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_DELAY_EXECUTE_TIMES = 10;
    /**
     * Url merge processor key
     */
    String URL_MERGE_PROCESSOR_KEY = "url-merge-processor";
    /**
     * use native image to compile dubbo's identifier
     */
    String NATIVE = "native";
    String DUBBO_MONITOR_ADDRESS = "dubbo.monitor.address";
    String SERVICE_NAME_MAPPING_KEY = "service-name-mapping";
    String SCOPE_MODEL = "scopeModel";
    String SERVICE_MODEL = "serviceModel";
    /**
     * The property name for {@link NetworkInterface#getDisplayName() the name of network interface} that
     * the Dubbo application will be ignored
     *
     * @since 2.7.6
     */
    String DUBBO_NETWORK_IGNORED_INTERFACE = "dubbo.network.interface.ignored";
    String OS_NAME_KEY = "os.name";
    String OS_LINUX_PREFIX = "linux";
    String OS_WIN_PREFIX = "win";
    String RECONNECT_TASK_TRY_COUNT = "dubbo.reconnect.reconnectTaskTryCount";
    int DEFAULT_RECONNECT_TASK_TRY_COUNT = 10;
    String RECONNECT_TASK_PERIOD = "dubbo.reconnect.reconnectTaskPeriod";
    int DEFAULT_RECONNECT_TASK_PERIOD = 1000;
    String RESELECT_COUNT = "dubbo.reselect.count";
    int DEFAULT_RESELECT_COUNT = 10;
    String ENABLE_CONNECTIVITY_VALIDATION = "dubbo.connectivity.validation";