Kubelet: Runtime Pod Cache

This proposal builds on top of the Pod Lifecycle Event Generator (PLEG) proposed in #12802. It assumes that Kubelet subscribes to the pod lifecycle event stream to eliminate periodic polling of pod states. Please see #12802. for the motivation and design concept for PLEG.

Runtime pod cache is an in-memory cache which stores the status of all pods, and is maintained by PLEG. It serves as a single source of truth for internal pod status, freeing Kubelet from querying the container runtime.

Motivation

With PLEG, Kubelet no longer needs to perform comprehensive state checking for all pods periodically. It only instructs a pod worker to start syncing when there is a change of its pod status. Nevertheless, during each sync, a pod worker still needs to construct the pod status by examining all containers (whether dead or alive) in the pod, due to the lack of the caching of previous states. With the integration of pod cache, we can further improve Kubelet's CPU usage by

Lowering the number of concurrent requests to the container runtime since pod workers no longer have to query the runtime individually.
Lowering the total number of inspect requests because there is no need to inspect containers with no state changes.

Don't we already have a container runtime cache?

The runtime cache is an optimization that reduces the number of GetPods() calls from the workers. However,

The cache does not store all information necessary for a worker to complete a sync (e.g., docker inspect); workers still need to inspect containers individually to generate api.PodStatus.
Workers sometimes need to bypass the cache in order to retrieve the latest pod state.

This proposal generalizes the cache and instructs PLEG to populate the cache, so that the content is always up-to-date.

Why can't each worker cache its own pod status?

The short answer is yes, they can. The longer answer is that localized caching limits the use of the cache content -- other components cannot access it. This often leads to caching at multiple places and/or passing objects around, complicating the control flow.

Runtime Pod Cache

Pod cache stores the PodStatus for all pods on the node. PodStatus encompasses all the information required from the container runtime to generate api.PodStatus for a pod.

// PodStatus represents the status of the pod and its containers.
// api.PodStatus can be derived from examining PodStatus and api.Pod.
type PodStatus struct {
    ID types.UID
    Name string
    Namespace string
    IP string
    ContainerStatuses []*ContainerStatus
}

// ContainerStatus represents the status of a container.
type ContainerStatus struct {
    ID ContainerID
    Name string
    State ContainerState
    CreatedAt time.Time
    StartedAt time.Time
    FinishedAt time.Time
    ExitCode int
    Image string
    ImageID string
    Hash uint64
    RestartCount int
    Reason string
    Message string
}

PodStatus is defined in the container runtime interface, hence is runtime-agnostic.

PLEG is responsible for updating the entries pod cache, hence always keeping the cache up-to-date.

Detect change of container state
Inspect the pod for details
Update the pod cache with the new PodStatus
- If there is no real change of the pod entry, do nothing
- Otherwise, generate and send out the corresponding pod lifecycle event

Note that in (3), PLEG can check if there is any disparity between the old and the new pod entry to filter out duplicated events if needed.

Evict cache entries

Note that the cache represents all the pods/containers known by the container runtime. A cache entry should only be evicted if the pod is no longer visible by the container runtime. PLEG is responsible for deleting entries in the cache.

Generate `api.PodStatus`

Because pod cache stores the up-to-date PodStatus of the pods, Kubelet can generate the api.PodStatus by interpreting the cache entry at any time. To avoid sending intermediate status (e.g., while a pod worker is restarting a container), we will instruct the pod worker to generate a new status at the beginning of each sync.

Cache contention

Cache contention should not be a problem when the number of pods is small. When Kubelet scales, we can always shard the pods by ID to reduce contention.

Disk management

The pod cache is not capable to fulfill the needs of container/image garbage collectors as they may demand more than pod-level information. These components will still need to query the container runtime directly at times. We may consider extending the cache for these use cases, but they are beyond the scope of this proposal.

Impact on Pod Worker Control Flow

A pod worker may perform various operations (e.g., start/kill a container) during a sync. They will expect to see the results of such operations reflected in the cache in the next sync. Alternately, they can bypass the cache and query the container runtime directly to get the latest status. However, this is not desirable since the cache is introduced exactly to eliminate unnecessary, concurrent queries. Therefore, a pod worker should be blocked until all expected results have been updated to the cache by PLEG.

Depending on the type of PLEG (see #12802) in use, the methods to check whether a requirement is met can differ. For a PLEG that solely relies on relisting, a pod worker can simply wait until the relist timestamp is newer than the end of the worker's last sync. On the other hand, if pod worker knows what events to expect, they can also block until the events are observed.

It should be noted that api.PodStatus will only be generated by the pod worker after the cache has been updated. This means that the perceived responsiveness of Kubelet (from querying the API server) will be affected by how soon the cache can be populated. For the pure-relisting PLEG, the relist period can become the bottleneck. On the other hand, A PLEG which watches the upstream event stream (and knows how what events to expect) is not restricted by such periods and should improve Kubelet's perceived responsiveness.

TODOs for v1.2

Redefine container runtime types (#12619): and introduce PodStatus. Refactor dockertools and rkt to use the new type.
Add cache and instruct PLEG to populate it.
Refactor Kubelet to use the cache.
Deprecate the old runtime cache.

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