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 static final String NAME = "dubbo";
    public static final String DUBBO_VERSION = Version.getProtocolVersion();
    public static final byte RESPONSE_WITH_EXCEPTION = 0;
    public static final byte RESPONSE_VALUE = 1;
    public static final byte RESPONSE_NULL_VALUE = 2;
    public static final byte RESPONSE_WITH_EXCEPTION_WITH_ATTACHMENTS = 3;
    public static final byte RESPONSE_VALUE_WITH_ATTACHMENTS = 4;
    public static final byte RESPONSE_NULL_VALUE_WITH_ATTACHMENTS = 5;
    public static final Object[] EMPTY_OBJECT_ARRAY = new Object[0];
    public static final Class<?>[] EMPTY_CLASS_ARRAY = new Class<?>[0];
    private static final ErrorTypeAwareLogger log = LoggerFactory.getErrorTypeAwareLogger(DubboCodec.class);
    private static final AtomicBoolean decodeInUserThreadLogged = new AtomicBoolean(false);
    private final CallbackServiceCodec callbackServiceCodec;
    private final FrameworkModel frameworkModel;
    public DubboCodec(FrameworkModel frameworkModel) {
        this.frameworkModel = frameworkModel;
        callbackServiceCodec = new CallbackServiceCodec(frameworkModel);
    }
    @Override
    protected Object decodeBody(Channel channel, InputStream is, byte[] header) throws IOException {
        byte flag = header[2], proto = (byte) (flag & SERIALIZATION_MASK);
        
        long id = Bytes.bytes2long(header, 4);
        if ((flag & FLAG_REQUEST) == 0) {
            
            Response res = new Response(id);
            if ((flag & FLAG_EVENT) != 0) {
                res.setEvent(true);
            }