The Gateway API has core support for cross Namespace routing. This is useful when more than one user or team is sharing the underlying networking infrastructure, yet control and configuration must be segmented to minimize access and fault domains.
Gateways and Routes can be deployed into different Namespaces and Routes can attach to Gateways across Namespace boundaries. This allows user access control to be applied differently across Namespaces for Routes and Gateways, effectively segmenting access and control to different parts of the cluster-wide routing configuration. The ability for Routes to attach to Gateways across Namespace boundaries are governed by Route Attachment. Route attachment is explored in this guide and demonstrates how independent teams can safely share the same Gateway.
In this guide there are two independent teams, store and site, operating
in the same Kubernetes cluster in the
site-ns Namespaces. These
are their goals and how they use Gateway API resources to accomplish them:
- The site team has two applications, home and login. The team wants to to isolate access and configuration across their apps as much as possible to minimize access and failure domains. They use separate HTTPRoutes attached to the same Gateway to isolate routing configurations, such as canary rollouts, and still share the same but share the same IP address, port, DNS domain, and TLS certificate.
- The store team has a single Service called store that they have deployed
store-nsNamespace which also needs to be exposed behind the same IP address and domain.
- The Foobar Corporation operates behind the
foo.example.comdomain for all apps. This is controlled by a central infrastructure team, operating in the
- Lastly, the security team controls the certificate for
foo.example.com. By managing this certificate through the single shared Gateway they are able to centrally control security without directly involving application teams.
The logical relationship between the Gateway API resources looks like this:
Cross-namespace Route Attachment¶
Route attachment is an important concept that dictates how Routes attach to Gateways and program their routing rules. It is especially relevant when there are Routes across Namespaces that share one or more Gateways. Gateway and Route attachment is bidirectional - attachment can only succeed if the Gateway owner and Route owner owner both agree to the relationship. This bi-directional relationship exists for two reasons:
- Route owners don't want to overexpose their applications through paths they are not aware of.
- Gateway owners don't want certain apps or teams using Gateways without permission. For example, an internal service shouldn't be accessible through an internet Gateway.
Gateways support attachment constraints which are fields on Gateway
listeners that restrict which Routes can be attached. Gateways support
Namespaces and Route types as attachment constraints. Any Routes that do not
meet the attachment constraints are not able to attach to that Gateway.
Similarly, Routes explicitly reference Gateways that they want to attach to
through the Route's
parentRef field. Together these create a handshake
between the infra owners and application owners that enables them to
independently define how applications are exposed through Gateways. This is
effectively a policy that reduces administrative overhead. App owners can
specify which Gateways their apps should use and infra owners can constrain
the Namespaces and types of Routes that a Gateway accepts.
The infrastructure team deploys the
shared-gateway Gateway into the
apiVersion: gateway.networking.k8s.io/v1alpha2 kind: Gateway metadata: name: shared-gateway namespace: infra-ns spec: gatewayClassName: shared-gateway-class listeners: - name: https hostname: "foo.example.com" protocol: HTTPS port: 443 allowedRoutes: namespaces: from: Selector selector: matchLabels: shared-gateway-access: "true" tls: certificateRef: name: foo-example-com
https listener in the above Gateway matches traffic for the
foo.example.com domain. This allows the infrastructure team to manage all
aspects of the domain. The HTTPRoutes below do not need to specify domains
and will match all traffic by default if
hostname is not set. This makes
it easier to manage HTTPRoutes because they can be domain agnostic, which is
helpful when application domains are not static.
This Gateway also configures HTTPS using the
infra-ns Namespace. This allows the infrastructure team to centrally
manage TLS on behalf of app owners. The
foo-example-com certificate will
terminate all traffic going to its attached Routes, without any TLS
configuration on the HTTPRoutes themselves.
This Gateway uses a Namespace selector to define which HTTPRoutes are allowed to attach. This allows the infrastructure team to constrain who or which apps can use this Gateway by allowlisting a set of Namespaces.
apiVersion: gateway.networking.k8s.io/v1alpha2 kind: Gateway spec: listeners: - allowedRoutes: namespaces: from: Selector selector: matchLabels: shared-gateway-access: "true" ...
Only Namespaces which are labelled
shared-gateway-access: "true" will be
able to attach their Routes to
shared-gateway. In the following set of
Namespaces, if an HTTPRoute existed in the
no-external-access Namespace with
infra-ns/shared-gateway, it would be ignored by the
Gateway because the attachment constraint (Namespace label) was not met.
apiVersion: v1 kind: Namespace metadata: name: infra-ns labels: shared-gateway-access: "true" --- apiVersion: v1 kind: Namespace metadata: name: site-ns labels: shared-gateway-access: "true" --- apiVersion: v1 kind: Namespace metadata: name: store-ns labels: shared-gateway-access: "true" --- apiVersion: v1 kind: Namespace metadata: name: no-external-access
Note that attachment constraints on the Gateway are not required, but they are
a best-practice if operating a cluster with many different teams and
Namespaces. In environments where all apps in a cluster have permission to
attach to a Gateway then the
listeners.routes field does not have to be
configured and all Routes can freely use the Gateway.
The store team deploys their route for the
store Service in the
apiVersion: gateway.networking.k8s.io/v1alpha2 kind: HTTPRoute metadata: name: store namespace: store-ns spec: parentRefs: - name: shared-gateway namespace: infra-ns rules: - matches: - path: value: /store backendRefs: - name: store port: 8080
This Route has straightforward routing logic as it just matches for
/store traffic which it sends to the
The site team now deploys Routes for their applications. They deploy two
HTTPRoutes into the
homeHTTPRoute acts as a default routing rule, matching for all traffic to
foo.example.com/*not matched by an existing routing rule and sending it to the
loginHTTPRoute routes traffic for
service/login-v2. It uses weights to granularly control traffic distribution between them.
Both of these Routes use the same Gateway attachment configuration which
gateway/shared-gateway in the
infra-ns Namespace as the only
Gateway that these Routes want to attach to.
apiVersion: gateway.networking.k8s.io/v1alpha2 kind: HTTPRoute metadata: name: home namespace: site-ns spec: parentRefs: - name: shared-gateway namespace: infra-ns rules: - backendRefs: - name: home port: 8080 --- apiVersion: gateway.networking.k8s.io/v1alpha2 kind: HTTPRoute metadata: name: login namespace: site-ns spec: parentRefs: - name: shared-gateway namespace: infra-ns rules: - matches: - path: value: /login backendRefs: - name: login-v1 port: 8080 weight: 90 - name: login-v2 port: 8080 weight: 10
After these three Routes are deployed, they will all be attached to the
shared-gateway Gateway. The Gateway merges these Routes into a single flat
list of routing rules. Routing precedence
between these routing rules is determined by most specific match and
conflicts are handled according to conflict
resolution. This provides predictable and
deterministic merging of routing rules between independent users.
Thanks to cross-Namespace routing, the Foobar Corporation can distribute ownership of their infrastructure more evenly, while still retaining centralized control. This gives them the best of both worlds, all delivered through declarative and open source APIs.