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Based on note & i/m exchange with scotthernandez:
- The configdb option (order of the config servers) is not fixed forever, only for a running cluster
- All mongos must use the same configdb option (same order)
- The first (avail) config server listed is used for reads
- The configdb option (with the order) can be changed, but only if all mongos (and shards) are brought down and then up with a new configdb string.
- Only mongos and the shards keep track of this setting
- The actual config servers know nothing about the order, or each other
http://docs.mongodb.org/manual/reference/mongos/?highlight=configdb#cmdoption-mongos--configdb
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