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

* the Dubbo application prefers
     *
     * @since 2.7.6
     */
    String DUBBO_PREFERRED_NETWORK_INTERFACE = "dubbo.network.interface.preferred";
    @Deprecated
    String SHUTDOWN_WAIT_SECONDS_KEY = "dubbo.service.shutdown.wait.seconds";
    String SHUTDOWN_WAIT_KEY = "dubbo.service.shutdown.wait";
    String DUBBO_PROTOCOL = "dubbo";
    String DUBBO_LABELS = "dubbo.labels";
    String DUBBO_ENV_KEYS = "dubbo.env.keys";
    String CONFIG_CONFIGFILE_KEY = "config-file";
    String CONFIG_ENABLE_KEY = "highest-priority";
    String CONFIG_NAMESPACE_KEY = "namespace";
    String CHECK_KEY = "check";
    String BACKLOG_KEY = "backlog";
    String HEARTBEAT_EVENT = null;
    String MOCK_HEARTBEAT_EVENT = "H";
    String READONLY_EVENT = "R";
    String REFERENCE_FILTER_KEY = "reference.filter";
    String HEADER_FILTER_KEY = "header.filter";
    String INVOCATION_INTERCEPTOR_KEY = "invocation.interceptor";
    String INVOKER_LISTENER_KEY = "invoker.listener";
    String REGISTRY_PROTOCOL_LISTENER_KEY = "registry.protocol.listener";
    String DUBBO_VERSION_KEY = "dubbo";
    String TAG_KEY = "dubbo.tag";
    /**
     * To decide whether to make connection when the client is created
     */
    String LAZY_CONNECT_KEY = "lazy";