Skip to content

Commit

Permalink
Tidy capitalization (#12603)
Browse files Browse the repository at this point in the history 
- Use “Job” to mean the Kubernetes object
- Use “job” where capitalization hinders readability
- Use “task”, or other synonymous concepts, to avoid confusion
- Use “Pod” to mean the Kubernetes object
- Use “pod” where capitalization hinders readability
- Use “container” in lowercase everywhere

Also:
- Clarify work queue example for Job controller
- Fix advice on running Job example
- Tweak emphasis
  • Loading branch information
@sftim @k8s-ci-robot
sftim authored and k8s-ci-robot committed Feb 18, 2019
1 parent eb5cac3 commit 5494391
Showing 1 changed file with 70 additions and 71 deletions.
141 changes: 70 additions & 71 deletions content/en/docs/concepts/workloads/controllers/jobs-run-to-completion.md
View file
Original file line number Diff line number Diff line change
Expand Up @@ -13,16 +13,16 @@ weight: 70

{{% capture overview %}}

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 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 task (ie, Job) is complete. Deleting a Job will clean up
the Pods it created.

A simple case is to create one Job object in order to reliably run one Pod to completion.
The Job object will start a new Pod if the first pod fails or is deleted (for example
The Job object will start a new Pod if the first Pod fails or is deleted (for example
due to a node hardware failure or a node reboot).

A Job can also be used to run multiple pods in parallel.
You can also use a Job to run multiple Pods in parallel.

{{% /capture %}}

Expand All @@ -36,14 +36,14 @@ It takes around 10s to complete.

{{< codenew file="controllers/job.yaml" >}}

Run the example job by downloading the example file and then running this command:
You can run the example with this command:

```shell
$ kubectl create -f https://k8s.io/examples/controllers/job.yaml
job "pi" created
```

Check on the status of the job using this command:
Check on the status of the Job with `kubectl`:

```shell
$ kubectl describe jobs/pi
Expand Down Expand Up @@ -78,18 +78,18 @@ Events:
1m 1m 1 {job-controller } Normal SuccessfulCreate Created pod: pi-dtn4q
```

To view completed pods of a job, use `kubectl get pods`.
To view completed Pods of a Job, use `kubectl get pods`.

