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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-69144] Add a command to allow querying local.oplog.rs through mongos for the head of oplog timestamp</title>
                <link>https://jira.mongodb.org/browse/SERVER-69144</link>
                <project id="10000" key="SERVER">Core Server</project>
                    <description>&lt;p&gt;There is a need for Mongosync to query the earliest timestamp on the source cluster, from which it can open a change stream. In the sharding cluster case, it would require Mongosync to know&lt;/p&gt;

&lt;p&gt;&lt;em&gt;max(&amp;lt;ts of oldest oplog entry&amp;gt; for each shard)&lt;/em&gt;&lt;/p&gt;

&lt;p&gt;After a discussion with Max and Bernard &lt;a href=&quot;https://mongodb.slack.com/archives/C0V2Z88D8/p1660672134181999&quot; class=&quot;external-link&quot; target=&quot;_blank&quot; rel=&quot;nofollow noopener&quot;&gt;here&lt;/a&gt; , we found there is currently no good way to do it. We will need a special command to allow mongos to return this value.&lt;/p&gt;</description>
                <environment></environment>
        <key id="2122301">SERVER-69144</key>
            <summary>Add a command to allow querying local.oplog.rs through mongos for the head of oplog timestamp</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="backlog-server-repl">Backlog - Replication Team</assignee>
                                    <reporter username="huan.li@mongodb.com">Huan Li</reporter>
                        <labels>
                            <label>former-quick-wins</label>
                    </labels>
                <created>Thu, 25 Aug 2022 17:00:51 +0000</created>
                <updated>Mon, 21 Aug 2023 18:57:55 +0000</updated>
                                                                                                <votes>0</votes>
                                    <watches>14</watches>
                                                                                                                <comments>
                            <comment id="4777502" author="huan.li" created="Thu, 25 Aug 2022 18:56:02 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=arun.banala%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;arun.banala@mongodb.com&quot;&gt;arun.banala@mongodb.com&lt;/a&gt; If we have an option to allow change stream to start at the earliest timestamp, I guess the first returned change event should carry this earliest timestamp?&#160;&lt;/p&gt;

&lt;p&gt;We could add option like &lt;/p&gt;

&lt;p&gt;{pipeline: &lt;span class=&quot;error&quot;&gt;&amp;#91;{$changeStream: 1}, {$limit: 1}&amp;#93;&lt;/span&gt;, cursor: {batchSize: 1}}  to make the change stream query more performant if it works.&#160;&lt;/p&gt;

&lt;p&gt;&#160;&lt;/p&gt;</comment>
                            <comment id="4777497" author="huan.li" created="Thu, 25 Aug 2022 18:52:43 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=arun.banala%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;arun.banala@mongodb.com&quot;&gt;arun.banala@mongodb.com&lt;/a&gt; We wanna provide a way for the user to be aware of the window that the replication would fail because of the oplog roll over. The error return of &lt;em&gt;ChangeStreamHistoryLost&lt;/em&gt; during change stream opening won&apos;t be good enough because that will only happen when we have finished all the collection copy.&#160;&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=wenbin.zhu%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;wenbin.zhu@mongodb.com&quot;&gt;wenbin.zhu@mongodb.com&lt;/a&gt; We could notify the user in this way if the falling off happens during collection copy, but that could only fail the replicator earlier instead of preventing it from happening. For instance, during live migration, the TSE used to increase the oplog size when they found the mongomirror has a potential risk to fell off the oplog.&lt;/p&gt;</comment>
                            <comment id="4777431" author="JIRAUSER1258790" created="Thu, 25 Aug 2022 18:31:22 +0000"  >&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=arun.banala%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;arun.banala@mongodb.com&quot;&gt;arun.banala@mongodb.com&lt;/a&gt; Since the current mongosync design only starts change stream after initial sync finishes, we want warn customers as early as possible if mongosync has fallen off the oplog while initial sync is still on-going.&lt;/p&gt;

&lt;p&gt;&lt;a href=&quot;https://jira.mongodb.org/secure/ViewProfile.jspa?name=huan.li%40mongodb.com&quot; class=&quot;user-hover&quot; rel=&quot;huan.li@mongodb.com&quot;&gt;huan.li@mongodb.com&lt;/a&gt; Instead of querying the oldest oplog, can we just periodically starts a dummy change stream at startAtTs in the background, and see if it returns ChangeStreamHistoryLost error to indicate we have fallen off oplog?&lt;/p&gt;</comment>
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