We are working on a dubbo gateway and need to support mutual access between different network clusters. This should be a common scenario. This requires dubbo to do two things for us:

1. Dynamically modify the ip and change the access of the local cluster to remote. This extension can be done, although there are some bugs, https://dubbo.apache.org/zh/docs3-v2/java-sdk/advanced-features-and-usage/service/specify-ip/

2. Forward the dubbo request. When the gateway receives the access request from cluster A, it should forward it to cluster B (now it returns an exception).

  From the code execution process.
  current:
  gateway-dubbo-provider receives the request -> parses -> judges whether the request interface has a corresponding cache URL -> no -> throws IOException -> netty returns

  ```java
  void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException {
          Response res = new Response(req.getId(), req.getVersion());
          if (req.isBroken()) {
              Object data = req.getData();
              String msg;
              if (data == null) {
                  msg = null;
              } else if (data instanceof Throwable) {
                  msg = StringUtils.toString((Throwable) data);
              } else {
                  msg = data.toString();
              }
              res.setErrorMessage("Fail to decode request due to: " + msg);
              res.setStatus(Response.BAD_REQUEST);

              channel.send(res);
              return;
          }
  }
  ```

  ​

  expect:
  gateway-dubbo-provider receives the request -> parses -> judges whether the request interface has a corresponding cache URL -> no -> obtains the exception handler through spi (the request can be forwarded here) -> netty returns

```java
  void handleRequest(final ExchangeChannel channel, Request req) throws RemotingException {
          Response res = new Response(req.getId(), req.getVersion());
          if (req.isBroken()) {
              Object data = req.getData();
           //expert begin
          List<ExceptionHandler> list = spiExtensions.listByType(ExceptionHandler); 
          list.forEach(handler.handle(path,version))
           //expert end
              String msg;
              if (data == null) {
                  msg = null;
              } else if (data instanceof Throwable) {
                  msg = StringUtils.toString((Throwable) data);
              } else {
                  msg = data.toString();
              }
              res.setErrorMessage("Fail to decode request due to: " + msg);
              res.setStatus(Response.BAD_REQUEST);

              channel.send(res);
              return;
          }
  }
```


Since dubbo.extension provides dynamic ip replacement, then we should consider whether the target ip can be accepted and processed. I think this exception handler is an extension function of the dubbo.extension package.
We can provide ExceptionHandler similar to springboot, which can be more flexible

import org.apache.dubbo.remoting.Channel;
import org.apache.dubbo.remoting.Codec;
import org.apache.dubbo.remoting.Decodeable;
import org.apache.dubbo.remoting.exchange.Request;
import org.apache.dubbo.remoting.transport.CodecSupport;
import org.apache.dubbo.rpc.RpcInvocation;
import org.apache.dubbo.rpc.model.ApplicationModel;
import org.apache.dubbo.rpc.model.FrameworkModel;
import org.apache.dubbo.rpc.model.FrameworkServiceRepository;
import org.apache.dubbo.rpc.model.MethodDescriptor;
import org.apache.dubbo.rpc.model.ModuleModel;
import org.apache.dubbo.rpc.model.ProviderModel;
import org.apache.dubbo.rpc.model.ServiceDescriptor;
import org.apache.dubbo.rpc.support.RpcUtils;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.List;
import java.util.Map;
import java.util.function.Supplier;
import static org.apache.dubbo.common.BaseServiceMetadata.keyWithoutGroup;
import static org.apache.dubbo.common.URL.buildKey;
import static org.apache.dubbo.common.constants.CommonConstants.DUBBO_VERSION_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.GROUP_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.PATH_KEY;
import static org.apache.dubbo.common.constants.CommonConstants.VERSION_KEY;
import static org.apache.dubbo.common.constants.LoggerCodeConstants.PROTOCOL_FAILED_DECODE;
import static org.apache.dubbo.rpc.Constants.SERIALIZATION_ID_KEY;
import static org.apache.dubbo.rpc.Constants.SERIALIZATION_SECURITY_CHECK_KEY;
public class DecodeableRpcInvocation extends RpcInvocation implements Codec, Decodeable {