Glossary
We use many terms in our Nephio discussions, coming from different domains including telco, Kubernetes, configuration management, and our own Nephio-specific terms. This glossary is intended to help clarify our usage of these terms.
Config
See Configuration.
Configuration
In Nephio, this usually refers to the Kubernetes resources used to provision and manage network functions, their underlying infrastructure, and their internal operation. Unfortunately this is a very general term and often is overloaded with multiple meanings.
Sometimes, folks will say network config or workload config to refer to the internal configuration of the network functions. Consider that most network functions today cannot be directly configured via Kubernetes resources. Instead, they are configured via a proprietary configuration file, netconf, or even an API. In that case, those terms usually refer to this proprietary configuration language rather than Kubernetes resources. It is a goal for Nephio to help vendors enable KRM-based management of this internal configuration, to allow leveraging all the techniques we are building for KRM-based configuration (this is part of the “Kubernetes Everywhere” principle).
As a community, we should try to use a common set of terminology for different types of configuration. See docs#4.
Config Injection
See Injector.
Controller
This term comes from Kubernetes where
controller is defined as a control
loop that watches the intended and actual state of the cluster, and attempts to make changes as needed to make the
actual state match the intended state. More specifically, this typically refers to software that processes Kubernetes
Resources residing in the Kubernetes API server, and either transforms them into new resources, or calls to other APIs
that change the state of some entity external to the API server. For example, kubelet itself is a controller that
processes Pod resources to create and manage containers on a Node.
See also: Operator, Injector, KRM function, Specializer
Controller Manager
This term comes from Kubernetes and refers to an executable that bundles many controllers into one binary.
See also: Controller, Operator
CR
See Custom Resource.
CRD
See Custom Resource Definition.
Custom Resource
A Custom Resource (CR) is a resource in a Kubernetes API server that has a Group/Version/Kind. It was added to the API server via a Custom Resource Definition. The relationship between a CR and a CRD is analogous to that of an object and a class in Object-Oriented Programming; the CRD defines the schema, and the CR is a particular instance.
Note that it is common for people to say “CRD” when in fact they mean “CR”, so be sure to ask for clarification if necessary.
See also: Custom Resource Definition
Custom Resource Definition
A Custom Resource Definition (CRD) is a built-in Kubernetes resource used to define custom resources within a Kubernetes API server. It is used to extend the functionality of a Kubernetes API server by adding new resource types. The CRD, identified by its Group/Version/Kind, defines the schema associated with the resource, as well as the resource API endpoints.
Note that it is common for people to say “CRD” when in fact they mean “CR”, so be sure to ask for clarification if necessary.
See also: Custom Resource
Dehydration
See Hydration.
DRY
This is a common software engineering term that stands for Don’t Repeat Yourself. DRY attempts to reduce repetition in software development. In the Kubernetes configuration management context, a good example is a Helm chart, which attempts to abstract the particular manifests for a given workload. A kpt package that is not yet ready to deploy is also an example of a DRY artifact. In general, any sort of “template” or “blueprint” is usually an attempt to capture some repeatable pattern, following this principle.
Fanout
This term refers to the process of taking a package and customizing it across a series of targets. It is a type of Variant Generation but more specific than that term. It is also an application of the DRY principle.
Some examples:
- A script that loops through an array, feeding values into Helm and rendering individually specialized manifests for each entry in the array.
- The PackageDeployment controller from the ONE Summit 2022 Workshop uses a label selector to identify target clusters, then clones a kpt package for each, creating one package revision per cluster.
- The PackageVariantSet controller in Porch can be used to clone a package across a set of repositories, or can create multiple clones of the same package with different names in a single repository, based on arbitrary object selectors.
See also: Hydration, Variant, Variant Generation
Hydration
A play on DRY and WET, this is the process by which a DRY artifact becomes ready for deployment. A familiar example is rendering a Helm chart. A lot of the effort in the configuration management aspects of Nephio are spent on making the hydration process scalable, collaborative, and manageable in Day 2 and beyond, all of which are challenges with current techniques.
Hydration may be out-of-place, where the source material (e.g., the Helm chart), is separate from the output of the hydration process (the manifests). This is probably the most familiar type of hydration, used by Helm and kustomize, for example. Think of it as a pipeline with an input artifact, input values, and output artifacts.
Hydration may also be in-place, where modifications are directly written to the manifests in question. There is no separate input artifact and output artifact. Rather, you may have a starting artifact, some operations you perform on that artifact to achieve your goal, but you store the results of those operations directly in the same artifact. Utilization of a version control system such as Git is critical in this case. This is the kind of hydration we typically use when operating on kpt packages.
With out-of-place hydration, the author of the template has to figure out, upfront, all the possible outcomes of the hydration process. Then, they have to make available inputs to the pipeline in order to make all of those different outcomes achievable. This leads to “over-parameterization” - where effectively every option possible in the outputs becomes an option in the input. At that point, you have mostly moved complexity rather than reduced complexity. In-place hydration can help with the over-parameterization, as values that are rarely changed by users can simply be edited in-place.
While related, DRY and WET are not exactly the same concepts as in-place and out-of-place hydration. The former two refer to principles, whereas the latter two are more about the operational pipeline.
Note that occasionally people say “dehydration” when they mean “hydration”, likely due to the fact that “dehydration” is a more familiar word in common language. Please offer folks some leeway in this, especially since we have many non-native English speakers.
Injection
See Injector.
Injector
We introduced this term during the Nephio ONE Summit 2022 Workshop. However, it has been renamed to specializer.
