xds: don't model all supported route actions as weighted_clusters

[easwars]

The config_selector inside xDS resolver has a list of routes. See:

grpc-go/internal/xds/resolver/serviceconfig.go

Line 157 in c7ec4d9

routes []route

The route struct is defined here:

grpc-go/internal/xds/resolver/serviceconfig.go

Line 111 in c7ec4d9

type route struct {

and has a matcher to match the incoming RPC and a clusters field which represents the cluters to which the matching RPC needs to be routed to. But the clusters field is of type wrr.WRR. See:

grpc-go/internal/xds/resolver/serviceconfig.go

Line 114 in c7ec4d9

clusters wrr.WRR // holds *routeCluster entries

This means that for the single-cluster case (which is the most common case) and the cluster_specifier_plugin case, we will still incur the cost of generating a random number for every RPC. At RPC time, SelectConfig is invoked which picks the cluster out of the matching route here:

grpc-go/internal/xds/resolver/serviceconfig.go

Line 190 in c7ec4d9

cluster, ok := rt.clusters.Next().(*routeCluster)

And the random WRR implementation has to generate a random number for every call to Next. See:

grpc-go/internal/wrr/random.go

Line 51 in c7ec4d9

func (rw *randomWRR) Next() (item any) {

This is something that should be avoided for performance reasons.

[easwars]

Fixing the issue in the xDS resolver would mean fixing the xDS client to not represent the single-cluster case as a weighted cluster. See: https://github.com/grpc/grpc-go/blob/c7ec4d9ae3281bc57a8adce59b572e56965fb728/internal/xds/xdsclient/xdsresource/unmarshal_rds.go#L322C5-L322C10