support rolling upgrades of a rails app

this gets complicated with schema changes :)

couple of methods... most promising is multiple named services for a single rails app...

i.e., two pools of app servers "myappA" and "myappB" that're all in normal rotation and sharing db replicas... until an upgrade.

    juju deploy mysql master-db
    juju deploy mysql slavedbA
    juju add-relation master-db:master slavedbA:slave
    juju deploy mysql slavedbB
    juju add-relation master-db:master slavedbB:slave

    juju deploy rails myappA
    juju add-relation myappA masterdb:db
    juju add-relation myappA slavedbA:slave

    juju deploy rails myappB
    juju add-relation myappB masterdb:db
    juju add-relation myappB slavedbB:slave

halt replication..

    juju set slavedbB replication=off

switch one pool to read-only via juju config... something like

    juju set myappB read-only=true

and upgrade a "segment" at a time.