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    <p>MongoDB does not offer master-master replication or multi-version concurrency. In other words, writes <em>always</em> go to the same server in a replica set. By default, even reads from secondaries are disabled so the default behavior is that you communicate only with one server at a time. Therefore, you do not need to worry about inconsistent results in safe mode if you use atomic modifiers (like <code>$inc, $push</code>, etc.).</p> <p>If you don't want to restrict yourself to these atomic modifiers, compare and swap as recommended by dcrosta (and the <a href="http://www.mongodb.org/display/DOCS/Atomic+Operations" rel="nofollow">mongo docs</a>) looks like a good idea. All this is not related to replica sets or sharding, however - <strong>it would be the same in a single-server scenario</strong>.</p> <p>If you need to ensure read consistency also in case of a database/node failure, you should make sure you're writing to the majority of servers in safe mode.</p> <p>The two approaches behave <em>different if you allow unsafe reads</em>: the atomic update operations would still work (but may give unexpected results), while the compare-and-swap approach would fail.</p>
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    1. POMongoDB Document Operations are Atomic and Isolated, but Are They Consistent?
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    1. COI don't see why the compare-and-swap as I've proposed would fail under replication but not in single-server. The key point in my suggestion is that you only write if the version hasn't changed from when you began editing it (i.e. in a web form), and let the user decide what to do if it has been modified (or otherwise manage the conflict). In single-server, you can still have multiple users, so conflicts can happen in both scenarios.
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    2. COIf you allow *unsafe reads* (i.e. reads from secondaries), the version read might not be up-to-date. In that scenario, the client believes the current version is, say 12, but the master has already seen a write, increasing the version to 13. Now the update would fail because we read from a secondary. This can't happen in single-server, and it can't happen when reading only from the master. The modifier operation would still execute, but might not give the expected result, because the older version is different from what was displayed to the user.
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    3. COIt can happen in single-server scenarios (or when not reading from secondaries) if another user updates the document between when a user reads a version (12 in your example) and when they try to write (i.e. on web page submission). In other words, build the code to expect conflicts and handle them appropriately (which is an application specific concern; you're right that MongoDB doesn't handle this for you in any way).
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