feat(server): add a storage-aware GET /readiness endpoint

hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/filter/AuthenticationFilter.java

@@ -77,6 +77,7 @@ public class AuthenticationFilter implements ContainerRequestFilter, ContainerRe
     private static final AntPathMatcher MATCHER = new AntPathMatcher();
     private static final Set<String> FIXED_WHITE_API_SET = ImmutableSet.of(
             "versions",
+            "readiness",
             "openapi.json"
     );
     /** Remove auth/login API from whitelist */

hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/filter/LoadDetectFilter.java

@@ -53,7 +53,8 @@ public class LoadDetectFilter implements ContainerRequestFilter {
             "",
             "apis",
             "metrics",
-            "versions"
+            "versions",
+            "readiness"
     );
 
     // Call gc every 30+ seconds if memory is low and request frequently

hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/filter/PathFilter.java

@@ -57,6 +57,7 @@ public class PathFilter implements ContainerRequestFilter {
             "apis",
             "metrics",
             "versions",
+            "readiness",
             "health",
             "gremlin",
             "graphs/auth",

hugegraph-server/hugegraph-api/src/main/java/org/apache/hugegraph/api/profile/ReadinessAPI.java

@@ -0,0 +1,69 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with this
+ * work for additional information regarding copyright ownership. The ASF
+ * licenses this file to You under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.hugegraph.api.profile;
+
+import java.util.Map;
+
+import org.apache.hugegraph.api.API;
+import org.apache.hugegraph.config.HugeConfig;
+import org.apache.hugegraph.config.ServerOptions;
+import org.apache.hugegraph.core.GraphManager;
+import org.apache.hugegraph.util.JsonUtil;
+
+import com.codahale.metrics.annotation.Timed;
+
+import io.swagger.v3.oas.annotations.tags.Tag;
+import jakarta.annotation.security.PermitAll;
+import jakarta.inject.Singleton;
+import jakarta.ws.rs.GET;
+import jakarta.ws.rs.Path;
+import jakarta.ws.rs.Produces;
+import jakarta.ws.rs.core.Context;
+import jakarta.ws.rs.core.Response;
+
+/**
+ * Storage-aware readiness for Kubernetes and load balancers: on hstore 200
+ * while at least one known Store answers this server, 503 while none does
+ * (or, before any Store list is known, while PD does not answer); on hbase
+ * 200 while the cluster answers an admin call within the budget. Unauthenticated, like
+ * /versions, so that an httpGet probe needs no credential; the body carries
+ * no addresses and no raw exception text.
+ */
+@Path("readiness")
+@Singleton
+@Tag(name = "ReadinessAPI")
+public class ReadinessAPI extends API {
+
+    @GET
+    @Timed
+    @Produces(APPLICATION_JSON_WITH_CHARSET)
+    @PermitAll
+    public Response get(@Context GraphManager manager, @Context HugeConfig conf) {
+        Map<String, Object> body = StorageReadiness.check(
+                manager, conf.get(ServerOptions.READINESS_TIMEOUT),
+                conf.get(ServerOptions.READINESS_CACHE_TTL),
+                conf.get(ServerOptions.READINESS_MAX_WAITERS));
+        Response.Status status = StorageReadiness.isReady(body) ?
+                                 Response.Status.OK :
+                                 Response.Status.SERVICE_UNAVAILABLE;
+        return Response.status(status)
+                       .type(APPLICATION_JSON_WITH_CHARSET)
+                       .entity(JsonUtil.toJson(body))
+                       .build();
+    }
+}