xds: refactor resource subscription implementation in XdsClient (#7458)

Introduce ResourceSubscriber for tracking the state of a single resource.

Every time newly subscribing to some resource, a corresponding ResourceSubscriber is created. Note it does not control the resource discovery RPCs. It is still the XdsClient that sends RPCs for with all subscribed resource names for each type. A ResourceSubscriber can have the following states:

  - When the initial resource fetch timer (respTimer) is pending, the resource is under discovery, the resource data is unknown. Even if the XdsClient receives a response not containing the corresponding resource, it does not mean the resource is absent. We still need to wait until a response containing the resource data coming or the timer being fired. The timer is scheduled when the ResourceSubscriber is created. So the XdsClient should always create the corresponding ResourceSubscriber when it starts to subscribe a new resource.

  - If the resource fetch timer is not pending, we must know the existence of the resource data. If data field is set, it is the most recently received resource data (aka, cached entry). Otherwise, absent field is set to true, indicating the resource does not exist. The exceptional case is when the ADS stream is closed and in the retry backoff period. During that period, respTimer is cancelled and the resource existence may or may not be known. Once the backoff finishes, the XdsClient will reschedule the respTimer when it recreates the ADS stream and re-request all the resources.

Watchers can be added to existing ResourceSubscribers. At the time the watcher is added, its callback will be invoked if we've already known the existence of the resource. Otherwise, the watcher will just sit there and wait data or absence to come in the future.

Author: voidzcy

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

@@ -103,7 +103,7 @@ ConfigUpdate build() {
     }
   }
 
-  static final class LdsUpdate {
+  static final class LdsUpdate implements ResourceUpdate {
     // Total number of nanoseconds to keep alive an HTTP request/response stream.
     private final long httpMaxStreamDurationNano;
     // The name of the route configuration to be used for RDS resource discovery.
@@ -134,6 +134,25 @@ List<VirtualHost> getVirtualHosts() {
       return virtualHosts;
     }
 
+    @Override
+    public int hashCode() {
+      return Objects.hash(httpMaxStreamDurationNano, rdsName, virtualHosts);
+    }
+
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) {
+        return true;
+      }
+      if (o == null || getClass() != o.getClass()) {
+        return false;
+      }
+      LdsUpdate that = (LdsUpdate) o;
+      return Objects.equals(httpMaxStreamDurationNano, that.httpMaxStreamDurationNano)
+          && Objects.equals(rdsName, that.rdsName)
+          && Objects.equals(virtualHosts, that.virtualHosts);
+    }
+
     @Override
     public String toString() {
       ToStringHelper toStringHelper = MoreObjects.toStringHelper(this);
@@ -182,7 +201,7 @@ LdsUpdate build() {
     }
   }
 
-  static final class RdsUpdate {
+  static final class RdsUpdate implements ResourceUpdate {
     // The list virtual hosts that make up the route table.
     private final List<VirtualHost> virtualHosts;
 
@@ -206,7 +225,7 @@ public String toString() {
     }
   }
 
-  static final class CdsUpdate {
+  static final class CdsUpdate implements ResourceUpdate {
     private final String clusterName;
     @Nullable
     private final String edsServiceName;
@@ -351,7 +370,7 @@ CdsUpdate build() {
     }
   }
 
-  static final class EdsUpdate {
+  static final class EdsUpdate implements ResourceUpdate {
     private final String clusterName;
     private final Map<Locality, LocalityLbEndpoints> localityLbEndpointsMap;
     private final List<DropOverload> dropPolicies;
@@ -497,10 +516,13 @@ ListenerUpdate build() {
     }
   }
 
+  interface ResourceUpdate {
+  }
+
   /**
    * Watcher interface for a single requested xDS resource.
    */
-  private interface ResourceWatcher {
+  interface ResourceWatcher {
 
     /**
      * Called when the resource discovery RPC encounters some transient error.
@@ -523,6 +545,14 @@ interface RdsResourceWatcher extends ResourceWatcher {
     void onChanged(RdsUpdate update);
   }
 
+  interface CdsResourceWatcher extends ResourceWatcher {
+    void onChanged(CdsUpdate update);
+  }
+
+  interface EdsResourceWatcher extends ResourceWatcher {
+    void onChanged(EdsUpdate update);
+  }
+
   /**
    * Config watcher interface. To be implemented by the xDS resolver.
    */
@@ -535,16 +565,6 @@ interface ConfigWatcher extends ResourceWatcher {
     void onConfigChanged(ConfigUpdate update);
   }
 
-  interface CdsResourceWatcher extends ResourceWatcher {
-
-    void onChanged(CdsUpdate update);
-  }
-
-  interface EdsResourceWatcher extends ResourceWatcher {
-
-    void onChanged(EdsUpdate update);
-  }
-
   /**
    * Listener watcher interface. To be used by {@link io.grpc.xds.internal.sds.XdsServerBuilder}.
    */