ZOOKEEPER-4956: Provide a HostProvider that uses DNS SRV record for dynamic server discovery

zookeeper-docs/src/main/resources/markdown/zookeeperAdmin.md

@@ -1315,6 +1315,14 @@ of servers -- that is, when deploying clusters of servers.
     leader election. If you want to test multiple servers on a single machine, then
     different ports can be used for each server.
 
+* *hostProvider.dnsSrvRefreshIntervalMs* :
+    (Java system property: **zookeeper.hostProvider.dnsSrvRefreshIntervalMs**)
+    **New in 3.10.0:**
+    The refresh interval in milliseconds for DNS SRV record lookups when using DnsSrvHostProvider.
+    This property controls how frequently the DNS SRV records are queried to update the server list.
+    A value of 0 disables periodic refresh. 
+
+    The default value is 60000 (60 seconds).
 
     <a name="id_multi_address"></a>
     Since ZooKeeper 3.6.0 it is possible to specify **multiple addresses** for each

zookeeper-server/pom.xml

@@ -190,6 +190,11 @@
       <artifactId>commons-io</artifactId>
       <scope>compile</scope>
     </dependency>
+    <dependency>
+      <groupId>dnsjava</groupId>
+      <artifactId>dnsjava</artifactId>
+      <version>3.5.1</version>
+    </dependency>
   </dependencies>
 
   <build>

zookeeper-server/src/main/java/org/apache/zookeeper/client/ConnectionType.java

@@ -0,0 +1,52 @@
+/*
+ * 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.zookeeper.client;
+
+import org.apache.yetus.audience.InterfaceAudience;
+
+/**
+ * Represents the type of connection resolution used by ZooKeeper clients.
+ * This is an internal enum used for connection string and provider validation.
+ */
+@InterfaceAudience.Private
+public enum ConnectionType {
+    /**
+     * DNS SRV record-based service discovery.
+     */
+    DNS_SRV("DNS SRV"),
+
+    /**
+     * Traditional host:port static server list.
+     */
+    HOST_PORT("Host:Port");
+
+    private final String name;
+
+    ConnectionType(String name) {
+        this.name = name;
+    }
+
+    /**
+     * Returns the name for this connection type.
+     * @return the name
+     */
+    public String getName() {
+        return name;
+    }
+}