githook" created="Thu, 12 Mar 2020 13:29:45 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;benety&apos;, &apos;name&apos;: &apos;Benety Goh&apos;, &apos;email&apos;: &apos;benety@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-46603&quot; title=&quot;disallow empty collection index build optimization on secondaries&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-46603&quot;&gt;&lt;del&gt;SERVER-46603&lt;/del&gt;&lt;/a&gt; disallow empty collection index build optimization on secondaries&lt;/p&gt;

&lt;p&gt;(cherry picked from commit a6bdcd9f7f1264f5161720bb174a0c81396e412c)&lt;br/&gt;
Branch: v4.4&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/fe38b2fde02cc113d0b2de8e7d8b998dbcfce0b8&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/fe38b2fde02cc113d0b2de8e7d8b998dbcfce0b8&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2966743" author="xgen-internal-githook" created="Wed, 11 Mar 2020 14:59:51 +0000"  >&lt;p&gt;Author:&lt;/p&gt;
{&apos;username&apos;: &apos;benety&apos;, &apos;name&apos;: &apos;Benety Goh&apos;, &apos;email&apos;: &apos;benety@mongodb.com&apos;}
&lt;p&gt;Message: &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-46603&quot; title=&quot;disallow empty collection index build optimization on secondaries&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-46603&quot;&gt;&lt;del&gt;SERVER-46603&lt;/del&gt;&lt;/a&gt; disallow empty collection index build optimization on secondaries&lt;br/&gt;
Branch: master&lt;br/&gt;
&lt;a href=&quot;https://github.com/mongodb/mongo/commit/a6bdcd9f7f1264f5161720bb174a0c81396e412c&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;https://github.com/mongodb/mongo/commit/a6bdcd9f7f1264f5161720bb174a0c81396e412c&lt;/a&gt;&lt;/p&gt;</comment>
                            <comment id="2933895" author="suganthi.mani" created="Wed, 4 Mar 2020 07:49:17 +0000"  >&lt;p&gt;The racy check has manifested in different ways, like in BF-16446 (see this for detailed analysis) and BF-16220. This can also lead to the original problem like in &lt;a href=&quot;https://jira.mongodb.org/browse/SERVER-45201&quot; title=&quot;Implicit collection creation from createIndexes can stall replication on secondaries by circumventing thread pool on primary&quot; class=&quot;issue-link&quot; data-issue-key=&quot;SERVER-45201&quot;&gt;&lt;del&gt;SERVER-45201&lt;/del&gt;&lt;/a&gt;. Consider the below event sequence for a 2 node replica set, assume your thread pool size is 1.&lt;/p&gt;

&lt;p&gt;1) Primary registers index build for index &apos;x_1&apos; on ns foor.bar.&lt;br/&gt;
2) Primary sees the collection is not empty and decides to use index builds coordinator thread pool.&lt;br/&gt;
3) Now, the documents in the collections gets deleted on primary and collection becomes empty.&lt;br/&gt;
4) Delete writes are replicated to secondary. &lt;br/&gt;
5) Then, primary, schedules the index build on the  thread pool and initializes it. This step generates the startIndexBuild oplog entry.&lt;br/&gt;
6) Secondary receives the startIndexBuild oplog entry and commits the index build as the index created on that collection is empty.&lt;br/&gt;
7) Current primary steps down and then the secondary steps up.&lt;br/&gt;
8) Two phase index build for &apos;x_1&apos; on old primary can survive state transition and runs continuing background.&lt;br/&gt;
9) New primary tries to create another index on &apos;y_1&apos; for non-empty collection. And, so, it uses the thread pool and does index building. This steps generates the startIndexBuild oplog entry.&lt;br/&gt;
10) When the old primary receives the startIndexBuild oplog entry. &lt;b&gt;The oplog applier applier gets stuck waiting for the index build coordinator thread to get freed up. But, the index build coordinator thread will keep waiting for the commit or abort oplog entry from the new primary for index &apos;x_1&apos;. And, the index build &apos;y_1&apos;  on new primary gets stuck waiting for votes from secondary&lt;/b&gt;&lt;/p&gt;</comment>
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