# Indexed Job for Parallel Processing with Static Work Assignment

Indexed Job for Parallel Processing with Static Work AssignmentFEATURE STATE: Kubernetes v1.24 [stable]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:Define a Job manifest using indexed completion. The downward API allows you to pass the pod index annotation as an environment variable or file to the container.Start an Indexed Job based on that manifest.Before you beginYou 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:iximiuz LabsKillercodaKodeKloudPlay with KubernetesYour Kubernetes server must be at or later than version v1.21.To check the version, enter kubectl version.Choose an approachTo access the work item from the worker program, you have a few options:Read the JOB_COMPLETION_INDEX environment variable. The Job controller automatically links this variable to the annotation containing the completion index.Read a file that contains the completion index.Assuming that you can't modify the program, you can wrap it with a script that reads the index using any of the methods above and converts it into something that the program can use as input.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.Define an Indexed JobHere 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']Running the JobNow 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.You can wait for the Job to succeed, with a timeout:# The check for condition name is case insensitive kubectl wait --for=condition=complete --timeout=300s job/indexed-job Now, describe the Job and check that it was successful.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