If the user doesn't specify a name for the 'next' controller, then the 'next' controller is renamed to
the name of the original controller.
Obviously there is a race here, where if you kill the client between delete foo, and creating the new version of 'foo' you might be surprised about what is there, but I think that's ok.
See [Recovery](#recovery) below
If the user does specify a name for the 'next' controller, then the 'next' controller is retained with its existing name,
and the old 'foo' controller is deleted. For the purposes of the rollout, we add a unique-ifying label ```kubernetes.io/deployment``` to both the ```foo``` and ```foo-next``` controllers.
The value of that label is the hash of the complete JSON representation of the```foo-next``` or```foo``` controller. The name of this label can be overridden by the user with the ```--deployment-label-key``` flag.
#### Recovery
If a rollout fails or is terminated in the middle, it is important that the user be able to resume the roll out.
To facilitate recovery in the case of a crash of the updating process itself, we add the following annotations to each replicaController in the ```kubernetes.io/``` annotation namespace:
* ```desired-replicas``` The desired number of replicas for this controller (either N or zero)
* ```update-partner``` A pointer to the replication controller resource that is the other half of this update (syntax ```<name>``` the namespace is assumed to be identical to the namespace of this replication controller.)
Whenever the rolling update command executes, the kubectl client looks for replication controllers called ```foo``` and ```foo-next```, if they exist, an attempt is
made to roll ```foo``` to ```foo-next```. If ```foo-next``` does not exist, then it is created, and the rollout is a new rollout. If ```foo``` doesn't exist, then
it is assumed that the rollout is nearly completed, and ```foo-next``` is renamed to ```foo```. Details of the execution flow are given below.
### Aborting a rollout
Abort is assumed to want to reverse a rollout in progress.
With the added detail that it moves the ```desired-replicas``` annotation from ```foo-v2``` to ```foo```
### Execution Details
For the purposes of this example, assume that we are rolling from ```foo``` to ```foo-next``` where the only change is an image update from `v1` to `v2`
If the user doesn't specify a ```foo-next``` name, then it is either discovered from the ```update-partner``` annotation on ```foo```. If that annotation doesn't exist,
then ```foo-next``` is synthesized using the pattern ```<controller-name>-<hash-of-next-controller-JSON>```
#### Initialization
* If ```foo``` and ```foo-next``` do not exist:
* Exit, and indicate an error to the user, that the specified controller doesn't exist.
* If ```foo``` exists, but ```foo-next``` does not:
* Create ```foo-next``` populate it with the ```v2``` image, set ```desired-replicas``` to ```foo.Spec.Replicas```
* Goto Rollout
* If ```foo-next``` exists, but ```foo``` does not:
* Assume that we are in the rename phase.
* Goto Rename
* If both ```foo``` and ```foo-next``` exist:
* Assume that we are in a partial rollout
* If ```foo-next``` is missing the ```desired-replicas``` annotation
* Populate the ```desired-replicas``` annotation to ```foo-next``` using the current size of ```foo```
* Goto Rollout
#### Rollout
* While size of ```foo-next``` < ```desired-replicas``` annotation on ```foo-next```
* increase size of ```foo-next```
* if size of ```foo``` > 0
decrease size of ```foo```
* Goto Rename
#### Rename
* delete ```foo```
* create ```foo``` that is identical to ```foo-next```
* delete ```foo-next```
#### Abort
* If ```foo-next``` doesn't exist
* Exit and indicate to the user that they may want to simply do a new rollout with the old version
* If ```foo``` doesn't exist
* Exit and indicate not found to the user
* Otherwise, ```foo-next``` and ```foo``` both exist
* Set ```desired-replicas``` annotation on ```foo``` to match the annotation on ```foo-next```
* Goto Rollout with ```foo``` and ```foo-next``` trading places.
