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I have a mongo shards cluster and a mongos instance. These servers are running on MongoDB 3.4.0. When I use mongo shell 3.4.0 to connect to the mongos, I got below output from sh.isBalancerRunning(),

mongos> sh.isBalancerRunning()
false

but I got a different result when I use mongo shell 3.2.13:

mongos> sh.isBalancerRunning()
true

Is this something not compatible between mongo versions? If yes, why other commands work fine just this command doesn't?

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The difference is that 3.4 balancing work is done by config server primary and at 3.2 it was done by one of the mongos services.

To prevent "older" (<3.4) mongos services to start balancing work, config server (>=3.4) makes "lock" document to the local collection, what "states" that one of the mongos services have "balancing lock" set.

That document doesn't prevent (>=3.4) config server to do balancing. That is the reason why older mongos "report" that there is always balancing ongoing!

  • Thanks for this explanation. One question is what you mean by ` config server (>=3.4) makes "lock" document to the local collection`. Does this mean the config server always lock the local collection? – Zhao Yi Oct 6 '17 at 22:59
  • With "local" I mean config servers config DB and there locks collection. docs.mongodb.com/manual/reference/config-database/#config.locks – JJussi Oct 7 '17 at 12:48

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