juju HA lands on tip

[roger peppe]

I've just landed some changes on juju tip that enable "highly available"
juju environments. In a HA environment, there are several state servers -
if one dies, one of the others should take over.

To make a HA environment, run the "ensure-availability" command. For
example to make a HA environment with three state servers, run:

$ juju ensure-availability -n 3

This will add new servers as necessary. It will also maintain the
current state server set if some servers have gone down.

A new field in the status output, state-server-member-status, present
only for state server machines, indicates the current status of each
state server.

When the status of a majority of state server machines is "has-vote",
the environment should be resilient to any one of them going down.

If a state server machine goes down and you run ensure-availability again,
it should detect that the machine has gone down and create a new machine
to replace it. When the new machine is available, the original (down)
machine's status will become "no-vote".

If you run ensure-availability again, it will see that the machine has
no vote and is down, and will remove its state server status. At that
point the machine can actually be removed (you'll need the --force flag).

Anyone that's interested, please try it out. It would be great if people
could see whether it works for them - there are bound to be some teething
problems!

There is one major caveat: upgrading an existing environment
to HA is not supported currently (Andrew Wilkins has a branch in review that
should fix this).

Please note that the transition times when mongo is failing over can be
quite long (on the order of minutes), so you'll need to be patient.

Many thanks to everyone that's pitched in to help get this done.

cheers,
rog.
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