Fixed - SearchDomainUnknownHostExceptionis thrown occasionally. #4576

redisson/src/main/java/org/redisson/config/Config.java

@@ -90,7 +90,7 @@ public class Config {
 
     private boolean useThreadClassLoader = true;
 
-    private AddressResolverGroupFactory addressResolverGroupFactory = new DnsAddressResolverGroupFactory();
+    private AddressResolverGroupFactory addressResolverGroupFactory = new SequentialDnsAddressResolverFactory();
 
     public Config() {
     }

redisson/src/main/java/org/redisson/connection/SequentialDnsAddressResolverFactory.java

@@ -0,0 +1,102 @@
+/**
+ * Copyright (c) 2013-2021 Nikita Koksharov
+ *
+ * Licensed 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.redisson.connection;
+
+import io.netty.channel.EventLoop;
+import io.netty.channel.socket.DatagramChannel;
+import io.netty.resolver.AddressResolver;
+import io.netty.resolver.AddressResolverGroup;
+import io.netty.resolver.InetSocketAddressResolver;
+import io.netty.resolver.NameResolver;
+import io.netty.resolver.dns.DnsAddressResolverGroup;
+import io.netty.resolver.dns.DnsServerAddressStreamProvider;
+import io.netty.util.concurrent.EventExecutor;
+import io.netty.util.concurrent.Promise;
+import org.redisson.pubsub.AsyncSemaphore;
+
+import java.net.InetAddress;
+import java.net.InetSocketAddress;
+import java.util.List;
+import java.util.concurrent.Callable;
+
+/**
+ * Address resolver which allows to control concurrency level of requests to DNS servers.
+ *
+ * @author Nikita Koksharov
+ *
+ */
+public class SequentialDnsAddressResolverFactory implements AddressResolverGroupFactory {
+
+    static class LimitedInetSocketAddressResolver extends InetSocketAddressResolver {
+
+        final AsyncSemaphore semaphore;
+
+        LimitedInetSocketAddressResolver(AsyncSemaphore semaphore, EventExecutor executor, NameResolver<InetAddress> nameResolver) {
+            super(executor, nameResolver);
+            this.semaphore = semaphore;
+        }
+
+        @Override
+        protected void doResolve(InetSocketAddress unresolvedAddress, Promise<InetSocketAddress> promise) throws Exception {
+            execute(() -> {
+                super.doResolve(unresolvedAddress, promise);
+                return null;
+            }, promise);
+        }
+
+        @Override
+        protected void doResolveAll(InetSocketAddress unresolvedAddress, Promise<List<InetSocketAddress>> promise) throws Exception {
+            execute(() -> {
+                super.doResolveAll(unresolvedAddress, promise);
+                return null;
+            }, promise);
+        }
+
+        private void execute(Callable<?> callable, Promise<?> promise) {
+            semaphore.acquire().thenAccept(s -> {
+                promise.addListener(r -> {
+                    semaphore.release();
+                });
+                try {
+                    callable.call();
+                } catch (Exception e) {
+                    promise.setFailure(e);
+                }
+            });
+        }
+    }
+
+    private final AsyncSemaphore asyncSemaphore;
+
+    public SequentialDnsAddressResolverFactory() {
+        this(8);
+    }
+
+    public SequentialDnsAddressResolverFactory(int concurrencyLevel) {
+        asyncSemaphore = new AsyncSemaphore(concurrencyLevel);
+    }
+
+    @Override
+    public AddressResolverGroup<InetSocketAddress> create(Class<? extends DatagramChannel> channelType, DnsServerAddressStreamProvider nameServerProvider) {
+        DnsAddressResolverGroup group = new DnsAddressResolverGroup(channelType, nameServerProvider) {
+            @Override
+            protected AddressResolver<InetSocketAddress> newAddressResolver(EventLoop eventLoop, NameResolver<InetAddress> resolver) throws Exception {
+                return new LimitedInetSocketAddressResolver(asyncSemaphore, eventLoop, resolver);
+            }
+        };
+        return group;
+    }
+}