Implementer's Guide

Everything you wanted to know about building a Gateway API implementation but were too afraid to ask.

This document is a place to collect tips and tricks for writing a Gateway API implementation that have no straightforward place within the godoc fields of the underlying types.

It's also intended to be a place to write down some guidelines to help implementers of this API to skip making common mistakes.

It may not be very relevant if you are intending to use this API as an end user as opposed to building something that uses it.

This is a living document, if you see something missing, PRs welcomed!

Important things to remember about Gateway API

Hopefully most of these are not surprising, but they sometimes have non-obvious implications that we'll try and lay out here.

Gateway API is a kubernetes.io API

Gateway API uses the gateway.networking.k8s.io API group. This means that, like APIs delivered in the core Kubernetes binaries, each time a release happens, the APIs have been reviewed by upstream Kubernetes reviewers, just like the APIs delivered in the core binaries.

Gateway API is delivered using CRDs

Gateway API is supplied as a set of CRDs, version controlled using our versioning policy.

The most important part of that versioning policy is that what appears to be the same object (that is, it has the same group,version, and kind) may have a slightly different schema. We make changes in ways that are compatible, so things should generally "just work", but there are some actions implementations need to take to make "just work"ing more reliable; these are detailed below.

The CRD-based delivery also means that if an implementation tries to use (that is get, list, watch, etc) Gateway API objects when the CRDs have not been installed, then it's likely that your Kubernetes client code will return serious errors. Tips to deal with this are also detailed below.

The CRD definitions for Gateway API objects all contain two specific annotations:

• gateway.networking.k8s.io/bundle-version: <semver-release-version>
• gateway.networking.k8s.io/channel: <channel-name>

The concepts of "bundle version" and "channel" (short for "release channel") are explained in our versioning documentation.

Implementations may use these to determine what schema versions are installed in the cluster, if any.

Changes to the Standard Channel CRDs are backwards compatible

Part of the contract for Standard Channel CRDs is that changes within an API version must be compatible. Note that CRDs that are part of Experimental Channel do not provide any backwards compatibility guarantees.

Although the Gateway API versioning policy largely aligns with upstream Kubernetes APIs, it does allow for "corrections to validation". For example, if the API spec stated that a value was invalid but the corresponding validation did not cover that, it's possible that a future release may add validation to prevent that invalid input.

This contract also means that an implementation will not fail with a higher version of the API than the version it was written with, because the newer schema being stored by Kubernetes will definitely be able to be serialized into the older version used in code by the implementation.

Similarly, if an implementation was written with a higher version, the newer values that it understands will simply never be used, as they are not present in the older version.

Implementation Rules and Guidelines

CRD Management

For information on how to manage Gateway API CRDs, including when it is acceptable to bundle CRD installation with your implementation, refer to our CRD Management Guide.

Conformance and Version Compatibility

A conformant Gateway API implementation is one that passes the conformance tests that are included in each Gateway API bundle version release.

An implementation MUST pass the conformance suite with no skipped tests to be conformant. Tests may be skipped during development, but a version you want to be conformant MUST have no skipped tests.

Extended features may, as per the contract for Extended status, be disabled.

Gateway API conformance is version-specific. An implementation that passes conformance for version N may not pass conformance for version N+1 without changes.

Implementations SHOULD submit a report from the conformance testing suite back to the Gateway API Github repo containing details of their testing.

The conformance suite output includes the Gateway API version supported.

Version Compatibility

Once v1.0 is released, for implementations supporting Gateway and GatewayClass, they MUST set a new Condition, SupportedVersion, with status: true meaning that the installed CRD version is supported, and status: false meaning that it is not.

Standard Status Fields and Conditions

Gateway API has many resources, but when designing this, we've worked to keep the status experience as consistent as possible across objects, using the Condition type and the status.conditions field.

Most resources have a status.conditions field, but some also have a namespaced field that contains a conditions field.

For the latter, Gateway's status.listeners and the Route status.parents fields are examples where each item in the slice identifies the Conditions associated with some subset of configuration.

