## Simple rolling update
This is a lightweight design document for simple rolling update in ```kubectl```

Complete execution flow can be found [here](#execution-details).

### Lightweight rollout
Assume that we have a current replication controller named ```foo``` and it is running image ```image:v1```

```kubectl rolling-update rc foo [foo-v2] --image=myimage:v2```

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 replicaiton 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.)

Recovery is achieved by issuing the same command again:

```
kubectl rolling-update rc foo [foo-v2] --image=myimage:v2
```

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.

```kubectl rolling-update rc foo [foo-v2] --rollback```

This is really just semantic sugar for:

```kubectl rolling-update rc foo-v2 foo```

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.
