Xds fallback

xds/src/main/java/io/grpc/xds/GrpcXdsTransportFactory.java

@@ -22,6 +22,7 @@
 import io.grpc.CallOptions;
 import io.grpc.ChannelCredentials;
 import io.grpc.ClientCall;
+import io.grpc.ConnectivityState;
 import io.grpc.Context;
 import io.grpc.Grpc;
 import io.grpc.ManagedChannel;
@@ -84,6 +85,12 @@ public void shutdown() {
       channel.shutdown();
     }
 
+    @Override
+    public boolean isConnected() {
+      ConnectivityState state = channel.getState(false);
+      return state == ConnectivityState.READY;
+    }
+
     private class XdsStreamingCall<ReqT, RespT> implements
         XdsTransportFactory.StreamingCall<ReqT, RespT> {
 

xds/src/main/java/io/grpc/xds/client/BootstrapperImpl.java

@@ -41,6 +41,9 @@
 @Internal
 public abstract class BootstrapperImpl extends Bootstrapper {
 
+  public static final String GRPC_EXPERIMENTAL_XDS_FALLBACK =
+      "GRPC_EXPERIMENTAL_XDS_FALLBACK";
+
   // Client features.
   @VisibleForTesting
   public static final String CLIENT_FEATURE_DISABLE_OVERPROVISIONING =
@@ -59,11 +62,18 @@
     logger = XdsLogger.withLogId(InternalLogId.allocate("bootstrapper", null));
   }
 
+  // Delayed initialization of xdsFallbackEnabled to allow for flag initialization.
+  public static boolean isEnabledXdsFallback() {
+    return GrpcUtil.getFlag(GRPC_EXPERIMENTAL_XDS_FALLBACK, false);
+  }
+
   protected abstract String getJsonContent() throws IOException, XdsInitializationException;
 
   protected abstract Object getImplSpecificConfig(Map<String, ?> serverConfig, String serverUri)
       throws XdsInitializationException;
 
+
+
   /**
    * Reads and parses bootstrap config. The config is expected to be in JSON format.
    */
@@ -102,6 +112,9 @@
       throw new XdsInitializationException("Invalid bootstrap: 'xds_servers' does not exist.");
     }
     List<ServerInfo> servers = parseServerInfos(rawServerConfigs, logger);
+    if (servers.size() > 1 && !isEnabledXdsFallback()) {
+      servers = ImmutableList.of(servers.get(0));
+    }
     builder.servers(servers);
 
     Node.Builder nodeBuilder = Node.newBuilder();
@@ -208,6 +221,9 @@
         if (rawAuthorityServers == null || rawAuthorityServers.isEmpty()) {
           authorityServers = servers;
         } else {
+          if (rawAuthorityServers.size() > 1 && !isEnabledXdsFallback()) {
+            rawAuthorityServers = ImmutableList.of(rawAuthorityServers.get(0));
+          }
           authorityServers = parseServerInfos(rawAuthorityServers, logger);
         }
         authorityInfoMapBuilder.put(