CODEWITHSUNDEEP
kubernetes
Akshit Bansal
Akshit Bansal

Reputation: 145

Difference between Always and IfNotPresent imagePullPolicy

I was checking Kubernetes documentation for pulling images. In that, I saw two policies IfNotPresent and Always. In "Always" its stated that

If the kubelet has a container image with that exact digest cached locally, the kubelet uses its cached image; otherwise, the kubelet pulls the image with the resolved digest, and uses that image to launch the container.

I am unable to understand what is local here. Is it a node, pod, or cluster? What is the difference between Always and IfNotPresent if it is at node level? It's very confusing.

Upvotes: 8

Views: 20499

Answers (5)

Viswesn
Viswesn

Reputation: 4880

When you use an image WITHOUT a tag, Kubernetes will assume that you want the latest version of the image, which is identified by the latest tag by default. If you have multiple versions of the same image in your repository with different tags, such as img1:1.0.0, img1:1.1.0, and img1:latest, Kubernetes will use the image with the tag specified in the pod specification.

If you use IfNotPresent and the image with the specified tag is already present on the worker node, Kubernetes will use that image to start the container, even if there is a newer version of the image available in the repository with the same tag.

If you use Always, however, Kubernetes will always attempt to download the latest version of the image with the specified tag from the repository, even if a cached copy of the image is already present on the worker node. This can be useful if you want to ensure that your containers are always running the latest version of the image.

consider a scenario where a container is running on a worker node with img1:latest as the latest tag, and then the container restarts or reschedules on another worker node with the same tag pointing to an older version of the image, IfNotPresent will use the local image present on the worker node, while Always will attempt to download the latest version of the image from the repository.

However, it's important to note that the behavior of Always is based on the digest of the image, not the tag. The digest is a unique identifier for a specific version of an image that is based on the content of the image. When you specify Always, Kubernetes will check the digest of the image on the worker node against the digest of the latest version of the image in the repository with the same tag. If the digests match, Kubernetes will use the cached copy of the image on the worker node. If the digests differ, Kubernetes will download the latest version of the image from the repository and use it to start the container.

Upvotes: 12

Reda E.
Reda E.

Reputation: 868

As described in Documentation: https://kubernetes.io/docs/concepts/containers/images/

The imagePullPolicy for a container and the tag of the image affect when the kubelet attempts to pull (download) the specified image.

IfNotPresent

the image is pulled only if it is not already present locally.

Always

every time the kubelet launches a container, the kubelet queries the container image registry to resolve the name to an image digest. If the kubelet has a container image with that exact digest cached locally, the kubelet uses its cached image; otherwise, the kubelet pulls the image with the resolved digest, and uses that image to launch the container.

Now, what's the best option ?

For production, IfNotPresent is the best option since your Kubernetes resources will need to restart sometimes and get re-deployed with different configurations, this will optimize time and network resources for your cluster.

Upvotes: 1

Aman_Bhala
Aman_Bhala

Reputation: 317

  • If imagePullPolicy is set to Always, Kubernetes will always pull the image from the Repository.
  • With IfNotPresent, Kubernetes will only pull the image when it does not already exist on the node.

Upvotes: 1

Blender Fox
Blender Fox

Reputation: 5625

Always at the name suggests will cause the container runtime to attempt to pull a new version of the image from the repository every time it tries to create the container.

In docker, this is like doing:

docker run --pull=always nginx

IfNotPresent will pull the image if it does not exist on the node that is attempting to create the container

This is like doing:

docker run --pull=missing nginx

or

docker run nginx

Upvotes: 4

SYN
SYN

Reputation: 5032

This refers to the cache of the node where that pod would be running. Each node has its own images cache.

If the image is present in cache, IfNotPresent policy would re-use it and skip image pull. While Always ensures to always pull the image.

Upvotes: 1

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