At one point, if you were interested in Version 6, you could take a look at Apollo subproject as it was expected to be the core of the 6.0 broker.  However, as of July 2015, Apollo is being unofficially declared dead.  There is currently no official decision about the path forward for 6.0, though it's possible that the Artemis subproject will instead become 6.0 at some point.

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