Support for "double barriers"

[stensonb]

As described here: http://zookeeper.apache.org/doc/trunk/recipes.html (search for "Double Barriers")

chef-client use case: ensure all nodes (or at least N nodes) have stopped the execution of a server until any are allowed to proceed...(due to some terrible schema, non-backwards compatible upgrade, for example)

[zuazo]

Yes, this would be very interesting.

I think this should be previously implemented in the zk gem, to prevent cookbook libraries become too complex.

There are already some related (abandoned?) tickets:

• Recipe: Barrier #42
• Recipe: Double Barrier #43