[mongodb-user] oplog count discrepancies

[Justin W]

Greetings,

We've recently been investigating using oplog tailing as a way for us to
export mongo data on-the-fly and have ran into something that is making us
second guess this solution. When using the 'local' db and running
'db.oplog.rs.count()' we get a lower number then running
'db.oplog.rs.find({'t': {$exists: true}}).count()'. Is this expected
behavior and if so can someone provide me with an explanation so I'm not
freaking about this anymore. :) I'm running mongo 3.2.

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Thank you,
Justin
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[John Murphy]

Hi Justin,

Is this expected behavior and if so can someone provide me with an
explanation so I’m not freaking about this anymore.

The count() <https://docs.mongodb.com/manual/reference/command/count/>
command is generally an estimate of the current document count, based upon
statistics provided by the storage engine.

After an unclean shutdown
<https://docs.mongodb.com/manual/reference/command/count/#accuracy-after-unexpected-shutdown>
of a mongod
<https://docs.mongodb.com/manual/reference/program/mongod/#bin.mongod>
using the WiredTiger <https://docs.mongodb.com/manual/core/wiredtiger/>
storage engine, count statistics reported by count may be inaccurate. If
this does occur you can run validate()
<https://docs.mongodb.com/manual/reference/command/validate/#dbcmd.validate>
on each collection to restore the correct statistics.

To accurately determine the current document count of a collection use the
$group
<https://docs.mongodb.com/manual/reference/operator/aggregation/group/#pipe._S_group>
stage of the db.collection.aggregate()
<https://docs.mongodb.com/manual/reference/method/db.collection.aggregate/#db.collection.aggregate>
method to $sum
<https://docs.mongodb.com/manual/reference/operator/aggregation/sum/#grp._S_sum>
the documents.

For example, the following query provides a real-time count of the
documents in the oplog.rs collection:

db.getSiblingDB('local').oplog.rs.aggregate([{ $group: { _id : null, count : { $sum: 1 } } }])

Regards,
John Murphy
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