There is still the concept of an injector, but it is limited to the PackageVariant and PackageVariantSet controllers. This process allows the author of the PackageVariant(Set) to configure the controller to pull in a resource from the management cluster, and copy it into the package. This allows us to combine upstream ( DRY) configuration with cluster-specific configuration based upon the target cluster.
kpt
Kpt is an open source tool for managing bundles of Kubernetes resource configurations, called kpt packages, using the Configuration-as-Data methodology.
The kpt command-line tool allows pulling, pushing, cloning and otherwise
managing packages stored in version control repositories (Git or OCI), as well
as execution of
KRM functions to perform consistent and
repeatable modifications to package resources.
Porch provides these package management, manipulation, and lifecycle operations in a Kubernetes-based API, allowing automation of these operations using standard Kubernetes controller techniques.
kpt Function
See KRM Function.
KRM
See Kubernetes Resource Model.
KRM Function
A KRM Function is an executable that takes Kubernetes resources as inputs, and produces Kubernetes resources as outputs. The function may add, remove, or modify the input resources to produce the outputs. This is similar to a Unix pipeline, but with KRM on the input and output, rather than simple streams.
Generally, best practices suggest KRM functions be hermetic (that is, they do not access the outside world).
In terms of the specification linked above, kustomize, kpt, and Porch are all orchestrators.
See also: Controller, kpt, Porch
Kubernetes Resource Model
The Kubernetes Resource Model (KRM) is the underlying declarative, intent-based API model and machinery for Kubernetes. It is the general name for what you likely think of when you hear “Kubernetes API”. Additional background:
Manifest
A file (or files) containing a representation of resources. Typically YAML files, but it could also be JSON or some other format.
Mutation
The act of changing the configuration. There are different processes that can be used for mutation, including controllers, specializers, KRM functions, web hooks, and manual in-place edits.
See also: Validation
Operator
An operator is a software component - usually a collection of one or more controller managers - that manages a particular type of workload. For example, a set of Kubernetes controllers to manage MySQL instances would be an operator.
Speaking loosely, controller and operator are often used interchangeably, though an operator always refers to code managing CRs rather than Kubernetes built-in types.
See CNFs and Operators for a thorough discussion.
Package
Generically, a logical grouping of Kubernetes resources or templated resources, for example representing a particular workload or network function installation.
For kpt packages, this specifically means well-formed Kubernetes resources along with a Kptfile. See the kpt package documentation.
This could also refer to a Helm chart, though generally we mean “kpt package” when we say “package”.
Package Revision
This specifically refers to the Porch PackageRevision resource. Porch adds
opinionated versioning and lifecycle management to packages, beyond what the
baseline kpt CLI expects. See the
Porch
documentation
for more information.
Porch
Porch is “kpt-as-a-service”, providing opinionated package management, manipulation, and lifecycle operations in a Kubernetes-based API. This allows automation of these operations using standard Kubernetes controller techniques.
Short for Package Orchestration.
See also: kpt
Resource
A Kubernetes term referring to a specific object stored in the API server, although we also use it to refer to the external representation of that object (for example text in a YAML file).
Also see REST.
Specializer
This refers to a software component that runs in the Nephio Management cluster,
and could be considered a type of
controller. However, it
specifically watches for PackageRevision resources in a Draft state, and
checks for the
conditions on those resources. When it finds
unsatisfied conditions of the type it handles, the specializer will
mutate (modify) the Draft package by adding or
changing resources.
For example, the IPAM specializer monitors package revision drafts for unresolved IP address claims. When it sees one, it takes information from the claim and uses it to allocate an IP address from the IP address management system. It writes the result back into the draft package, where a KRM function can process the result and copy ( propagate) it to the correct resources in the package.
Validation
The act of verifying that the configuration is syntactical correct, and that it matches a set of rules (or policies). Those rules or policies may be for internal consistency (e.g., matching Deployment and Service label selectors), or they may be organizationally related (e.g., all Deployments must contain a label indicating cost allocation center).
Value Propagation
The same value in a configuration is often used in more than one place. Value propagation is the technique of setting or generating the value once, and then copying (or propagating) it to different places in the configuration. For example, setting a Helm value in the values.yaml file, and then having it used in multiple places across different resources.
Variant
A variant is an modified version of a package. Sometimes it is the output of the hydration process, particularly when using out-of-place hydration. For example, if you use the same Helm chart with different inputs to create per-cluster workloads, you are generating variants.
In Nephio, we use kpt packages to help keep an association between a package and the variants of that package. When you clone a kpt package, an association is maintained with the upstream package. Every deployable variant of a package is a clone of the original, upstream package. This assists greatly in Day 2 operations; when you update the original package, you can identify all variants and merge the updates from the upstream into the downstream. This behavior is automated via the PackageVariant controller.
Variant Generation
The process of creating variants, typically in an automated way. Variants could be created across different dimensions - for example, you could create a package per cluster. Alternatively, you may create a variant per environment - for example, development, staging, and production variants.
Different methods may be warranted depending on the reason for your variants. In the ONE Summit 2022 Workshop, the PackageDeployment controller generated variants based upon the target clusters. The Porch PackageVariantSet allows more general-purpose generation of variants, based upon an explicitly list, a label selector on repositories, or an arbitrary object selector. As we develop Nephio, we may build new types of variant generators, and may even compose them (for example, to produce variants that are affected by both environment and cluster).
WET
This term, which we use as an acronym for “Write Every Time”, comes from software engineering, and is a somewhat pejorative term in contrast to DRY. However, in the context of configuration-as-data, rather than code, the idea of storing the configuration as fully-formed data enables automation and the use of data-management techniques to manage the configuration at scale.
Workload
A workload is any application running on Kubernetes, including network functions.