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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-54711] Invariant on shardVersion, atClusterTime relationship for sharded write transactions</title>
                <link>https://jira.mongodb.org/browse/SERVER-54711</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;When a moveChunk on a collection commits, the shardVersion of the collection is advanced and a timestamp is chosen such that:&lt;/p&gt;
&lt;ul&gt;
	&lt;li&gt;Reads of moved documents after that timestamp on the recipient are considered &quot;owned&quot;&lt;/li&gt;
	&lt;li&gt;Reads of moved documents after that timestamp on the donor are considered &quot;orphaned&quot;&lt;/li&gt;
&lt;/ul&gt;


&lt;p&gt;Also note that a donor does not eagerly delete or mark its orphans in a way that would generate a storage engine write conflict. The orphans are eventually deleted at some arbitrary point in the future.&lt;/p&gt;

&lt;p&gt;Updating documents that are concurrently participating in a moveChunk result in those updates being transferred (&quot;xfer mods&quot;) from the donor -&amp;gt; recipient. Those updates must be applied in a critical section before the moveChunk can be committed.&lt;/p&gt;

&lt;p&gt;Sharded transactions, however operate against a snapshot defined by the &lt;tt&gt;atClusterTime&lt;/tt&gt; &lt;a href=&quot;https://github.com/mongodb/mongo/blob/ad726654e555009747c5c361039c0c44edcf00d1/src/mongo/s/transaction_router.cpp#L858-L879&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;attached by a mongoS&lt;/a&gt;. Performing a transactional sharded update on a document that&apos;s concurrently being moved with a stale &lt;tt&gt;atClusterTime&lt;/tt&gt; can result in write skew and lost data.&lt;/p&gt;

&lt;p&gt;This correctness problem is protected by the use of a shard version. But first consider this scenario that uses a stale read timestamp without a shard versioning protocol:&lt;/p&gt;
&lt;p/&gt;
&lt;div id=&quot;syntaxplugin&quot; class=&quot;syntaxplugin&quot; style=&quot;border: 1px dashed #bbb; border-radius: 5px !important; overflow: auto; max-height: 30em;&quot;&gt;
&lt;table cellspacing=&quot;0&quot; cellpadding=&quot;0&quot; border=&quot;0&quot; width=&quot;100%&quot; style=&quot;font-size: 1em; line-height: 1.4em !important; font-weight: normal; font-style: normal; color: black;&quot;&gt;
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				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;  margin-top: 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;| Client                                                 | Config                              | Shard0            | Shard1            |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;|--------------------------------------------------------+-------------------------------------+-------------------+-------------------|&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;| Insert A                                               |                                     | Insert A @ TS(10) |                   |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;|                                                        | MoveChunkStarts                     |                   |                   |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;|                                                        |                                     |                   | Insert A @ TS(20) |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;|                                                        | MoveChunkCommits ValidAfter: TS(30) |                   |                   |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
				&lt;tr id=&quot;syntaxplugin_code_and_gutter&quot;&gt;
						&lt;td  style=&quot; line-height: 1.4em !important; padding: 0em; vertical-align: top;&quot;&gt;
					&lt;pre style=&quot;font-size: 1em; margin: 0 10px;   margin-bottom: 10px;  width: auto; padding: 0;&quot;&gt;&lt;span style=&quot;color: black; font-family: &apos;Consolas&apos;, &apos;Bitstream Vera Sans Mono&apos;, &apos;Courier New&apos;, Courier, monospace !important;&quot;&gt;| Delete A (broadcast transaction) atClusterTime: TS(15) |                                     | Delete A @ 40     | `A` Not Found     |&lt;/span&gt;&lt;/pre&gt;
			&lt;/td&gt;
		&lt;/tr&gt;
			&lt;/tbody&gt;
&lt;/table&gt;
&lt;/div&gt;
&lt;p/&gt;

&lt;p&gt;In this example, a client inserts and deletes a document and received a success for both requests. However the document still remains as it wasn&apos;t deleted from the proper shard.&lt;/p&gt;

&lt;p&gt;In this example, the move chunk committing generates a new shard version which we&apos;ll call &lt;tt&gt;SV&lt;/tt&gt;. If the client request is operating with an &lt;tt&gt;atClusterTime: TS(15)&lt;/tt&gt;, but also claims to be using the fresh shard version: &lt;tt&gt;SV&lt;/tt&gt;, the same bug would manifest.&lt;/p&gt;

&lt;p&gt;Thus we can conclude there is a relationship that must be maintained for correctness of the system with chunk migrations and transactions that do updates:&lt;br/&gt;
For a shard receiving a &lt;tt&gt;startTransaction&lt;/tt&gt; request with a shardVersion &lt;tt&gt;SV&lt;/tt&gt;, the request&apos;s &lt;tt&gt;atClusterTime&lt;/tt&gt; must be at least the &lt;tt&gt;validAfter&lt;/tt&gt; timestamp associated &lt;tt&gt;SV&lt;/tt&gt;.&lt;/p&gt;</description>
                <environment></environment>
        <key id="1629997">SERVER-54711</key>
            <summary>Invariant on shardVersion, atClusterTime relationship for sharded write transactions</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="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="sergi.mateo-bellido@mongodb.com">Sergi Mateo Bellido</assignee>
                                    <reporter username="daniel.gottlieb@mongodb.com">Daniel Gottlieb</reporter>
                        <labels>
                            <label>sharding-wfbf-day</label>
                    </labels>
                <created>Mon, 22 Feb 2021 19:52:03 +0000</created>
                <updated>Sun, 29 Oct 2023 21:57:10 +0000</updated>
                            <resolved>Mon, 8 Mar 2021 15:45:33 +0000</resolved>
                                                    <fixVersion>4.9.0</fixVersion>
                                    <component>Sharding</component>
                                        <votes>0</votes>
                                    <watches>4</watches>
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