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Implementation guidelines

There are some general design guidelines used throughout this API.

Note

Throughout the Gateway API documentation and specification, keywords such as "MUST", "MAY", and "SHOULD" are used broadly. These should be interpreted as described in RFC 2119.

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.

Gracefully Handling Future API Versions

An important consideration when implementing this API is how it might change in the future. Similar to the Ingress API before it, this API is designed to be implemented by a variety of different products within the same cluster. That means that the API version your implementation was developed with may be different than the API version it is used with. At a minimum, the following requirements must be met to ensure future versions of the API do not break your implementation:

  • Handle fields with loosened validation without crashing
  • Handle fields that have transitioned from required to optional without crashing

Limitations of CRD and Webhook Validation

CRD and webhook validation is not the final validation i.e. webhook is "nice UX" but not schema enforcement. This validation is intended to provide immediate feedback to users when they provide an invalid configuration. Write code defensively with the assumption that at least some invalid input (Gateway API resources) will reach your controller. Both Webhook and CRD validation is not fully reliable because it:

  • May not be deployed correctly.
  • May be loosened in future API releases. (Fields may contain values with less restrictive validation in newer versions of the API).

Note: These limitations are not unique to Gateway API and apply more broadly to any Kubernetes CRDs and webhooks.

Implementers should ensure that, even if unexpected values are encountered in the API, their implementations are still as secure as possible and handle this input gracefully. The most common response would be to reject the configuration as malformed and signal the user via a condition in the status block. To avoid duplicating work, Gateway API maintainers are considering adding a shared validation package that implementations can use for this purpose. This is tracked by #926.

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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