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

```shell
$ pods=$(kubectl get pods --selector=job-name=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.
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:

Expand All @@ -110,7 +110,7 @@ The `.spec.template` is the only required field of the `.spec`.

The `.spec.template` is a [pod template](/docs/concepts/workloads/pods/pod-overview/#pod-templates). It has exactly the same schema as a [pod](/docs/user-guide/pods), 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
In addition to required fields for a Pod, a pod template in a Job must specify appropriate
labels (see [pod selector](#pod-selector)) and an appropriate restart policy.

Only a [`RestartPolicy`](/docs/concepts/workloads/pods/pod-lifecycle/#restart-policy) equal to `Never` or `OnFailure` is allowed.
Expand All @@ -123,31 +123,30 @@ See section [specifying your own pod selector](#specifying-your-own-pod-selector

### Parallel Jobs

There are three main types of jobs:
There are three main types of task suitable to run as a Job:

1. Non-parallel Jobs
- normally only one pod is started, unless the pod fails.
- job is complete as soon as Pod terminates successfully.
- normally, only one Pod is started, unless the Pod fails.
- the Job is complete as soon as its Pod terminates successfully.
1. Parallel Jobs with a *fixed completion count*:
- specify a non-zero positive value for `.spec.completions`.
- the job is complete when there is one successful pod for each value in the range 1 to `.spec.completions`.
- **not implemented yet:** Each pod passed a different index in the range 1 to `.spec.completions`.
- the Job represents the overall task, and is complete when there is one successful Pod for each value in the range 1 to `.spec.completions`.
- **not implemented yet:** Each Pod is passed a different index in the range 1 to `.spec.completions`.
1. Parallel Jobs with a *work queue*:
- do not specify `.spec.completions`, default to `.spec.parallelism`.
- the pods must coordinate with themselves or an external service to determine what each should work on.
- each pod is independently capable of determining whether or not all its peers are done, thus the entire Job is done.
- when _any_ pod terminates with success, no new pods are created.
- once at least one pod has terminated with success and all pods are terminated, then the job is completed with success.
- once any pod has exited with success, no other pod should still be doing any work or writing any output. They should all be
in the process of exiting.

For a Non-parallel job, you can leave both `.spec.completions` and `.spec.parallelism` unset. When both are
- the Pods must coordinate amongst themselves or an external service to determine what each should work on. For example, a Pod might fetch a batch of up to N items from the work queue.
- each Pod is independently capable of determining whether or not all its peers are done, and thus that the entire Job is done.
- when _any_ Pod from the Job terminates with success, no new Pods are created.
- once at least one Pod has terminated with success and all Pods are terminated, then the Job is completed with success.
- once any Pod has exited with success, no other Pod should still be doing any work for this task or writing any output. They should all be in the process of exiting.

For a _non-parallel_ Job, you can leave both `.spec.completions` and `.spec.parallelism` unset. When both are
unset, both are defaulted to 1.

For a Fixed Completion Count job, you should set `.spec.completions` to the number of completions needed.
For a _fixed completion count_ Job, you should set `.spec.completions` to the number of completions needed.
You can set `.spec.parallelism`, or leave it unset and it will default to 1.

For a Work Queue Job, you must leave `.spec.completions` unset, and set `.spec.parallelism` to
For a _work queue_ Job, you must leave `.spec.completions` unset, and set `.spec.parallelism` to
a non-negative integer.

For more information about how to make use of the different types of job, see the [job patterns](#job-patterns) section.
Expand All @@ -162,28 +161,28 @@ If it is specified as 0, then the Job is effectively paused until it is increase
Actual parallelism (number of pods running at any instant) may be more or less than requested
parallelism, for a variety of reasons:

- For Fixed Completion Count jobs, the actual number of pods running in parallel will not exceed the number of
- For _fixed completion count_ Jobs, the actual number of pods running in parallel will not exceed the number of
remaining completions. Higher values of `.spec.parallelism` are effectively ignored.
- For work queue jobs, no new pods are started after any pod has succeeded -- remaining pods are allowed to complete, however.
- For _work queue_ Jobs, no new Pods are started after any Pod has succeeded -- remaining Pods are allowed to complete, however.
- If the controller has not had time to react.
- If the controller failed to create pods for any reason (lack of ResourceQuota, lack of permission, etc.),
- If the controller failed to create Pods for any reason (lack of `ResourceQuota`, lack of permission, etc.),
then there may be fewer pods than requested.
- The controller may throttle new pod creation due to excessive previous pod failures in the same Job.
- When a pod is gracefully shutdown, it takes time to stop.
- The controller may throttle new Pod creation due to excessive previous pod failures in the same Job.
- When a Pod is gracefully shut down, it takes time to stop.

## Handling Pod and Container Failures

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
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.spec.restartPolicy = "OnFailure"`, then the Pod stays
on the node, but the Container is re-run. Therefore, your program needs to handle the case when it is
on the node, but the container is re-run. Therefore, your program needs to handle the case when it is
restarted locally, or else specify `.spec.template.spec.restartPolicy = "Never"`.
See [pods-states](/docs/concepts/workloads/pods/pod-lifecycle/#example-states) for more information on `restartPolicy`.
See [pod lifecycle](/docs/concepts/workloads/pods/pod-lifecycle/#example-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, deleted, etc.), or if a container of the Pod fails and the
`.spec.template.spec.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
starts a new Pod. This means that your application 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.

Expand All @@ -194,7 +193,7 @@ 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.

### Pod Backoff failure policy
### Pod backoff failure policy

There are situations where you want to fail a Job after some amount of retries
due to a logical error in configuration etc.
Expand Down Expand Up @@ -244,7 +243,7 @@ spec:
restartPolicy: Never
```
Note that both the Job Spec and the [Pod Template Spec](https://kubernetes.io/docs/concepts/workloads/pods/init-containers/#detailed-behavior) within the Job have an `activeDeadlineSeconds` field. Ensure that you set this field at the proper level.
Note that both the Job spec and the [Pod template spec](https://kubernetes.io/docs/concepts/workloads/pods/init-containers/#detailed-behavior) within the Job have an `activeDeadlineSeconds` field. Ensure that you set this field at the proper level.

## Clean Up Finished Jobs Automatically

Expand Down Expand Up @@ -316,7 +315,7 @@ The tradeoffs are:
- One Job object for each work item, vs. a single Job object for all work items. The latter is
better for large numbers of work items. The former creates some overhead for the user and for the
system to manage large numbers of Job objects.
- Number of pods created equals number of work items, vs. each pod can process multiple work items.
- Number of pods created equals number of work items, vs. each Pod can process multiple work items.
The former typically requires less modification to existing code and containers. The latter
is better for large numbers of work items, for similar reasons to the previous bullet.
- Several approaches use a work queue. This requires running a queue service,
Expand All @@ -336,7 +335,7 @@ The pattern names are also links to examples and more detailed description.

