Table of Contents

• Jobs
  • What is a job?
  • Running an example Job
  • Writing a Job Spec
    • Pod Template
    • Pod Selector
    • Multiple Completions
    • Parallelism
  • Handling Pod and Container Failures
  • Alternatives to Job
    • Bare Pods
    • Replication Controller
  • Caveats
  • Future work

A job creates one or more pods and ensures that a specified number of them successfully terminate. As pods successfully complete, the job tracks the successful completions. When a specified number of successful completions is reached, the job itself is complete. Deleting a Job will cleanup the pods it created.

A simple case is to create 1 Job object in order to reliably run one Pod to completion. A Job can also be used to run multiple pods in parallel.

Here is an example Job config. It computes π to 2000 places and prints it out. It takes around 10s to complete.

apiVersion: extensions/v1beta1
kind: Job
metadata:
  name: pi
spec:
  selector:
    matchLabels:
      app: pi
  template:
    metadata:
      name: pi
      labels:
        app: pi
    spec:
      containers:
      - name: pi
        image: perl
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: Never

Download example

Run the example job by downloading the example file and then running this command:

$ kubectl create -f ./job.yaml
jobs/pi

Check on the status of the job using this command:

$ kubectl describe jobs/pi
Name:		pi
Namespace:	default
Image(s):	perl
Selector:	app=pi
Parallelism:	2
Completions:	1
Labels:		<none>
Pods Statuses:	1 Running / 0 Succeeded / 0 Failed
Events:
  FirstSeen	LastSeen	Count	From	SubobjectPath	Reason			Message
  ─────────	────────	─────	────	─────────────	──────			───────
  1m		1m		1	{job }			SuccessfulCreate	Created pod: pi-z548a

To view completed pods of a job, use kubectl get pods --show-all. The --show-all will show completed pods too.

To list all the pods that belong to job in a machine readable form, you can use a command like this:

$ pods=$(kubectl get pods --selector=app=pi --output=jsonpath={.items..metadata.name})
echo $pods
pi-aiw0a

Here, the selector is the same as the selector for the job. The --output=jsonpath option specifies an expression that just gets the name from each pod in the returned list.

View the standard output of one of the pods:

$ kubectl logs pi-aiw0a
3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095505822317253594081284811174502841027019385211055596446229489549303819644288109756659334461284756482337867831652712019091456485669234603486104543266482133936072602491412737245870066063155881748815209209628292540917153643678925903600113305305488204665213841469519415116094330572703657595919530921861173819326117931051185480744623799627495673518857527248912279381830119491298336733624406566430860213949463952247371907021798609437027705392171762931767523846748184676694051320005681271452635608277857713427577896091736371787214684409012249534301465495853710507922796892589235420199561121290219608640344181598136297747713099605187072113499999983729780499510597317328160963185950244594553469083026425223082533446850352619311881710100031378387528865875332083814206171776691473035982534904287554687311595628638823537875937519577818577805321712268066130019278766111959092164201989380952572010654858632788659361533818279682303019520353018529689957736225994138912497217752834791315155748572424541506959508295331168617278558890750983817546374649393192550604009277016711390098488240128583616035637076601047101819429555961989467678374494482553797747268471040475346462080466842590694912933136770289891521047521620569660240580381501935112533824300355876402474964732639141992726042699227967823547816360093417216412199245863150302861829745557067498385054945885869269956909272107975093029553211653449872027559602364806654991198818347977535663698074265425278625518184175746728909777727938000816470600161452491921732172147723501414419735685481613611573525521334757418494684385233239073941433345477624168625189835694855620992192221842725502542568876717904946016534668049886272327917860857843838279679766814541009538837863609506800642251252051173929848960841284886269456042419652850222106611863067442786220391949450471237137869609563643719172874677646575739624138908658326459958133904780275901

As with all other Kubernetes config, a Job needs apiVersion, kind, and metadata fields. For general information about working with config files, see here, here, and here.

A Job also needs a .spec section.

