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    <p>There may be other ways to accomplish this, but one approach is to version your documents, and issue updates against only the version that the user had previously read (i.e., ensure that no one else has updated the document since it was last read). Here's a brief example of this technique using pymongo:</p> <pre><code>&gt;&gt;&gt; db.foo.save({'_id': 'a', 'version': 1, 'things': []}, safe=True) 'a' &gt;&gt;&gt; db.foo.update({'_id': 'a', 'version': 1}, {'$push': {'things': 'thing1'}, '$inc': {'version': 1}}, safe=True) {'updatedExisting': True, 'connectionId': 112, 'ok': 1.0, 'err': None, 'n': 1} </code></pre> <p>note in the above, key "n" is 1, indicating that the document was updated</p> <pre><code>&gt;&gt;&gt; db.foo.update({'_id': 'a', 'version': 1}, {'$push': {'things': 'thing2'}, '$inc': {'version': 1}}, safe=True) {'updatedExisting': False, 'connectionId': 112, 'ok': 1.0, 'err': None, 'n': 0} </code></pre> <p>here where we tried to update against the wrong version, key "n" is 0</p> <pre><code>&gt;&gt;&gt; db.foo.update({'_id': 'a', 'version': 2}, {'$push': {'things': 'thing2'}, '$inc': {'version': 1}}, safe=True) {'updatedExisting': True, 'connectionId': 112, 'ok': 1.0, 'err': None, 'n': 1} &gt;&gt;&gt; db.foo.find_one() {'things': ['thing1', 'thing2'], '_id': 'a', 'version': 3} </code></pre> <p>Note that this technique relies on using safe writes, otherwise we don't get an acknowledgement indicating the number of documents updated. A variation on this would use the <a href="http://www.mongodb.org/display/DOCS/findAndModify+Command"><code>findAndModify</code></a> command, which will either return the document, or <code>None</code> (in Python) if no document matching the query was found. <code>findAndModify</code> allows you to return either the new (i.e. after updates are applied) or old version of the document.</p>
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    1. POMongoDB Document Operations are Atomic and Isolated, but Are They Consistent?
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    1. CO+1 Good answer but worth noting that this approach puts the responsibility on the client (your application code) to check to see what is the result of the findAndModify command and re-issue the update if the findAndModify fails (as an indication that the document has already been updated by someone else). I think this is a fair trade off to make.
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    2. CO@dcrosta That makes sense... so basically as long as we use the highest level of "safe write" (WriteConcern.REPLICAS_SAFE for us Java drinkers) there's virtually no chance of two clients accessing different replicas at the exact same time and both (successfuly) writing conflicting "version: 1" updates?
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    3. CO@nomizzz See also: http://www.mongodb.org/display/DOCS/Atomic+Operations#AtomicOperations-%22UpdateifCurrent%22
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