Implementation guidelines

There are some general design guidelines used throughout this API.

Single resource consistency

The Kubernetes API guarantees consistency only on a single resource level. There are a couple of consequences for complex resource graphs as opposed to single resources:

  • Error checking of properties spanning multiple resource will be asynchronous and eventually consistent. Simple syntax checks will be possible at the single resource level, but cross resource dependencies will need to be handled by the controller.
  • Controllers will need to handle broken links between resources and/or mismatched configuration.

Conflicts

Separation and delegation of responsibility among independent actors (e.g between cluster ops and application developers) can result in conflicts in the configuration. For example, two application teams may inadvertently submit configuration for the same HTTP path.

In most cases, guidance for conflict resolution is provided along with the documentation for fields that may have a conflict. If a conflict does not have a prescribed resolution, the following guiding principles should be applied:

  • Prefer not to break things that are working.
  • Drop as little traffic as possible.
  • Provide a consistent experience when conflicts occur.
  • Make it clear which path has been chosen when a conflict has been identified. Where possible, this should be communicated by setting appropriate status conditions on relevant resources.
  • More specific matches should be given precedence over less specific ones.
  • The resource with the oldest creation timestamp wins.
  • If everything else is equivalent (including creation timestamp), precedences should be given to the resource appearing first in alphabetical order (namespace/name). For example, foo/bar would be given precedence over foo/baz.

Conformance

As this API aims to cover a wide set of implementations and use cases, it will not be possible for all implementations to support all features at the present. However, we do expect the set of features supported to converge eventually. For a given feature, users will be guaranteed that features in the API will be portable between providers if the feature is supported.

To model this in the API, we are taking a similar approach as with sig-arch work on conformance profiles. Features as described in the API spec will be divided into three major categories:

  • CORE features will be portable and we expect that there is a reasonable roadmap for ALL implementations towards support of APIs in this category.
  • EXTENDED features are those that are portable but not universally supported across implementations. Those implementations that support the feature will have the same behavior and semantics. It is expected that some number of EXTENDED features will eventually migrate into the CORE. EXTENDED features will be part of the API types and schema.
  • CUSTOM features are those that are not portable and are vendor-specific. CUSTOM features will not have API types and schema except via generic extension points.

Behavior and feature in the CORE and EXTENDED set will be defined and validated via behavior-driven conformance tests. CUSTOM features will not be covered by conformance tests.

By including and standardizing EXTENDED features in the API spec, we expect to be able to converge on portable subsets of the API among implementations without compromising overall API support. Lack of universal support will not be a blocker towards developing portable feature sets. Standardizing on spec will make it easier to eventually graduate to CORE when support is widespread.

Conformance expectations

We expect there will be varying levels of conformance among the different providers in the early days of this API. Users can use the results of the conformance tests to understand areas where there may be differences in behavior from the spec.

Implementation-specific

In some aspects of the API, we give the user an ability to specify usage of the feature, however, the exact behavior may depend on the underlying implementation. For example, regular expression matching is present in all implementations but specifying an exact behavior is impossible due to subtle differences between the underlying libraries used (e.g. PCRE, ECMA, Re2). It is still useful for our users to spec out the feature as much as possible, but we acknowledge that the behavior for some subset of the API may still vary (and that's ok).

These cases will be specified as defining delimited parts of the API "implementation-specific".

The "implementation-specific" designation allows a CORE or EXTENDED feature to be well-defined taking into account the realities of some features that are mostly but not entirely portable.

API Conventions

Service APIs follow Kubernetes API conventions. These conventions are intended to ease client development and ensure that configuration mechanisms can consistently be implemented across a diverse set of use cases. One variation from the Kubernetes conventions is for fields with default values. These fields are considered optional but are not pointers since they will never be empty.