velero/design/wait-for-additional-items.md

Wait for AdditionalItems to be ready on Restore

When a velero RestoreItemAction plugin returns a list of resources via AdditionalItems, velero restores these resources before restoring the current resource. There is a race condition here, as it is possible that after running the restore on these returned items, the current item's restore might execute before the additional items are available. Depending on the nature of the dependency between the current item and the additional items, this could cause the restore of the current item to fail.

Goals

• Enable Velero to ensure that Additional items returned by a restore plugin's Execute func are ready before restoring the current item
• Augment the RestoreItemAction plugin interface to allow the plugins to determine when an additional item is ready, since doing so requires knowledge specific to the resource type.

Background

Because Velero does not wait after restoring additional items to restore the current item, in some cases the current item restore will fail if the additional items are not yet ready. Velero (and the RestoreItemAction plugins) need to implement this "wait until ready" functionality.

High-Level Design

After each RestoreItemAction execute call (and following restore of any returned additional items) , we need to wait for these returned Additional Items to be ready before restoring the current item. In order to do this, we also need to extend the RestoreItemActionExecuteOutput struct to allow the plugin which returned additional items to determine whether they are ready.

Detailed Design

restoreItem Changes

When each RestoreItemAction Execute() call returns, the RestoreItemActionExecuteOutput struct contains a slice of AdditionalItems which must be restored before this item can be restored. After restoring these items, Velero needs to be able to wait for them to be ready before moving on to the next item. Right after looping over the additional items at https://github.com/vmware-tanzu/velero/blob/main/pkg/restore/restore.go#L960-L991

we still have a reference to the additional items (GroupResource and namespaced name), as well as a reference to the RestoreItemAction plugin which required it.

At this point, if the RestoreItemActionExecuteOutput WaitForAdditionalItems field is set to true we need to call a func similar to crdAvailable which we will call itemsAvailable https://github.com/vmware-tanzu/velero/blob/main/pkg/restore/restore.go#L623 This func should also be defined within restore.go

Instead of the one minute CRD timeout, we'll use a timeout specific to waiting for additional items. There will be a new field added to serverConfig, additionalItemsReadyTimeout, with a defaultAdditionalItemsReadyTimeout const set to 10 minutes. In addition, each plugin will be able to define an override for the global server-level value, which will be added as another optional field in the RestoreItemActionExecuteOutput struct. Instead of the IsUnstructuredCRDReady call, we'll call AreAdditionalItemsReady on the plugin, passing in the same AdditionalItems slice as an argument (with items which failed to restore filtered out). If this func returns an error, then itemsAvailable will propagate the error, and restoreItem will handle it the same way it handles an error return on restoring an additional item. If the timeout is reached without ready returning true, velero will continue on to attempt restore of the current item.

RestoreItemAction plugin interface changes

In order to implement the AreAdditionalItemsReady plugin func, a new function will be added to the RestoreItemAction interface.

type RestoreItemAction interface {
        // AppliesTo returns information about which resources this action should be invoked for.
        // A RestoreItemAction's Execute function will only be invoked on items that match the returned
        // selector. A zero-valued ResourceSelector matches all resources.
        AppliesTo() (ResourceSelector, error)

        // Execute allows the ItemAction to perform arbitrary logic with the item being restored,
        // including mutating the item itself prior to restore. The item (unmodified or modified)
        // should be returned, along with an optional slice of ResourceIdentifiers specifying additional
        // related items that should be restored, a warning (which will be logged but will not prevent
        // the item from being restored) or error (which will be logged and will prevent the item
        // from being restored) if applicable.
        Execute(input *RestoreItemActionExecuteInput) (*RestoreItemActionExecuteOutput, error)

	// AreAdditionalItemsReady allows the ItemAction to communicate whether the passed-in
	// slice of AdditionalItems (previously returned by Execute())
	// are ready. Returns true if all items are ready, and false
	// otherwise. The second return value is an error string if an
	// error occurred.
	AreAdditionalItemsReady(restore *api.Restore, AdditionalItems []ResourceIdentifier) (bool, string)
}

RestoreItemActionExecuteOutput changes

Two new fields will be added to RestoreItemActionExecuteOutput, both optional. AdditionalItemsReadyTimeout, if non-zero, will override serverConfig.additionalItemsReadyTimeout. If WaitForAdditionalItems is true, then restoreItem will call itemsAvailable which will invoke the plugin func AreAdditionalItemsReady and wait until the func returns true or the timeout is reached. If WaitForAdditionalItems is false (the default case), then current velero behavior will be followed. Existing plugins which do not need to signal to wait for AdditionalItems won't need to change their Execute() functions.

