Proposal: Volcano job support scale up and down

docs/design/job-scale-up-down.md

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+# Volcano Job scale up and down
+
+@hzxuzhonghu; April 24, 2020
+
+## Motivation
+
+Currently, Volcano does not support Job update. It is not allowed to update the `Job.Spec` on the fly.
+However, many users show appeal to run ML training jobs in a elastic manner. For example ModelArts want to dynamically adjust Job's replicas according to the cluster idle capacity 
+in order to achieve most high efficiency on GPU card.
+
+I propose to support volcano job dynamical scale up/down before more intelligent elasticity in the first step.
+
+## Design
+
+Before this design, let's recall the current Job's initialization
+
+### Job Initialization
+
+When a Volcano job is created, the job controller does the following to run/manage all of its tasks.
+
+1. all the plugins execute OnJobAdd callbacks to create service and hosts configmap, etc
+
+2. create pvc for the job
+
+3. create PodGroup for the job
+
+4. execute plugins' OnPodAdd callbacks to set pod related env, mount hostfile, etc
+
+5. call the kube-apiserver to create pods equals the replicas of the job
+
+All above steps are run in `syncJob`, which is called when external events happen, for this it happens when Job is newly created.
+
+### Volcano Job Scale Up/Down
+
+The Job's scale up and down correlates to reconciling of the resources the job owns, like PVC/PodGroup/Service/HostFile ConfigMap
+so the procedure is kind of similar to the [Job Initialization](#Job Initialization).
+
+The differences are:
+
+1. job plugins' callbacks：only the `svc` plugin should update the configmap including the job tasks
+
+2. create pods when scale up
+
+3. delete pods when scale down
+
+However, only when the job is not started, the initialization is run. 
+So we need a way to know whether it is a scale up/down event that triggered this round of sync.
+
+The way I propose is to add a new event `JobUpdatedEvent` to indicate that the job is updated(here only cares about the scale up/down).
+And accordingly add a new action `UpdateJobAction` to run `UpdateJob` function. And the overall workflow is:
+![workflow](images/Job-scale-up-down.PNG)
+
+To scale up/down on the fly, Volcano should be responsible to notify the original pods the current status, including the hosts of all the pods.
+This is done by plugins, so to distinguish from the initialization phase, a new `OnJobUpdate` is introduced. 
+It is to reconcile all the associated configs of the job. Currently, the `svc` plugin should update the configmap of all the hosts.
+
+**NOTE**: Users should watch the `/etc/volcano` to get the up-to-date hosts files if they want to be aware of the training workers.
+
+```
+type PluginInterface interface {
+	// The unique name of Plugin.
+	Name() string
+
+	// for all pod when createJobPod
+	OnPodCreate(pod *v1.Pod, job *vcbatch.Job) error
+
+	// do once when syncJob
+	OnJobAdd(job *vcbatch.Job) error
+
+	// do once when killJob
+	OnJobDelete(job *vcbatch.Job) error
+
+	OnJobUpdate(job *vcbatch.Job) error
+}
+```
+
+`UpdateJob` is much like the current `SyncJob`, and it's workflow is:
+
+1. all the plugins execute OnJobUpdate callbacks, which is to update all the envs, service and hosts configmap.
+
+2. create pvc for the job if necessary
+
+3. update PodGroup for the job if necessary
+
+4. execute plugins' OnPodAdd callbacks to set pod related env, mount hostfile, etc
+
+5. call the kube-apiserver to create/delete pods equals the replicas of the job
+
+
+**Note**: when scale down, the pod delete order is from the larger indexed to the lower indexed. But this is not guaranteed as Kubernetes is a eventual consistent system.
+
+
+
+### Admission webhook
+
+Should prevent invalid mutating Job Spec on the fly. In this proposal, we only allow `replicas` and `minAvailable` update. Any other spec changes will be prohibited.
+It is also not allowed if the number of total replicas is less than the `minAvailable`.
+
+`minAvailable` must be greater than zero, we depend on it to maintain the job status.
+
+
+
+