kubernetes/docs/design/selector-generation.md

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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>

If you are using a released version of Kubernetes, you should
refer to the docs that go with that version.

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<strong>
The latest release of this document can be found
[here](http://releases.k8s.io/release-1.3/docs/design/selector-generation.md).

Documentation for other releases can be found at
[releases.k8s.io](http://releases.k8s.io).
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Design
=============

# Goals

Make it really hard to accidentally create a job which has an overlapping
selector, while still making it possible to chose an arbitrary selector, and
without adding complex constraint solving to the APIserver.

# Use Cases

1. user can leave all label and selector fields blank and system will fill in
reasonable ones: non-overlappingness guaranteed.
2. user can put on the pod template some labels that are useful to the user,
without reasoning about non-overlappingness. System adds additional label to
assure not overlapping.
3. If user wants to reparent pods to new job (very rare case) and knows what
they are doing, they can completely disable this behavior and specify explicit
selector.
4. If a controller that makes jobs, like scheduled job, wants to use different
labels, such as the time and date of the run, it can do that.
5.  If User reads v1beta1 documentation or reuses v1beta1 Job definitions and
just changes the API group, the user should not automatically be allowed to
specify a selector, since this is very rarely what people want to do and is
error prone.
6. If User downloads an existing job definition, e.g. with
`kubectl get jobs/old -o yaml` and tries to modify and post it, he should not
create an overlapping job.
7. If User downloads an existing job definition, e.g. with
`kubectl get jobs/old -o yaml` and tries to modify and post it, and he
accidentally copies the uniquifying label from the old one, then he should not
get an error from a label-key conflict, nor get erratic behavior.
8. If user reads swagger docs and sees the selector field, he should not be able
to set it without realizing the risks.
8. (Deferred requirement:) If user wants to specify a preferred name for the
non-overlappingness key, they can pick a name.

# Proposed changes

## API

`extensions/v1beta1 Job` remains the same. `batch/v1 Job` changes change as
follows.

Field `job.spec.manualSelector` is added. It controls whether selectors are
automatically generated. In automatic mode, user cannot make the mistake of
creating non-unique selectors. In manual mode, certain rare use cases are
supported.

Validation is not changed. A selector must be provided, and it must select the
pod template.

Defaulting changes. Defaulting happens in one of two modes:

### Automatic Mode

- User does not specify `job.spec.selector`.
- User is probably unaware of the `job.spec.manualSelector` field and does not
think about it.
- User optionally puts labels on pod template (optional). User does not think
about uniqueness, just labeling for user's own reasons.
- Defaulting logic sets `job.spec.selector` to
`matchLabels["controller-uid"]="$UIDOFJOB"`
- Defaulting logic  appends 2 labels to the `.spec.template.metadata.labels`.
  - The first label is controller-uid=$UIDOFJOB.
  - The second label is "job-name=$NAMEOFJOB".

### Manual Mode

- User means User or Controller for the rest of this list.
- User does specify `job.spec.selector`.
- User does specify `job.spec.manualSelector=true`
- User puts a unique label or label(s) on pod template (required). User does
think carefully about uniqueness.
- No defaulting of pod labels or the selector happen.

### Rationale

UID is better than Name in that:
- it allows cross-namespace control someday if we need it.
- it is unique across all kinds.  `controller-name=foo` does not ensure
uniqueness across Kinds `job` vs `replicaSet`.  Even `job-name=foo` has a
problem: you might have a `batch.Job` and a `snazzyjob.io/types.Job` -- the
latter cannot use label `job-name=foo`, though there is a temptation to do so.
- it uniquely identifies the controller across time.  This prevents the case
where, for example, someone deletes a job via the REST api or client
(where cascade=false), leaving pods around. We don't want those to be picked up
unintentionally. It also prevents the case where a user looks at an old job that
finished but is not deleted, and tries to select its pods, and gets the wrong
impression that it is still running.

Job name is more user friendly.  It is self documenting

Commands like  `kubectl get pods -l job-name=myjob` should do exactly what is
wanted 99.9% of the time.  Automated control loops should still use the
controller-uid=label.

Using both gets the benefits of both, at the cost of some label verbosity.

The field is a `*bool`.  Since false is expected to be much more common,
and since the feature is complex, it is better to leave it unspecified so that
users looking at a stored pod spec do not need to be aware of this field.

### Overriding Unique Labels

If user does specify `job.spec.selector` then the user must also specify
`job.spec.manualSelector`. This ensures the user knows that what he is doing is
not the normal thing to do.

To prevent users from copying the `job.spec.manualSelector` flag from existing
jobs, it will be optional and default to false, which means when you ask GET and
existing job back that didn't use this feature, you don't even see the
`job.spec.manualSelector` flag, so you are not tempted to wonder if you should
fiddle with it.

## Job Controller

No changes

## Kubectl

No required changes. Suggest moving SELECTOR to wide output of `kubectl get
jobs` since users do not write the selector.

## Docs

Remove examples that use selector and remove labels from pod templates.
Recommend `kubectl get jobs -l job-name=name` as the way to find pods of a job.

# Conversion

The following applies to Job, as well as to other types that adopt this pattern:

- Type `extensions/v1beta1` gets a field called `job.spec.autoSelector`.
- Both the internal type and the `batch/v1` type will get
`job.spec.manualSelector`.
- The fields `manualSelector` and `autoSelector` have opposite meanings.
- Each field defaults to false when unset, and so v1beta1 has a different
default than v1 and internal. This is intentional: we want new uses to default
to the less error-prone behavior, and we do not want to change the behavior of
v1beta1.

*Note*: since the internal default is changing, client library consumers that
create Jobs may need to add "job.spec.manualSelector=true" to keep working, or
switch to auto selectors.

Conversion is as follows:
- `extensions/__internal` to `extensions/v1beta1`: the value of
`__internal.Spec.ManualSelector` is defaulted to false if nil, negated,
defaulted to nil if false, and written `v1beta1.Spec.AutoSelector`.
- `extensions/v1beta1` to `extensions/__internal`: the value of
`v1beta1.SpecAutoSelector` is defaulted to false if nil, negated, defaulted to
nil if false, and written to `__internal.Spec.ManualSelector`.

This conversion gives the following properties.

1. Users that previously used v1beta1 do not start seeing a new field when they
get back objects.
2. Distinction between originally unset versus explicitly set to false is not
preserved (would have been nice to do so, but requires more complicated
solution).
3. Users who only created v1beta1 examples or v1 examples, will not ever see the
existence of either field.
4. Since v1beta1 are convertable to/from v1, the storage location (path in etcd)
does not need to change, allowing scriptable rollforward/rollback.

# Future Work

Follow this pattern for Deployments, ReplicaSet, DaemonSet when going to v1, if
it works well for job.

Docs will be edited to show examples without a `job.spec.selector`.

We probably want as much as possible the same behavior for Job and
ReplicationController.


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