Today, we want to share an error we encountered while deploying an application to Kubernetes (the standalone version included in Docker Desktop). This experience helped us understand how Kubernetes pulls images from a repository. Let's start by detailing the error itself. We created the image:
❯ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
registry/demoapi latest 110fb82f1b68 23 hours ago 212MB
Using the following configuration, we created a deployment in Kubernetes:
apiVersion: apps/v1
kind: Deployment
metadata:
name: demoapi-test-deployment
labels:
app: demoapi-test
spec:
replicas: 1
selector:
matchLabels:
app: demoapi-test
template:
metadata:
labels:
app: demoapi-test
spec:
containers:
- name: demoapi-test-container
image: registry/demoapi:latest
ports:
- containerPort: 80
We encountered an error while trying to pull the image:
❯ kubectl get pods
NAME READY STATUS RESTARTS AGE
demoapi-test-deployment-7b8f5d4fc7-8fd7t 0/1 ErrImagePull 0 14s
Then a question came to us: why is Kubernetes trying to pull the image if it's already on our machine? Looking at the pod information, we found that Kubernetes set the image pull policy to Always:
❯ kubectl get pod demoapi-test-deployment-7b8f5d4fc7-8fd7t -o yaml
apiVersion: v1
kind: Pod
metadata:
creationTimestamp: "2022-04-17T15:51:47Z"
generateName: demoapi-test-deployment-7b8f5d4fc7-
labels:
app: demoapi-test
pod-template-hash: 7b8f5d4fc7
name: demoapi-test-deployment-7b8f5d4fc7-8fd7t
namespace: default
ownerReferences:
- apiVersion: apps/v1
blockOwnerDeletion: true
controller: true
kind: ReplicaSet
name: demoapi-test-deployment-7b8f5d4fc7
uid: 7a4cdd35-8f56-4e5e-ab11-efc1e8994423
resourceVersion: "70292"
uid: 4c430c05-f007-4a81-ad6a-396231675567
spec:
containers:
- image: registry/demoapi:latest
imagePullPolicy: Always
name: demoapi-test-container
ports:
- containerPort: 80
protocol: TCP
Checking the Kubernetes documentation, we found the logic used to assign the default image pull policy:
If you omit the imagePullPolicy field, and the tag for the container image is :latest, imagePullPolicy is automatically set to Always.
If you omit the imagePullPolicy field, and you don't specify the tag for the container image, imagePullPolicy is automatically set to Always.
If you omit the imagePullPolicy field, and you specify the tag for the container image that isn't :latest, the imagePullPolicy is automatically set to IfNotPresent.
After reading that, the fix was straightforward. We decided to specify an image pull policy:
apiVersion: apps/v1
kind: Deployment
metadata:
name: demoapi-test-deployment
labels:
app: demoapi-test
spec:
replicas: 1
selector:
matchLabels:
app: demoapi-test
template:
metadata:
labels:
app: demoapi-test
spec:
containers:
- name: demoapi-test-container
image: registry/demoapi:latest
imagePullPolicy: IfNotPresent
ports:
- containerPort: 80
And problem fixed:
❯ kubectl get deployments
NAME READY UP-TO-DATE AVAILABLE AGE
demoapi-test-deployment 1/1 1 1 2s
Thank you, and happy coding.
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