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.
The verb apply in the context of k8s refers to a kubectl command and an in-progress API endpoint 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.
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.
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 (DAM) is a set of ideas 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.
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.
A kustomization is a file called kustomization.yaml that
describes a configuration consumable by kustomize.
Here's an example.
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:
- (TBD) Standard k8s API kind-version fields.
Kubernetes is an open-source system for automating deployment, scaling, and management of containerized applications.
It's often abbreviated as k8s.
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 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.
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.
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.
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.
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.
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 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.
A sub-whatever is not a thing. There are only bases and overlays.
The target is the argument to kustomize build, e.g.:
kustomize build $target
$target must be a path 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.
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.