In addition, a new func, WithItemsWait() will be added to RestoreItemActionExecuteOutput similar to WithoutRestore() which will set WaitForAdditionalItems to true. This will allow a plugin to include waiting for AdditionalItems like this:

func AreAdditionalItemsReady (restore *api.Restore, additionalItems []ResourceIdentifier) (bool, string) {
	...
	return true, ""
}
func (p *RestorePlugin) Execute(input *velero.RestoreItemActionExecuteInput) (*velero.RestoreItemActionExecuteOutput, error) {
	...
	return velero.NewRestoreItemActionExecuteOutput(input.Item).WithItemsWait(), nil
}

// RestoreItemActionExecuteOutput contains the output variables for the ItemAction's Execution function.
type RestoreItemActionExecuteOutput struct {
	// UpdatedItem is the item being restored mutated by ItemAction.
	UpdatedItem runtime.Unstructured

	// AdditionalItems is a list of additional related items that should
	// be restored.
	AdditionalItems []ResourceIdentifier

	// SkipRestore tells velero to stop executing further actions
	// on this item, and skip the restore step. When this field's
	// value is true, AdditionalItems will be ignored.
	SkipRestore bool

        // WaitForAdditionalItems determines whether velero will wait
	// until AreAdditionalItemsReady returns true before restoring
	// this item. If this field's value is true, then after restoring
	// the returned AdditionalItems, velero will not restore this item
	// until AreAdditionalItemsReady returns true or the timeout is
        // reached. Otherwise, AreAdditionalItemsReady is not called.
        WaitForAdditionalItems bool

	// AdditionalItemsReadyTimeout will override serverConfig.additionalItemsReadyTimeout
	// if specified. This value specifies how long velero will wait
	// for additional items to be ready before moving on.
	AdditionalItemsReadyTimeout time.Duration
}

// WithItemsWait returns RestoreItemActionExecuteOutput with WaitForAdditionalItems set to true.
func (r *RestoreItemActionExecuteOutput) WithItemsWait()
) *RestoreItemActionExecuteOutput {
	r.WaitForAdditionalItems = true
	return r
}

New design iteration (Feb 2021)

In starting the implementation based on the originally approved design, I've run into an unexpected snag. When adding the wait func pointer to the RestoreItemActionExecuteOutput struct, I had forgotten about the protocol buffer message format that's used for passing args to the plugin methods. Funcs are predefined RPC calls with autogenerated go code, so we can't just pass a regular golang func pointer in the struct. I've modified the above design to instead use an explicit AreAdditionalItemsReady func. Since this will break backwards compatibility with current RestoreItemAction plugins, implementation of this feature should wait until Velero plugin versioning, as described in https://github.com/vmware-tanzu/velero/issues/3285 is implemented. With plugin versioning in place, existing (non-versioned or 1.0-versioned) RestoreItemAction plugins which do not define AreAdditionalItemsReady would be able to coexist with a to-be-implemented RestoreItemAction plugin version 2.0 (or 1.1, etc.) which defines this new interface method. Without plugin versioning, implementing this feature would break all existing plugins until they define AreAdditionalItemsReady.

Also note that when moving to the new plugin version, the vast majority of plugins will probably not need to wait for additional items. All they will need to do to react to this plugin interface change would be to define the following in the plugin:

func AreAdditionalItemsReady (restore *api.Restore, additionalItems []ResourceIdentifier) (bool, string) {
	return true, ""
}

As long as they never set WaitForAdditionalItems to true, this function won't be called anyway, but if it is called, there will be no waiting, since it will always return true.