When you specify completions with `.spec.completions`, each Pod created by the Job controller
has an identical [`spec`](https://git.k8s.io/community/contributors/devel/api-conventions.md#spec-and-status). This means that
all pods will have the same command line and the same
all pods for a task will have the same command line and the same
image, the same volumes, and (almost) the same environment variables. These patterns
are different ways to arrange for pods to work on different things.

Expand All @@ -355,29 +354,29 @@ Here, `W` is the number of work items.

### Specifying your own pod selector

Normally, when you create a job object, you do not specify `.spec.selector`.
The system defaulting logic adds this field when the job is created.
Normally, when you create a Job object, you do not specify `.spec.selector`.
The system defaulting logic adds this field when the Job is created.
It picks a selector value that will not overlap with any other jobs.

However, in some cases, you might need to override this automatically set selector.
To do this, you can specify the `.spec.selector` of the job.
To do this, you can specify the `.spec.selector` of the Job.

Be very careful when doing this. If you specify a label selector which is not
unique to the pods of that job, and which matches unrelated pods, then pods of the unrelated
job may be deleted, or this job may count other pods as completing it, or one or both
of the jobs may refuse to create pods or run to completion. If a non-unique selector is
chosen, then other controllers (e.g. ReplicationController) and their pods may behave
unique to the pods of that Job, and which matches unrelated Pods, then pods of the unrelated
job may be deleted, or this Job may count other Pods as completing it, or one or both
Jobs may refuse to create Pods or run to completion. If a non-unique selector is
chosen, then other controllers (e.g. ReplicationController) and their Pods may behave
in unpredictable ways too. Kubernetes will not stop you from making a mistake when
specifying `.spec.selector`.

Here is an example of a case when you might want to use this feature.

Say job `old` is already running. You want existing pods
to keep running, but you want the rest of the pods it creates
to use a different pod template and for the job to have a new name.
You cannot update the job because these fields are not updatable.
Therefore, you delete job `old` but leave its pods
running, using `kubectl delete jobs/old --cascade=false`.
Say Job `old` is already running. You want existing Pods
to keep running, but you want the rest of the Pods it creates
to use a different pod template and for the Job to have a new name.
You cannot update the Job because these fields are not updatable.
Therefore, you delete Job `old` but _leave its pods
running_, using `kubectl delete jobs/old --cascade=false`.
Before deleting it, you make a note of what selector it uses:

```
Expand All @@ -392,11 +391,11 @@ spec:
...
```

Then you create a new job with name `new` and you explicitly specify the same selector.
Since the existing pods have label `job-uid=a8f3d00d-c6d2-11e5-9f87-42010af00002`,
they are controlled by job `new` as well.
Then you create a new Job with name `new` and you explicitly specify the same selector.
Since the existing Pods have label `job-uid=a8f3d00d-c6d2-11e5-9f87-42010af00002`,
they are controlled by Job `new` as well.

You need to specify `manualSelector: true` in the new job since you are not using
You need to specify `manualSelector: true` in the new Job since you are not using
the selector that the system normally generates for you automatically.

```
Expand All @@ -420,33 +419,33 @@ mismatch.
### Bare Pods
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.
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.
### Replication Controller
Jobs are complementary to [Replication Controllers](/docs/user-guide/replication-controller).
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).
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 tasks).
As discussed in [Pod Lifecycle](/docs/concepts/workloads/pods/pod-lifecycle/), `Job` is *only* appropriate for pods with
`RestartPolicy` equal to `OnFailure` or `Never`. (Note: If `RestartPolicy` is not set, the default
value is `Always`.)
As discussed in [Pod Lifecycle](/docs/concepts/workloads/pods/pod-lifecycle/), `Job` is *only* appropriate
for pods with `RestartPolicy` equal to `OnFailure` or `Never`.
(Note: If `RestartPolicy` is not set, the default value is `Always`.)
### Single Job starts Controller Pod
Another pattern is for a single Job to create a pod which then creates other pods, acting as a sort
of custom controller for those pods. This allows the most flexibility, but may be somewhat
Another pattern is for a single Job to create a Pod which then creates other Pods, acting as a sort
of custom controller for those Pods. This allows the most flexibility, but may be somewhat
complicated to get started with and offers less integration with Kubernetes.
One example of this pattern would be a Job which starts a Pod which runs a script that in turn
starts a Spark master controller (see [spark example](https://github.com/kubernetes/examples/tree/{{< param "githubbranch" >}}/staging/spark/README.md)), runs a spark
driver, and then cleans up.
An advantage of this approach is that the overall process gets the completion guarantee of a Job
object, but complete control over what pods are created and how work is assigned to them.
object, but complete control over what Pods are created and how work is assigned to them.
## Cron Jobs {#cron-jobs}
Expand Down

0 comments on commit 5494391

Please sign in to comment.