# 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 [JSONMergePatch]: https://tools.ietf.org/html/rfc7386 [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]). Note that for custom resources, SMPs are treated as [json merge patches][JSONMergePatch]. 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.