Users of the server-side garbage collection need to determine if the garbage collection is done. For example:
* Currently `kubectl delete rc` blocks until all the pods are terminating. To convert to use server-side garbage collection, kubectl has to be able to determine if the garbage collection is done.
* [#19701](https://github.com/kubernetes/kubernetes/issues/19701#issuecomment-236997077) is a use case where the user needs to wait for all service dependencies garbage collected and their names released, before she recreates the dependencies.
We define the garbage collection as "done" when all the dependents are deleted from the key-value store, rather than merely in the terminating state. There are two reasons: *i)* for `Pod`s, the most usual garbage, only when they are deleted from the key-value store, we know kubelet has released their resources; *ii)* some users need to recreate objects with the same names, they need to wait for the old objects to be deleted from the key-value store. (This limitation is because we index objects by their names in the key-value store today.)
Synchronous Garbage Collection is a best-effort (see [unhandled cases](#unhandled-cases)) mechanism that allows user to determine if the garbage collection is done: after the API server receives a deletion request of an owning object, the object keeps existing in the key-value store until all its dependents are deleted from the key-value store by the garbage collector.
// If SynchronousGarbageCollection is set, the object will not be deleted immediately. Instead, a CollectingGarbage finalizer will be placed on the object. The garbage collector will remove the finalizer from the object when all depdendents are deleted.
* The request is rejected with 400 if both `DeleteOptions.SynchronousGarbageCollection` and `DeleteOptions.OrphanDependents` are true (or default to true, see [DefaultGarbageCollectionPolicy](https://github.com/kubernetes/kubernetes/blob/release-1.4/pkg/registry/generic/registry/store.go#L500)).
`processEvent()` manages GC’s internal owner-dependency relationship graph, `uidToNode`. It updates `uidToNode` according to the Add/Update/Delete events in the cluster. To support synchronous GC, it has to:
* handle Add or Update events where `obj.Finalizers.Has(GCFinalizer) && obj.DeletionTimestamp != nil`. The object will be added into the `synchronousGC queue`. The object will be marked as “GC in progress” in `uidToNode`.
* Upon receiving the deletion event of an object, put its owner into the `synchronousGC queue`. This is to force the `GCFinalizer` (described next) to re-check if all dependents of the owner is deleted.
**Addition of GCFinalizer() routine**
* Pops an object from the `synchronousGC queue`.
* Ignores the object if it doesn’t exist in `uidToNode`, or if the object is not marked as “GC in progress” in `uidToNode`.
* To avoid racing with another controller, it requeues the object if `observedGeneration < Generation`. This is best-effort, see [unhandled cases](#unhandled-cases).
* If so, then add all dependents to the `dirtryQueue`; we need bookkeeping to avoid adding the dependents repeatedly if the owner gets in the `synchronousGC queue` multiple times.
`processItem()` consumes the `dirtyQueue`, requests the API server to delete an item if all of its owners do not exist. To support synchronous GC, it has to:
* treat an owner as "not exist" if `owner.DeletionTimestamp != nil && !owner.Finalizers.Has(OrphanFinalizer)`, otherwise Synchronous GC will not progress because the owner keeps existing in the key-value store.
* if an object has multiple owners, some owners still exist while other owners are in the synchronous GC stage, then according to the existing logic of GC, the object wouldn't be deleted. To unblock the synchronous GC of owners, `processItem()` has to remove the ownerReferences pointing to them.
SynchronousGC will enter a deadlock in the presence of circular dependencies. The garbage collector can break the circle by lazily breaking circular dependencies: when `processItem()` processes an object, if it finds the object and all of its owners have the `GCFinalizer`, it removes the `GCFinalizer` from the object.
Note that the approach is not rigorous and thus having false positives. For example, if a user first sends a SynchronousGC delete request for an object, then sends the delete request for its owner, then `processItem()` will be fooled to believe there is a circle. We expect user not to do this. We can make the circle detection more rigorous if needed.
Circular dependencies are regarded as user error. If needed, we can add more guarantees to handle such cases later.
* If the GC observes the owning object with the `GCFinalizer` before it observes the creation of all the dependents, GC will remove the finalizer from the owning object before all dependents are gone. Hence, “Synchronous GC” is best-effort, though we guarantee that the dependents will be deleted eventually. We face a similar case when handling OrphanFinalizer, see [GC known issues](https://github.com/kubernetes/kubernetes/issues/26120).
Finalizer breaks an assumption that many Kubernetes components have: a deletion request with `grace period=0` will immediately remove the object from the key-value store. This is not true if an object has pending finalizers, the object will continue to exist, and currently the API server will not return an error in this case.
**Namespace controller** suffered from this [problem](https://github.com/kubernetes/kubernetes/issues/32519) and was fixed in [#32524](https://github.com/kubernetes/kubernetes/pull/32524) by retrying every 15s if there are objects with pending finalizers to be removed from the key-value store. Object with pending `GCFinalizer` might take arbitrary long time be deleted, so namespace deletion might time out.
**kubelet** deletes the pod from the key-value store after all its containers are terminated ([code](../../pkg/kubelet/status/status_manager.go#L441-L443)). It also assumes that if the API server does not return an error, the pod is removed from the key-value store. Breaking the assumption will not break `kubelet` though, because the `pod` must have already been in the terminated `phase`, `kubelet` will not care to manage it.
**Node controller** forcefully deletes pod if the pod is scheduled to a node that does not exist ([code](../../pkg/controller/node/nodecontroller.go#L474)). The pod will continue to exist if it has pending finalizers. The node controller will futilely retry the deletion. Also, the `node controller` forcefully deletes pods before deleting the node ([code](../../pkg/controller/node/nodecontroller.go#L592)). If the pods have pending finalizers, the `node controller` will go ahead deleting the node, leaving those pods behind. These pods will be deleted from the key-value store when the pending finalizers are removed.
**Podgc** deletes terminated pods if there are too many of them in the cluster. We need to make sure finalizers on Pods are taken off quickly enough so that the progress of `Podgc` is not affected.
**Deployment controller** adopts existing `ReplicaSet` (RS) if its template matches. If a matching RS has a pending `GCFinalizer`, deployment should adopt it, take its pods into account, but shouldn't try to mutate it, because the RS controller will ignore a RS that's being deleted. Hence, `deployment controller` should wait for the RS to be deleted, and then create a new one.
**Replication controller manager**, **Job controller**, and **ReplicaSet controller** ignore pods in terminated phase, so pods with pending finalizers will not block these controllers.
**kubectl**: synchronous GC can simplify the **kubectl delete** reapers. Let's take the `deployment reaper` as an example, since it's the most complicated one. Currently, the reaper finds all `RS` with matching labels, scales them down, polls until `RS.Status.Replica` reaches 0, deletes the `RS`es, and finally deletes the `deployment`. If using the synchronous GC, `kubectl delete deployment` is as easy as sending a synchronous GC delete request for the deployment, and polls until the deployment is deleted from the key-value store.
Note that this **changes the behavior** of `kubectl delete`. The command will be blocked until all pods are deleted from the key-value store, instead of being blocked until pods are in the terminating state. This means `kubectl delete` blocks for longer time, but it has the benefit that the resources used by the pods are released when the `kubectl delete` returns.
Old kubectl is not affected, because `DeleteOptions.SynchronousGarbageCollection` defaults to false.
A user who is authorized to update one object can affect the synchronous GC behavior of another object. Specifically, by setting an object as a pod's owner, and setting a very long grace termination period for the pod, a user can make the synchronous GC of the owner to take long time.
