Virtual Host Discovery Service (VHDS)

    By default in RDS, all routes for a cluster are sent to every Envoy instance in the mesh. This causes scaling issues as the size of the cluster grows. The majority of this complexity can be found in the virtual host configurations, of which most are not needed by any individual proxy.

    In order to fix this issue, the Virtual Host Discovery Service (VHDS) protocol uses the delta xDS protocol to allow a route configuration to be subscribed to and the necessary virtual hosts to be requested as needed. Instead of sending all virtual hosts with a route config, using VHDS will allow an Envoy instance to subscribe and unsubscribe from a list of virtual hosts stored internally in the xDS management server. The xDS management server will monitor this list and use it to filter the configuration sent to an individual Envoy instance to only contain the subscribed virtual hosts.

    Virtual hosts in VHDS are identified by a combination of the name of the route configuration to which the virtual host belongs as well as the HTTP host header (:authority for HTTP2) entry. Resources should be named as follows:

    Note that matching should be done from right to left since a host entry cannot contain slashes while a route configuration name can.

    Subscribing to Resources

    VHDS allows resources to be to using a DeltaDiscoveryRequest with the set to type.googleapis.com/envoy.config.route.v3.VirtualHost and resource_names_subscribe set to a list of virtual host resource names for which it would like configuration.

    If a route for the contents of a host/authority header cannot be resolved, the active stream is paused while a is sent. When a DeltaDiscoveryResponse is received where one of the or the name in the response exactly matches the entry from the DeltaDiscoveryRequest, the route configuration is updated, the stream is resumed, and processing of the filter chain continues.

    When a route configuration entry is updated, if the has changed, the virtual host table for that route configuration is cleared, which will require that all virtual hosts be sent again.

    VHDS shouldn’t present any compatibility issues with scoped RDS. Route configuration names can still be used for virtual host matching, but with scoped RDS configured it would point to a scoped route configuration.

    However, it is important to note that using on-demand and VHDS together will require two on-demand subscriptions per routing scope.

    Statistics

    VHDS has a statistics tree rooted at http.<stat_prefix>.vhds.<virtual_host_name>.. Any character in the name gets replaced with in the stats tree. The stats tree contains the following statistics: