Kubernetes xDS server

This small server can be used for:

• gRPC client-side Load Balancing
• Envoy cluster CDS

Configuration in the client (supported clients as of september 2020: C-core, Java & Go) goes like this:

• 1. Set the environment variable GRPC_XDS_BOOTSTRAP=/bootstrap.json (doc)
• 2. Add this file /bootstrap.json containing these contents:

  {
    "xds_servers": [{
      "server_uri": "xds-server-address:8080"
    }],
    "node": {
      "id": "$HOSTNAME",
      "metadata": {
          "SOME_KEY": "SOME_VALUE"
      },
      "locality": {
          "zone": "europe-west4-a"
      }
    }
  } 

• 3. Create the gRPC client like this:

  import (
    _ "google.golang.org/grpc/xds" // To install the xds resolvers and balancers.
  )
  
  grpc.Dial("xds:///upstream-service", grpc.WithInsecure())

References

1. Guide to the xDS protocol
2. Original proposal: https://github.com/grpc/proposal/blob/master/A27-xds-global-load-balancing.md
3. https://itnext.io/proxyless-grpc-load-balancing-in-kubernetes-ca1a4797b742 & https://github.com/asishrs/proxyless-grpc-lb
4. GoogleBlog: efficient multi-zone and topology aware routing
5. This is an (more flexible) alternative to the TopologyAwareHints beta-feature in Kubernetes 1.23+
6. Using xDS v3: server_features <- xds_v3
7. gRPC xDS Loadbalancing
8. debug gRPC xDS clients