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

public void encode(Channel channel, OutputStream output, Object message) throws IOException {
        throw new UnsupportedOperationException();
    }
    private void checkSerializationTypeFromRemote() {
    }
    @Override
    public Object decode(Channel channel, InputStream input) throws IOException {
        ObjectInput in = CodecSupport.getSerialization(serializationType)
            .deserialize(channel.getUrl(), input);
        this.put(SERIALIZATION_ID_KEY, serializationType);
        String dubboVersion = in.readUTF();
        request.setVersion(dubboVersion);
        setAttachment(DUBBO_VERSION_KEY, dubboVersion);
        String path = in.readUTF();
        setAttachment(PATH_KEY, path);
        String version = in.readUTF();
        setAttachment(VERSION_KEY, version);
        setMethodName(in.readUTF());
        String desc = in.readUTF();
        setParameterTypesDesc(desc);
        ClassLoader originClassLoader = Thread.currentThread().getContextClassLoader();
        try {
            if (Boolean.parseBoolean(System.getProperty(SERIALIZATION_SECURITY_CHECK_KEY, "true"))) {
                CodecSupport.checkSerialization(frameworkModel.getServiceRepository(), path, version, serializationType);
            }
            Object[] args = DubboCodec.EMPTY_OBJECT_ARRAY;
            Class<?>[] pts = DubboCodec.EMPTY_CLASS_ARRAY;
            if (desc.length() > 0) {
                FrameworkServiceRepository repository = frameworkModel.getServiceRepository();
                List<ProviderModel> providerModels = repository.lookupExportedServicesWithoutGroup(keyWithoutGroup(path, version));