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traefik-route-k8s-operator

Description

Traefik-route-k8s provides separation of concerns between the general needs of the ingress and ingress-per-unit relation interfaces, and the need to specify per-relation routing logic at runtime which in this case is specific to Traefik. The traefik-k8s charm provides support for a few built-in routing types and optionally setting an external hostname.

The traefik-route-k8s charm proposed here addresses advanced use-cases, providing the cloud admin full control over the routing template.

This charm allows to preserve the generality of the ingress_per_unit relation interface of the traefik-k8s charm, while exposing Traefik-specific routing configurations through a configuration charm that sits between traefik-k8s and the proxied application.

Usage

The idea is that this charm sits between the charm in need of ingress (let's call it Charm), and traefik-k8s. For example a juju topology for providing ingress to a prometheus-k8s charm could look like:

classDiagram
    direction LR
    traefik_k8s "1" --|> "*" traefik_route_k8s : traefik_route
    traefik_route_k8s "1" --|> "1" prometheus_k8s : ingress_per_unit
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This way the complexity of configuring the ingress is hidden from the (developers/maintainers of the) end consumer, prometheus-k8s in this example, while at the same time all advanced configuration options remain available to the cloud admin.

The information flow can be visualized as:

sequenceDiagram
    traefik --> traefik-route: traefik_route
    note left of traefik-route: user relates traefik and traefik-route

    traefik-route --> charm: ingress_per_unit
    note left of charm: user relates charm and traefik-route
    note left of traefik-route: user configures traefik-route (config template and root_url)

    charm ->> traefik-route: (model & unit name)
    note left of traefik-route: traefik-route renders the config
    traefik-route ->>  traefik: (rendered) config yaml
    traefik-route ->>  charm: ingress (root_url)
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Configuration fields:

You can configure TraefikRoute with the following parameters:

  • rule: contains a Jinja 2 template that is used to populate the Traefik router’s rule field. it allows the cloud admin to override the template that traefik's router uses to generate ingress urls. Cfr. traefik's documentation for how that works more precisely.
    The following global variables will be injected when evaluating the template:

    • {{juju_model}} resolves to the model name of the downstream proxied application.

    • {{juju_application}} resolves to the application name of the downstream proxied application.

    • {{juju_unit}} resolves to the unit name of the downstream proxied unit (this is applicable only when the ingress is performed per-unit, and in the future we plan to support also application-level routing); to avoid issues when used together with the Host directive or similar, the slash character between application name and unit index is replaced with a dash.

      For example, the rule value:

      rule=Host(`foo.bar/{{juju_unit}}`)

      will generate the following Traefik configuration file when related via the ingress_per_unit relation interface with a Juju application called prometheustest from the costest model with two units:

      http:
    routers:
        juju-costest-prometheustest-0-router:
            rule: Host(`foo.bar/prometheustest-0`)
            service: juju-costest-prometheustest-0
            entrypoint: web
    services:
        juju-costest-prometheustest-0-service:
            loadBalancer:
                servers:
                - url: http://<unit_ingress_address>:<unit_port>

      The <unit_ingress_address> and <unit_port> tokens are provided by each unit of the downstream proxied application over the ingress_per_unit relation interface.

  • root_url: The url to advertise to the unit in need of ingress.

    The value of the field is going to be processed in exactly the same way as the rule field. The assumption is that the url is going to match the rule; however, we have no way to validate and/or enforce this condition; so beware! For example, given a downstream unit called prometheus/0 in the cos model, the following configuration is valid:

    rule="Host(`{{juju_unit}}.{{juju_model}}.foo.bar`)"
root_url="http://{{juju_unit}}.{{juju_model}}.foo.bar/baz"

    while the following configuration is not:

    rule="Host(`{{juju_model}}-{{juju_unit}}.foo.bar`) || 
     HostRegexp(`{subdomain:[a-z]+}.foo.bar`) || 
     Host(`doobadooba.com`)"
root_url="ka-ching.com"

    The reason why this is not valid is that the url does not match the rule: so the url advertised to the unit will not in fact be routed correctly by Traefik. Note that Traefik will still work 'correctly', i.e. the application will be reachable at (for example) http://doobadooba.com. Examples of 'good' root_url values for this case would be:

    root_url="{{juju_model}}-{{juju_unit}}.foo.bar/baz"
root_url="baz.foo.bar" 
root_url="doobadooba.com/choo"

Relations

Provides an “ingress-per-unit” relation using the “ingress_per_unit” relation interface, with limit 1.

Requires a “traefik-route” relation using a “traefik_route” relation interface, with limit 1.

OCI Images

This is a workload-less charm.

Contributing

Please see the Juju SDK docs for guidelines on enhancements to this charm following best practice guidelines, and CONTRIBUTING.md for developer guidance.