# Glossary [DAM]: #declarative-application-management [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 [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. ## declarative application management _Declarative Application Management_ (DAM) is a [set of ideas](https://goo.gl/T66ZcD) aiming to ease management of k8s clusters. * Works with any configuration, be it bespoke, off-the-shelf, stateless, stateful, etc. * Supports common customizations, and creation of [variants] (dev vs. staging vs. production). * Exposes and teaches native k8s APIs, rather than hiding them. * No friction integration with version control to support reviews and audit trails. * Composable with other tools in a unix sense. * Eschews 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: * Immediate customization declarations, e.g. _namePrefix_, _commonLabels_, etc. * Resource _generators_ for configmaps and secrets. * References to _external files_ in these categories: * [resources] - completely specified k8s API objects, e.g. `deployment.yaml`, `configmap.yaml`, etc. * [patches] - _partial_ resources that modify full resources defined in a [base] (only meaningful in an [overlay]). * [bases] - path to a directory containing a [kustomization] (only meaningful in an [overlay]). * (_TBD_) Standard k8s API kind-version fields. ## 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 A _patch_ is a partially defined k8s resource with a name that must match a resource already known per traversal rules built into [kustomize]. _Patch_ is a field in the kustomization, distinct from resources, because a patch file looks like a resource file, but has different semantics. A patch depends on (modifies) a resource, whereas a resource has no dependencies. Since any resource file can be used as a patch, one cannot reliably distinguish a resource from a patch just by looking at the file's [YAML]. ## patchJson6902 A _patchJson6902_ refers to a kubernetes object and a [JSONPatch] that can patch the object. A [JSONPatch] contains a list of operations to change the object's field directly. This is different from [patch], which is applied to a target kubernetes object by [strategic merge]. _patchesJson6902_ is a field in kustomization. It contains a list of _patchJson6902_. ## resource A _resource_, in the context of kustomize, is a path to a [YAML] or [JSON] file that completely defines a functional k8s API object, like a deployment or a configmap. 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. A _resource_ in the content of a REST-ful API is the target of an HTTP operation like _GET_, _PUT_ or _POST_. k8s offers a RESTful API surface to interact with clients. ## 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.