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

int DEFAULT_TIMEOUT = 1000;
    String SESSION_KEY = "session";
    
    
    String TIMEOUT_ATTACHMENT_KEY = "_TO";
    String TIMEOUT_ATTACHMENT_KEY_LOWER = "_to";
    String TIME_COUNTDOWN_KEY = "timeout-countdown";
    String ENABLE_TIMEOUT_COUNTDOWN_KEY = "enable-timeout-countdown";
    String REMOVE_VALUE_PREFIX = "-";
    String PROPERTIES_CHAR_SEPARATOR = "-";
    String UNDERLINE_SEPARATOR = "_";
    String SEPARATOR_REGEX = "_|-";
    String GROUP_CHAR_SEPARATOR = ":";
    String HIDE_KEY_PREFIX = ".";
    String DOT_REGEX = "\\.";
    String DEFAULT_KEY_PREFIX = "default.";
    String DEFAULT_KEY = "default";
    String PREFERRED_KEY = "preferred";
    /**
     * Default timeout value in milliseconds for server shutdown
     */
    int DEFAULT_SERVER_SHUTDOWN_TIMEOUT = 10000;
    String SIDE_KEY = "side";
    String PROVIDER_SIDE = "provider";
    String CONSUMER_SIDE = "consumer";
    String ANYHOST_KEY = "anyhost";
    String ANYHOST_VALUE = "0.0.0.0";
    String LOCALHOST_KEY = "localhost";
    String LOCALHOST_VALUE = "127.0.0.1";
    String METHODS_KEY = "methods";