kustomize/docs/glossary.md

# Glossary
[CRD spec]: https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/
[CRD]: #custom-resource-definition
[DAM]: #declarative-application-management
[Declarative Application Management]: https://github.com/kubernetes/community/blob/master/contributors/design-proposals/architecture/declarative-application-management.md
[JSON]: https://www.json.org/
[JSONPatch]: https://tools.ietf.org/html/rfc6902
[Resource]: #resource
[YAML]: http://www.yaml.org/start.html
[application]: #application
[apply]: #apply
[apt]: https://en.wikipedia.org/wiki/APT_(Debian)
[base]: #base
[bases]: #base
[bespoke]: #bespoke-configuration
[gitops]: #gitops
[k8s]: #kubernetes
[kubernetes]: #kubernetes
[kustomize]: #kustomize
[kustomization]: #kustomization
[off-the-shelf]: #off-the-shelf-configuration
[overlay]: #overlay
[overlays]: #overlay
[patch]: #patch
[patches]: #patch
[patchJson6902]: #patchjson6902
[patchExampleJson6902]: https://github.com/kubernetes-sigs/kustomize/blob/master/examples/jsonpatch.md
[patchesJson6902]: #patchjson6902
[proposal]: https://github.com/kubernetes/community/pull/1629
[rebase]: https://git-scm.com/docs/git-rebase
[resource]: #resource
[resources]: #resource
[rpm]: https://en.wikipedia.org/wiki/Rpm_(software)
[strategic-merge]: https://github.com/kubernetes/community/blob/master/contributors/devel/strategic-merge-patch.md
[target]: #target
[variant]: #variant
[variants]: #variant
[workflow]: workflows.md

## application

An _application_ is a group of k8s resources related by
some common purpose, e.g.  a load balancer in front of a
webserver backed by a database.
[Resource] labelling, naming and metadata schemes have
historically served to group resources together for
collective operations like _list_ and _remove_.

This [proposal] describes a new k8s resource called
_application_ to more formally describe this idea and
provide support for application-level operations and
dashboards.

[kustomize] configures k8s resources, and the proposed
application resource is just another resource.


## apply

The verb _apply_ in the context of k8s refers to a
kubectl command and an in-progress [API
endpoint](https://goo.gl/UbCRuf) for mutating a
cluster.

One _applies_ a statement of what one wants to a
cluster in the form of a complete resource list.

The cluster merges this with the previously applied
state and the actual state to arrive at a new desired
state, which the cluster's reconcilation loop attempts
to create.  This is the foundation of level-based state
management in k8s.

## base

A _base_ is a [target] that some [overlay] modifies.

Any target, including an [overlay], can be a base to
another target.

A base has no knowledge of the overlays that refer to it.

For simple [gitops] management, a base configuration
could be the _sole content of a git repository
dedicated to that purpose_.  Same with [overlays].
Changes in a repo could generate a build, test and
deploy cycle.


## bespoke configuration

A _bespoke_ configuration is a [kustomization] and some
[resources] created and maintained internally by some
organization for their own purposes.

The [workflow] associated with a _bespoke_ config is
simpler than the workflow associated with an
[off-the-shelf] config, because there's no notion of
periodically capturing someone else's upgrades to the
[off-the-shelf] config.

## custom resource definition

One can extend the k8s API by making a
Custom Resource Definition ([CRD spec]).

This defines a custom [resource] (CD), an entirely
new resource that can be used alongside _native_
resources like ConfigMaps, Deployments, etc.

Kustomize can customize a CD, but to do so
kustomize must also be given the corresponding CRD
so that it can interpret the structure correctly.

## declarative application management

Kustomize aspires to support [Declarative Application Management],
a set of best practices around managing k8s clusters.

In brief, kustomize should

 * Work with any configuration, be it bespoke,
   off-the-shelf, stateless, stateful, etc.
 * Support common customizations, and creation of
   [variants] (e.g. _development_ vs.
   _staging_ vs. _production_).
 * Expose and teach native k8s APIs, rather than
   hide them.
 * Add no friction to version control integration to
   support reviews and audit trails.
 * Compose with other tools in a unix sense.
 * Eschew crossing the line into templating, domain
   specific languages, etc., frustrating the other
   goals.

## gitops

Devops or CICD workflows that use a git repository as a
single source of truth and take action (e.g., build,
test or deploy) when that truth changes.

## kustomization

A _kustomization_ is a file called `kustomization.yaml` that
describes a configuration consumable by [kustomize].


Here's an [example](kustomization.yaml).

A kustomization contains fields falling into these categories:

 * _Customization operators_ for modifying operands, e.g.
   _namePrefix_, _nameSuffix_, _commonLabels_, _patches_, etc.

 * _Customization operands_:
   * [resources] - completely specified k8s API objects,
      e.g. `deployment.yaml`, `configmap.yaml`, etc.
   * [bases] - paths or github URLs specifying directories
     containing a [kustomization].  These bases may
     be subjected to more customization, or merely
     included in the output.
   * [CRD]s - custom resource definition files, to allow use
     of _custom_ resources in the _resources_ list.
     Not an actual operand - but allows the use of new operands.

 * Generators, for creating more resources
   (configmaps and secrets) which can then be
   customized.

