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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.

Overlapping Support Levels

It is possible for support levels to overlap. When this occurs, the minimum expressed support level should be interpreted. For example, an identical struct may be embedded in two different places. In one of those places, the struct is considered to have CORE support while the other place only includes EXTENDED support. Fields within this struct may express separate CORE and EXTENDED support levels, but those levels may never be interpreted as exceeding the support level of the parent struct they are embedded in.

For a more concrete example, HTTPRoute includes CORE support for filters defined within a Rule and EXTENDED support when defined within ForwardTo. Those filters may separately define support levels for each field. When interpreting overlapping support levels, the minimum value should be interpreted. That means if a field has a CORE support level but is in a filter attached in a place with EXTENDED support, the interpreted support level should be EXTENDED.

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.

Kind vs. Resource

Similar to other Kubernetes APIs, Gateway API uses "Kind" instead of "Resource" in object references throughout the API. This pattern should be familiar to most Kubernetes users.

Per the Kubernetes API conventions, this means that all implementations of this API should have a predefined mapping between kinds and resources. Relying on dynamic resource mapping is not safe.

API Conventions

Gateway API follows 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. In addition to the Kubernetes API conventions, Gateway API has the following conventions:

List Names

Another convention this project uses is for plural field names for lists in our CRDs. We use the following rules:

  • If the field name is a noun, use a plural value.
  • If the field name is a verb, use a singular value.

So for example, in HTTPRoute, hostnames uses a plural, but forwardTo is singular, although they are both lists.

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