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 default limit number for callback service instances
     *
     * @see #CALLBACK_INSTANCES_LIMIT_KEY
     */
    int DEFAULT_CALLBACK_INSTANCES = 1;
    String LOADBALANCE_KEY = "loadbalance";
    String DEFAULT_LOADBALANCE = "random";
    String RETRIES_KEY = "retries";
    String FORKS_KEY = "forks";
    int DEFAULT_RETRIES = 2;
    int DEFAULT_FAILBACK_TIMES = 3;
    String INTERFACES = "interfaces";
    String SSL_ENABLED_KEY = "ssl-enabled";
    String SERVICE_PATH_PREFIX = "service.path.prefix";
    String PROTOCOL_SERVER_SERVLET = "servlet";
    String PROTOCOL_SERVER = "server";
    String IPV6_KEY = "ipv6";
    /**
     * The parameter key for the class path of the ServiceNameMapping {@link Properties} file
     *
     * @since 2.7.8
     */
    String SERVICE_NAME_MAPPING_PROPERTIES_FILE_KEY = "service-name-mapping.properties-path";
    /**
     * The default class path of the ServiceNameMapping {@link Properties} file
     *
     * @since 2.7.8
     */
    String DEFAULT_SERVICE_NAME_MAPPING_PROPERTIES_PATH = "META-INF/dubbo/service-name-mapping.properties";
    String CLASS_DESERIALIZE_BLOCK_ALL = "dubbo.security.serialize.blockAllClassExceptAllow";