The .spec.template is the only required field of the .spec.

The .spec.template is a pod template. It has exactly the same schema as a pod, except it is nested and does not have an apiVersion or kind.

In addition to required fields for a Pod, a pod template in a job must specify appropriate lables (see pod selector and an appropriate restart policy.

Only a RestartPolicy equal to Never or OnFailure are allowed.

The .spec.selector field is a label query over a set of pods.

The spec.selector is an object consisting of two fields:

• matchLabels - works the same as the .spec.selector of a ReplicationController
• matchExpressions - allows to build more sophisticated selectors by specyfing key, list of values and an operator that relates the key and values.

When the two are specified the result is ANDed.

If .spec.selector is unspecified, .spec.selector.matchLabels will be defaulted to .spec.template.metadata.labels.

Also you should not normally create any pods whose labels match this selector, either directly, via another Job, or via another controller such as ReplicationController. Otherwise, the Job will think that those pods were created by it. Kubernetes will not stop you from doing this.

By default, a Job is complete when one Pod runs to successful completion. You can also specify that this needs to happen multiple times by specifying .spec.completions with a value greater than 1. When multiple completions are requested, each Pod created by the Job controller has an identical spec. In particular, all pods will have the same command line and the same image, the same volumes, and mostly the same environment variables. It is up to the user to arrange for the pods to do work on different things. For example, the pods might all access a shared work queue service to acquire work units.

To create multiple pods which are similar, but have slightly different arguments, environment variables or images, use multiple Jobs.

You can suggest how many pods should run concurrently by setting .spec.parallelism to the number of pods you would like to have running concurrently. This number is a suggestion. The number running concurrently may be lower or higher for a variety of reasons. For example, it may be lower if the number of remaining completions is less, or as the controller is ramping up, or if it is throttling the job due to excessive failures. It may be higher for example if a pod is gracefully shutdown, and the replacement starts early.

If you do not specify .spec.parallelism, then it defaults to .spec.completions.

A Container in a Pod may fail for a number of reasons, such as because the process in it exited with a non-zero exit code, or the Container was killed for exceeding a memory limit, etc. If this happens, and the .spec.template.containers[].restartPolicy = "OnFailure", then the Pod stays on the node, but the Container is re-run. Therefore, your program needs to handle the the case when it is restarted locally, or else specify .spec.template.containers[].restartPolicy = "Never". See pods-states for more information on restartPolicy.

An entire Pod can also fail, for a number of reasons, such as when the pod is kicked off the node (node is upgraded, rebooted, delelted, etc.), or if a container of the Pod fails and the .spec.template.containers[].restartPolicy = "Never". When a Pod fails, then the Job controller starts a new Pod. Therefore, your program needs to handle the case when it is restarted in a new pod. In particular, it needs to handle temporary files, locks, incomplete output and the like caused by previous runs.

Note that even if you specify .spec.parallelism = 1 and .spec.completions = 1 and .spec.template.containers[].restartPolicy = "Never", the same program may sometimes be started twice.

If you do specify .spec.parallelism and .spec.completions both greater than 1, then there may be multiple pods running at once. Therefore, your pods must also be tolerant of concurrency.

When the node that a pod is running on reboots or fails, the pod is terminated and will not be restarted. However, a Job will create new pods to replace terminated ones. For this reason, we recommend that you use a job rather than a bare pod, even if your application requires only a single pod.

Jobs are complementary to Replication Controllers. A Replication Controller manages pods which are not expected to terminate (e.g. web servers), and a Job manages pods that are expected to terminate (e.g. batch jobs).

As discussed in life of a pod, Job is only appropriate for pods with RestartPolicy equal to OnFailure or Never. (Note: If RestartPolicy is not set, the default value is Always.)

Job objects are in the extensions API Group.

Job objects have API version v1beta1. Beta objects may undergo changes to their schema and/or semantics in future software releases, but similar functionality will be supported.

Support for creating Jobs at specified times/dates (i.e. cron) is expected in the next minor release.