Virtual Host Discovery Service (VHDS)

The virtual host discovery service (VHDS) API is an optional API that Envoy will call to dynamically fetch virtual hosts. A virtual host includes a name and set of domains that get routed to it based on the incoming request’s host header.

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 Host Resource Naming Convention

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:

<route configuration name>/<host entry>

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 subscribed to using a DeltaDiscoveryRequest with the type_url 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 DeltaDiscoveryRequest is sent. When a DeltaDiscoveryResponse is received where one of the aliases or the name in the response exactly matches the resource_names_subscribe entry from the DeltaDiscoveryRequest, the route configuration is updated, the stream is resumed, and processing of the filter chain continues.

Updates to virtual hosts occur in two ways. If a virtual host was originally sent over RDS, then the virtual host should be updated over RDS. If a virtual host was subscribed to over VHDS, then updates will take place over VHDS.

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

Compatibility with Scoped RDS

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 scoped RDS and VHDS together will require two on-demand subscriptions per routing scope.

• v3 API reference

Statistics

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

Name	Type	Description
config_reload	Counter	Total API fetches that resulted in a config reload due to a different config
empty_update	Counter	Total count of empty updates received