For the Gateway case, it's to allow Conditions per Listener, and in the Route case, it's to allow Conditions per implementation (since Route objects can be used in multiple Gateways, and those Gateways can be reconciled by different implementations).

In all of these cases, there are some relatively-common Condition types that have similar meanings:

• Accepted - the resource or part thereof contains acceptable config that will produce some configuration in the underlying data plane that the implementation controls. This does not mean that the whole configuration is valid, just that enough is valid to produce some effect.
• Programmed - this represents a later phase of operation, after Accepted, when the resource or part thereof has been Accepted and programmed into the underlying dataplane. Users should expect the configuration to be ready for traffic to flow at some point in the near future. This Condition does not say that the dataplane is ready when it's set, just that everything is valid and it will become ready soon. "Soon" may have different meanings depending on the implementation.
• ResolvedRefs - this Condition indicates that all references in the resource or part thereof were valid and pointed to an object that both exists and allows that reference. If this Condition is set to status: false, then at least one reference in the resource or part thereof is invalid for some reason, and the message field should indicate which one are invalid.

Implementers should check the godoc for each type to see the exact details of these Conditions on each resource or part thereof.

Additionally, the upstream Conditions struct contains an optional observedGeneration field - implementations MUST use this field and set it to the metadata.generation field of the object at the time the status is generated. This allows users of the API to determine if the status is relevant to the current version of the object.

TLS

TLS is a large topic in Gateway API, with the set of capabilities continuing to expand. There is a TLS guide that covers this topic in more depth from a user-facing perspective, but this section attempts to fill in some gaps from an implementer's perspective.

Listener Isolation

Within Gateways, TLS config is currently tied exclusively to Listeners. To make that approach manageable, we're encouraging all implementations to work towards the goal of providing full "Listener Isolation" as defined below:

Requests SHOULD match at most one Listener. For example, if Listeners are defined for "foo.example.com" and ".example.com", a request to "foo.example.com" SHOULD only be routed using routes attached to the "foo.example.com" Listener (and not the ".example.com" Listener).

Implementations that do not support Listener Isolation MUST clearly document this. In the future, we plan to add HTTPS Listener Isolation conformance tests to ensure this behavior is consistent across implementations that claim support for the feature. See #2803 for the latest updates on these tests.

Indirect Configuration

There are a variety of instances where TLS Certificates may not be managed directly by the owner of a Gateway. Although this is not meant to be an exhaustive list, it documents some of the approaches we expect to see used to manage TLS Certificates with Gateway API:

1. Certificates from other places

Some providers offer the ability to configure and host TLS Certificates outside of Kubernetes altogether. Implementations that can connect to those external providers may expose that capability via a TLS option on the Gateway Listener, for example:

  listeners:
  - name: https
    protocol: HTTPS
    port: 443
    tls:
      mode: Terminate
      options:
        vendor.example.com/certificate-name: store-example-com

In this example, the store-example-com name would refer to the name of a certificate stored by the external vendor.example.com TLS Certificate provider.

2. Automatically generated TLS certs that are populated later

Many users would prefer that TLS certs will be automatically generated on their behalf. One potential implementation of that would involve a controller that watches Gateways and HTTPRoutes, generates TLS certs, and attaches them to the Gateway. Depending on the implementation details, Gateway owners may need to configure something at the Gateway or Listener level to explicitly opt-in to this feature. For example, let's say someone created acme-cert-generator to generate certs following this pattern. That generator may choose to only generate and populate certs on Gateway Listeners with acme.io/cert-generator set in tls.options or a similar annotation set for the entire Gateway.

Note that this is actually fairly similar to how Cert Manager works today, but that requires Gateway owners to reference a Kubernetes Secret that it will then populate. This specific approach was required because TLS CertificateRefs were required to be specified until Gateway API v1.1.

With the relaxing of Gateway API validation in v1.1, TLS Certificates can be left unspecified on creation, allowing for less configuration when working with generated TLS certificates.

3. Certs that are specified by other personas

In some organizations, Application Developers are responsible for managing TLS Certificates (see Roles and Personas for more on this and other roles).

