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

String STUB_EVENT_KEY = "dubbo.stub.event";
    String REFERENCE_INTERCEPTOR_KEY = "reference.interceptor";
    String SERVICE_FILTER_KEY = "service.filter";
    String EXPORTER_LISTENER_KEY = "exporter.listener";
    /**
     * After simplify the registry, should add some parameter individually for provider.
     *
     * @since 2.7.0
     */
    String EXTRA_KEYS_KEY = "extra-keys";
    String GENERIC_SERIALIZATION_NATIVE_JAVA = "nativejava";
    String GENERIC_SERIALIZATION_GSON = "gson";
    String GENERIC_SERIALIZATION_DEFAULT = "true";
    String GENERIC_SERIALIZATION_BEAN = "bean";
    String GENERIC_RAW_RETURN = "raw.return";
    String GENERIC_SERIALIZATION_PROTOBUF = "protobuf-json";
    String GENERIC_WITH_CLZ_KEY = "generic.include.class";
    /**
     * Whether to cache locally, default is true
     */
    String REGISTRY_LOCAL_FILE_CACHE_ENABLED = "file-cache";
    String METADATA_INFO_CACHE_EXPIRE_KEY = "metadata-info-cache.expire";
    int DEFAULT_METADATA_INFO_CACHE_EXPIRE = 10 * 60 * 1000;
    String METADATA_INFO_CACHE_SIZE_KEY = "metadata-info-cache.size";
    int DEFAULT_METADATA_INFO_CACHE_SIZE = 16;
    /**
     * The limit of callback service instances for one interface on every client
     */
    String CALLBACK_INSTANCES_LIMIT_KEY = "callbacks";
    /**