## kubernetes

[Kubernetes](https://kubernetes.io) is an open-source
system for automating deployment, scaling, and
management of containerized applications.

It's often abbreviated as _k8s_.

## kubernetes-style object

[fields required]: https://kubernetes.io/docs/concepts/overview/working-with-objects/kubernetes-objects/#required-fields

An object, expressed in a YAML or JSON file, with the
[fields required] by kubernetes.  Basically just a
_kind_ field to identify the type, a _metadata/name_
field to identify the particular instance, and an
_apiVersion_ field to identify the version (if there's
more than one version).

## kustomize

_kustomize_ is a command line tool supporting template-free
customization of declarative configuration targetted to
k8s-style objects.

_Targetted to k8s means_ that kustomize may need some
limited understanding of API resources, k8s concepts
like names, labels, namespaces, etc. and the semantics
of resource patching.

kustomize is an implementation of [DAM].


## off-the-shelf configuration

An _off-the-shelf_ configuration is a kustomization and
resources intentionally published somewhere for others
to use.

E.g. one might create a github repository like this:

> ```
> github.com/username/someapp/
>   kustomization.yaml
>   deployment.yaml
>   configmap.yaml
>   README.md
> ```

Someone could then _fork_ this repo (on github) and
_clone_ their fork to their local disk for
customization.

This clone could act as a [base] for the user's
own [overlays] to do further customization.

## overlay

An _overlay_ is a [target] that modifies (and thus
depends on) another target.

The [kustomization] in an overlay refers to (via file
path, URI or other method) some other kustomization,
known as its [base].

An overlay is unusable without its base.

An overlay may act as a base to another overlay.

Overlays make the most sense when there is _more than
one_, because they create different [variants] of a
common base - e.g.  _development_, _QA_, _staging_ and
_production_ environment variants.

These variants use the same overall resources, and vary
in relatively simple ways, e.g. the number of replicas
in a deployment, the CPU to a particular pod, the data
source used in a configmap, etc.

One configures a cluster like this:

> ```
>  kustomize build someapp/overlays/staging |\
>      kubectl apply -f -
>
>  kustomize build someapp/overlays/production |\
>      kubectl apply -f -
> ```

Usage of the base is implicit - the overlay's
kustomization points to the base.


## package

The word _package_ has no meaning in kustomize, as
kustomize is not to be confused with a package
management tool in the tradition of, say, [apt] or
[rpm].

## patch

General instructions to modify a resource.

There are two alternative techniques with similar
power but different notation - the
[strategic merge patch](#patchstrategicmerge)
and the [JSON patch](#patchjson6902).

## patchStrategicMerge

A _patchStrategicMerge_ is [strategic-merge]-style patch (SMP).

An SMP looks like an incomplete YAML specification of
a k8s resource.  The SMP includes `TypeMeta`
fields to establish the group/version/kind/name of the
[resource] to patch, then just enough remaining fields
to step into a nested structure to specify a new field
value, e.g. an image tag.

By default, an SMP _replaces_ values.  This
usually desired when the target value is a simple
string, but may not be desired when the target 
value is a list.

To change this 
default behavior, add a _directive_.  Recognized 
directives include _replace_ (the default), _merge_
(avoid replacing a list), _delete_ and a few more
(see [these notes][strategic-merge]).

Fun fact - any resource file can be used as
an SMP, overwriting matching fields in another
resource with the same group/version/kind/name,
but leaving all other fields as they were.

TODO(monopole): add ptr to example.

## patchJson6902

A _patchJson6902_ refers to a kubernetes [resource] and
a [JSONPatch] specifying how to change the resource.

A _patchJson6902_ can do almost everything a
_patchStrategicMerge_ can do, but with a briefer
syntax.  See this [example][patchExampleJson6902].

## resource

A _resource_ in the context of a REST-ful API is the
target object of an HTTP operation like _GET_, _PUT_ or
_POST_.  k8s offers a REST-ful API surface to interact
with clients.

A _resource_, in the context of kustomization file,
is a path to a [YAML] or [JSON] file describing
a k8s API object, like a Deployment or a
ConfigmMap.

More generally, a resource can be any correct YAML file
that [defines an object](https://kubernetes.io/docs/concepts/overview/working-with-objects/kubernetes-objects/#required-fields)
with a _kind_ and a _metadata/name_ field.


## sub-target / sub-application / sub-package

A _sub-whatever_ is not a thing. There are only
[bases] and [overlays].

## target

The _target_ is the argument to `kustomize build`, e.g.:

> ```
>  kustomize build $target
> ```

`$target` must be a path or a url to a directory that
immediately contains a [kustomization].

The target contains, or refers to, all the information
needed to create customized resources to send to the
[apply] operation.

A target is a [base] or an [overlay].

## variant

A _variant_ is the outcome, in a cluster, of applying
an [overlay] to a [base].

E.g., a _staging_ and _production_ overlay both modify
some common base to create distinct variants.

The _staging_ variant is the set of resources exposed
to quality assurance testing, or to some external users
who'd like to see what the next version of production
will look like.

The _production_ variant is the set of resources
exposed to production traffic, and thus may employ
deployments with a large number of replicas and higher
cpu and memory requests.