To enable this use case, a new controller and CRD would be created. This CRD would link hostnames to user-provided certs, and then the controller would populate the certs specified by that CRD on Gateway listeners that matched those hostnames. This would also likely benefit from a Listener or Gateway-level opt-in for the behavior.

Overall Guidelines for TLS Extensions

When building TLS extensions on top of Gateway API, it's important to follow the following guidelines:

1. Use domain-prefixed names that are unique to your implementation for any TLS Options or Annotations. (For example, use example.com/certificate-name instead of just certificate-name).
2. Do not encode sensitive information like certificates in the option or annotation value. Instead favor references via concise names that are easy to understand. Although these values can technically be as long as 253 characters, we strongly recommend keeping the values below 50 characters to maintain overall readability and UX.
3. To enable these extensions, Gateway API v1.1+ will no longer require TLS Config to be specified on Gateway Listeners. When a Gateway Listener does not have sufficient TLS configuration specified, implementations MUST set the Programmed condition to False for that Listener with the InvalidTLSConfig reason.
4. Regardless of any extensions you may choose to support, it is important to support the core TLS configuration that is intended to be portable across all implementations. Extensions have their place in this API, but all implementations MUST still support the core capabilities of the API.

Resource Details

For each currently available conformance profile, there are a set of resources that implementations are expected to reconcile.

The following section goes through each Gateway API object and indicates expected behaviors.

GatewayClass

GatewayClass has one main spec field - controllerName. Each implementation is expected to claim a domain-prefixed string value (like example.com/example-ingress) as its controllerName.

Implementations MUST watch all GatewayClasses, and reconcile GatewayClasses that have a matching controllerName. The implementation must choose at least one compatible GatewayClass out of the set of GatewayClasses that have a matching controllerName, and indicate that it accepts processing of that GatewayClass by setting an Accepted Condition to status: true in each. Any GatewayClasses that have a matching controllerName but are not Accepted must have the Accepted Condition set to status: false.

Implementations MAY choose only one GatewayClass out of the pool of otherwise acceptable GatewayClasses if they can only reconcile one, or, if they are capable of reconciling multiple GatewayClasses, they may also choose as many as they like.

If something in the GatewayClass renders it incompatible (at the time of writing, the only possible reason for this is that there is a pointer to a paramsRef object that is not supported by the implementation), then the implementation SHOULD mark the incompatible GatewayClass as not Accepted.

Gateway

Gateway objects MUST refer in the spec.gatewayClassName field to a GatewayClass that exists and is Accepted by an implementation for that implementation to reconcile them.

Gateway objects that fall out of scope (for example, because the GatewayClass they reference was deleted) for reconciliation MAY have their status removed by the implementation as part of the delete process, but this is not required.

Routes

All Route objects share some properties:

• They MUST be attached to an in-scope parent for the implementation to consider them reconcilable.
• The implementation MUST update the status for each in-scope Route with the relevant Conditions, using the namespaced parents field. See the specific Route types for details, but this usually includes Accepted, Programmed and ResolvedRefs Conditions.
• Routes that fall out of scope SHOULD NOT have status updated, since it's possible that these updates may overwrite any new owners. The observedGeneration field will indicate that any remaining status is out of date.

HTTPRoute

HTTPRoutes route HTTP traffic that is unencrypted and available for inspection. This includes HTTPS traffic that's terminated at the Gateway (since that is then decrypted), and allows the HTTPRoute to use HTTP properties, like path, method, or headers in its routing directives.

TLSRoute

TLSRoutes route encrypted TLS traffic using the SNI header, without decrypting the traffic stream, to the relevant backends.

TCPRoute

TCPRoutes route a TCP stream that arrives at a Listener to one of the given backends.

UDPRoute

UDPRoutes route UDP packets that arrive at a Listener to one of the given backends.

ReferenceGrant

ReferenceGrant is a special resource that is used by resource owners in one namespace to selectively allow references from Gateway API objects in other namespaces.

A ReferenceGrant is created in the same namespace as the thing it's granting reference access to, and allows access from other namespaces, from other Kinds, or both.

Implementations that support cross-namespace references MUST watch ReferenceGrant and reconcile any ReferenceGrant that points to an object that's referred to by an in-scope Gateway API object.