[Java] Implement GcsClient (#4601)

java/api/src/main/java/org/ray/api/Ray.java

@@ -1,6 +1,8 @@
 package org.ray.api;
 
 import java.util.List;
+
+import org.ray.api.gcs.GcsClient;
 import org.ray.api.id.UniqueId;
 import org.ray.api.runtime.RayRuntime;
 import org.ray.api.runtime.RayRuntimeFactory;
@@ -127,4 +129,11 @@ public final class Ray extends RayCall {
   public static RuntimeContext getRuntimeContext() {
     return runtime.getRuntimeContext();
   }
+
+  /**
+   * Get gcs client.
+   */
+  public static GcsClient getGcsClient() {
+    return runtime.getGcsClient();
+  }
 }

java/api/src/main/java/org/ray/api/gcs/GcsClient.java

@@ -0,0 +1,15 @@
+package org.ray.api.gcs;
+
+import java.util.List;
+
+/**
+ * The client used to interface with the GCS.
+ */
+public interface GcsClient {
+
+  /**
+   * Get all node information in Ray cluster.
+   */
+  List<NodeInfo> getAllNodeInfo();
+
+}

java/api/src/main/java/org/ray/api/gcs/NodeInfo.java

@@ -0,0 +1,36 @@
+package org.ray.api.gcs;
+
+import java.util.Map;
+import org.ray.api.id.UniqueId;
+
+/**
+ * A class that represents the information of a node.
+ */
+public class NodeInfo {
+
+  public final UniqueId nodeId;
+
+  public final String nodeAddress;
+
+  public final boolean isAlive;
+
+  public final Map<String, Double> resources;
+
+  public NodeInfo(UniqueId nodeId,  String nodeAddress,
+                  boolean isAlive, Map<String, Double> resources) {
+    this.nodeId = nodeId;
+    this.nodeAddress = nodeAddress;
+    this.isAlive = isAlive;
+    this.resources = resources;
+  }
+
+  public String toString() {
+    return "NodeInfo{"
+        + "nodeId='" + nodeId + '\''
+        + ", nodeAddress='" + nodeAddress + "\'"
+        + ", isAlive=" + isAlive
+        + ", resources=" + resources
+        + "}";
+  }
+
+}

java/api/src/main/java/org/ray/api/runtime/RayRuntime.java

@@ -7,6 +7,7 @@ import org.ray.api.RayPyActor;
 import org.ray.api.RuntimeContext;
 import org.ray.api.WaitResult;
 import org.ray.api.function.RayFunc;
+import org.ray.api.gcs.GcsClient;
 import org.ray.api.id.UniqueId;
 import org.ray.api.options.ActorCreationOptions;
 import org.ray.api.options.CallOptions;
@@ -131,4 +132,6 @@ public interface RayRuntime {
    */
   RayPyActor createPyActor(String moduleName, String className, Object[] args,
       ActorCreationOptions options);
+
+  GcsClient getGcsClient();
 }