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The primary is running mongod 3.0.6 and the secondaries running mongod 3.0.7

is a mixing of versions allowed in the same replica set?

For upgrade it's best practise to not all nodes the same time.

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For minor versions its not an issue, although i would suggest to keep the same minor version across all nodes to avoid different behavior (due to different bugs and optimization patches between versions) after a stepdown. For major versions is fine when the primary is one major version behind the secondaries. Its not a good idea to have a primary on different major version from a secondary.

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    This seems to be more heresay... Not a good idea most likely but how else do you upgrade from 3.0 to say 3.2, you don't upgrade all of them at the same time. – runxc1 Bret Ferrier Mar 21 '16 at 17:16
  • Keeping the mixed mode for couple of minutes/hours during upgrade is acceptable but primary should be upgraded last. Are you saying that you going to upgrade one node to 3.2 and then promote it as primary leaving the other nodes to 3.0? – Antonios Mar 21 '16 at 21:19
  • No I am adding new nodes and removing old nodes slowly bringing them up to 3.2 but there is going to be a non nominal time where the nodes are mixed, it shouldn't be left that way indefinitely but it is designed to be able to operate like that. – runxc1 Bret Ferrier Mar 23 '16 at 17:52
  • In fact, I tried and met a problem auth, 2.4 and 3.6 use different auth mechanics. – tim Jul 8 '17 at 22:42

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