Kubernetes v1.22 [beta]
In this example, you will run a Kubernetes Job that uses multiple parallel worker processes. Each worker is a different container running in its own Pod. The Pods have an index number that the control plane sets automatically, which allows each Pod to identify which part of the overall task to work on.
The pod index is available in the annotation batch.kubernetes.io/job-completion-index
as a string representing its decimal value. In order for the containerized task process to obtain this index, you can publish the value of the annotation using the downward API mechanism. For convenience, the control plane automatically sets the downward API to expose the index in the JOB_COMPLETION_INDEX
environment variable.
Here is an overview of the steps in this example:
Indexed
Job based on that manifest.You should already be familiar with the basic, non-parallel, use of Job.
You need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:
Your Kubernetes server must be at or later than version v1.21. To check the version, enterkubectl version
. To access the work item from the worker program, you have a few options:
JOB_COMPLETION_INDEX
environment variable. The Job controller automatically links this variable to the annotation containing the completion index.For this example, imagine that you chose option 3 and you want to run the rev utility. This program accepts a file as an argument and prints its content reversed.
rev data.txt
You'll use the rev
tool from the busybox
container image.
As this is only an example, each Pod only does a tiny piece of work (reversing a short string). In a real workload you might, for example, create a Job that represents the task of producing 60 seconds of video based on scene data. Each work item in the video rendering Job would be to render a particular frame of that video clip. Indexed completion would mean that each Pod in the Job knows which frame to render and publish, by counting frames from the start of the clip.
Here is a sample Job manifest that uses Indexed
completion mode:
application/job/indexed-job.yaml
apiVersion: batch/v1
kind: Job
metadata:
name: 'indexed-job'
spec:
completions: 5
parallelism: 3
completionMode: Indexed
template:
spec:
restartPolicy: Never
initContainers:
- name: 'input'
image: 'docker.io/library/bash'
command:
- "bash"
- "-c"
- |
items=(foo bar baz qux xyz)
echo ${items[$JOB_COMPLETION_INDEX]} > /input/data.txt
volumeMounts:
- mountPath: /input
name: input
containers:
- name: 'worker'
image: 'docker.io/library/busybox'
command:
- "rev"
- "/input/data.txt"
volumeMounts:
- mountPath: /input
name: input
volumes:
- name: input
emptyDir: {}
In the example above, you use the builtin JOB_COMPLETION_INDEX
environment variable set by the Job controller for all containers. An init container maps the index to a static value and writes it to a file that is shared with the container running the worker through an emptyDir volume. Optionally, you can define your own environment variable through the downward API to publish the index to containers. You can also choose to load a list of values from a ConfigMap as an environment variable or file.
Alternatively, you can directly use the downward API to pass the annotation value as a volume file, like shown in the following example:
application/job/indexed-job-vol.yaml
apiVersion: batch/v1
kind: Job
metadata:
name: 'indexed-job'
spec:
completions: 5
parallelism: 3
completionMode: Indexed
template:
spec:
restartPolicy: Never
containers:
- name: 'worker'
image: 'docker.io/library/busybox'
command:
- "rev"
- "/input/data.txt"
volumeMounts:
- mountPath: /input
name: input
volumes:
- name: input
downwardAPI:
items:
- path: "data.txt"
fieldRef:
fieldPath: metadata.annotations['batch.kubernetes.io/job-completion-index']
Now run the Job:
# This uses the first approach (relying on $JOB_COMPLETION_INDEX)
kubectl apply -f https://kubernetes.io/examples/application/job/indexed-job.yaml
When you create this Job, the control plane creates a series of Pods, one for each index you specified. The value of .spec.parallelism
determines how many can run at once whereas .spec.completions
determines how many Pods the Job creates in total.
Because .spec.parallelism
is less than .spec.completions
, the control plane waits for some of the first Pods to complete before starting more of them.
Once you have created the Job, wait a moment then check on progress:
kubectl describe jobs/indexed-job
The output is similar to:
Name: indexed-job
Namespace: default
Selector: controller-uid=bf865e04-0b67-483b-9a90-74cfc4c3e756
Labels: controller-uid=bf865e04-0b67-483b-9a90-74cfc4c3e756
job-name=indexed-job
Annotations: <none>
Parallelism: 3
Completions: 5
Start Time: Thu, 11 Mar 2021 15:47:34 +0000
Pods Statuses: 2 Running / 3 Succeeded / 0 Failed
Completed Indexes: 0-2
Pod Template:
Labels: controller-uid=bf865e04-0b67-483b-9a90-74cfc4c3e756
job-name=indexed-job
Init Containers:
input:
Image: docker.io/library/bash
Port: <none>
Host Port: <none>
Command:
bash
-c
items=(foo bar baz qux xyz)
echo ${items[$JOB_COMPLETION_INDEX]} > /input/data.txt
Environment: <none>
Mounts:
/input from input (rw)
Containers:
worker:
Image: docker.io/library/busybox
Port: <none>
Host Port: <none>
Command:
rev
/input/data.txt
Environment: <none>
Mounts:
/input from input (rw)
Volumes:
input:
Type: EmptyDir (a temporary directory that shares a pod's lifetime)
Medium:
SizeLimit: <unset>
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal SuccessfulCreate 4s job-controller Created pod: indexed-job-njkjj
Normal SuccessfulCreate 4s job-controller Created pod: indexed-job-9kd4h
Normal SuccessfulCreate 4s job-controller Created pod: indexed-job-qjwsz
Normal SuccessfulCreate 1s job-controller Created pod: indexed-job-fdhq5
Normal SuccessfulCreate 1s job-controller Created pod: indexed-job-ncslj
In this example, you run the Job with custom values for each index. You can inspect the output of one of the pods:
kubectl logs indexed-job-fdhq5 # Change this to match the name of a Pod from that Job
The output is similar to:
xuq
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Documentation Distributed under CC BY 4.0.
https://kubernetes.io/docs/tasks/job/indexed-parallel-processing-static/