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Container Runtime Interface special cases

The target of this document is to outline corner cases and common pitfalls in conjunction with the Container Runtime Interface (CRI). This document outlines CRI-O's interpretation of certain aspects of the interface, which may not be completely formalized.

The main documentation of the CRI can be found in the corresponding protobuf definition, whereas this document follows it on the service/rpc level.

ListImages

ListImages lists existing images. Its response consists of an array of Image types. Besides other information, an Image contains repo_tags and repo_digests, which are defined as:

// Other names by which this image is known.
repeated string repo_tags = 2;

// Digests by which this image is known.
repeated string repo_digests = 3;

Both tags and digests will be used by:

  • The kubelet, which displays them in the node status as a flat list, for example:

    > kubectl get node 127.0.0.1 -o json | jq .status.images

    [
  {
    "names": [
      "k8s.gcr.io/pause@sha256:4a1c4b21597c1b4415bdbecb28a3296c6b5e23ca4f9feeb599860a1dac6a0108",
      "k8s.gcr.io/pause@sha256:927d98197ec1141a368550822d18fa1c60bdae27b78b0c004f705f548c07814f",
      "k8s.gcr.io/pause:3.2"
    ],
    "sizeBytes": 688049
  }
]

    Right now, the amount of images shown is limited by the kubelet flag --node-status-max-images (). The scheduler uses this list to score nodes based on the information if a container image already exists.

  • crictl, which is able to output the image list in a human readable way:

    > sudo crictl images --digests
IMAGE                 TAG       DIGEST           IMAGE ID         SIZE
k8s.gcr.io/pause      3.2       4a1c4b21597c1    80d28bedfe5de    688kB

CRI-O implements the ConvertImage function to follow the following self-defined rules:

  • always return at least one repo_digests value
  • return zero or more repo_tags values

There are multiple use-cases where this behavior is relevant. Those will be covered separately by using real world examples.

Pulling an image from a remote registry

This is the standard behavior and already shown in the pause image example above. crictl is able to display all information, like the image name, tag and digest. There are multiple digests available for this image, which gets a correct representation within the kubelet's node status.

Pulling an updated version of an image with the same tag

Let's assume we pulled the image quay.io/saschagrunert/hello-world and afterwards its latest tag got updated. Now we pull the image again, which results in untagging the local image in favor of the new remote one.

CRI-O would now have no available RepoTags nor RepoDigests within the storage.ImageResult. In this case, CRI-O uses an assembled repoDigests value from the PreviousName and the image digest:

repoDigests = []string{from.PreviousName + "@" + string(from.Digest)}

This allows tools like crictl to output the image name by adding a <none> placeholder for the tag:

> sudo crictl images --digests
IMAGE                               TAG       DIGEST           IMAGE ID         SIZE
quay.io/saschagrunert/hello-world   <none>    2403474085c1e    14c28051b743c    5.88MB
quay.io/saschagrunert/hello-world   latest    ca810c5740f66    d1165f2212346    17.7kB

The kubelet is still able to list the image by its digest, which could be referenced by a Kubernetes container:

> kubectl get node 127.0.0.1 -o json | jq .status.images

{
  "names": [
    "quay.io/saschagrunert/hello-world@sha256:2403474085c1e68c0aa171eb1b2b824a841a4aa636a4f2500c8d2e2f6d3cb422"
  ],
  "sizeBytes": 5884835
}

Building container images locally

We assume that we consecutively build a container image locally like this:

> sudo podman build --no-cache -t test .

The previous image tag gets removed by Podman and applied to the current build. In that case CRI-O will use the PreviousName in the same way as described in the use-case above.

Pulling images by digest

If we pull a container image by its digest like this:

> sudo crictl pull docker.io/alpine@sha256:2a8831c57b2e2cb2cda0f3a7c260d3b6c51ad04daea0b3bfc5b55f489ebafd71

Then CRI-O will not be able to provide a RepoTags result, but a single entry in RepoDigests. The output for tools like crictl will be the same as described in the examples above. In the same way the node status receives the single digest entry:

{
  "names": [
    "docker.io/library/alpine@sha256:2a8831c57b2e2cb2cda0f3a7c260d3b6c51ad04daea0b3bfc5b55f489ebafd71"
  ],
  "sizeBytes": 5850080
}