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POdb.collection.count() returns a lot more documents for sharded collection in MongoDB
 

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  1. POdb.collection.count() returns a lot more documents for sharded collection in MongoDB
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    <p>I have 2 shards with replication sets (3 instances each). When I do <code>count()</code> on a sharded collection, I get a lot more than the real number of documents (more than 2.5 millions documents difference). Same when I just do <code>find()</code> and incrementing counter in <code>forEach()</code> loop.</p> <p>How do I know real number of documents? First of all, I know the trend of increase, i.e. it can not increase so radically. Secondly, when I count documents with the following M/R script, I get real number of documents (as I assume). I use this script to see duplicate documents. Number of duplicates is several thousands not millions. And the count on <code>test_duplicate_collection</code> minus duplicates gives me real number of documents. </p> <pre><code>var map = function(){ emit(this.doc_id, 1); }; var reduce = function(key, values){ var result = 0; values.forEach(function(value) { result += value; }); return result; }; db.test_collection.mapReduce(map, reduce, "test_duplicate_collection",null ); </code></pre> <p>Now, I understand that during balancing it can happen that some chunks are not deleted yet while transferring them to another shard. But I see in the status (<code>sh.status()</code>) that all chunks are equally distributed. I have also tried to pause write operations to see if it takes some time, but nothing happened. </p> <p>You might say deletion of moved chunks is still going on, and indeed when I just started to use sharding I saw slight decreases (with no write operations) for sharded collection. But currently, there is no change over time, it just stands still. I tried also to use <code>orphanage.js</code> with the hope to find orphaned documents (using the script from <a href="https://groups.google.com/forum/#!topic/mongodb-user/OKH5_KDO04I" rel="nofollow">https://groups.google.com/forum/#!topic/mongodb-user/OKH5_KDO04I</a>) but no such documents have been found.</p> <p>My question is what can be the reason that <code>count()</code> and <code>find().forEach()</code> give more than real number of documents (i.e. vs M/R script). </p> <p>Appreciate your help.</p> <p><strong>EDIT1</strong></p> <p>There was a problem with the configuration of the replication set in one of the shards. Specifically, no master has been set in the configuration file. In MMS dashboard instead of <code>Primary</code> I always saw <code>Slave</code> for host who was listened by other replication hosts. When we fixed it, <code>forEach</code> loop count started to show the same number of documents as in M/R script above. So the only problem currently is with the <code>count()</code> itself. </p> <p>In MongoDB JIRA I found the following unresolved bug with count() in sharded environment <a href="https://jira.mongodb.org/browse/SERVER-3645" rel="nofollow">https://jira.mongodb.org/browse/SERVER-3645</a> But it really relates to count() during balancing, i.e. count may count chunks which are currently moved by the balancer. As a workaround this bug proposes to put query which is always true. I tried it also but still it returns count as before.</p>
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    1. COdid you check if the insertion/updation query is correct (sharing that would be useful)?
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    2. COYou are executing all queries via mongos and not directly against the shards, correct?
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    3. CO@Sai I will double check regarding insert/update.
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