plusmancn opened a new issue #8698:
URL: https://github.com/apache/dubbo/issues/8698
## The logic of preferredProtocol was not user-friendly, we maybe should
optimize it.
### What's the problem?
When we only use the `triple` protocol in dubbo with setting provider's
register-mode as `instance`, the consumer can't find the service.
The source is in
`org.apache.dubbo.config.metadata.ServiceInstanceHostPortCustomizer`, let's
have a look:
```java
@Override
public void customize(ServiceInstance serviceInstance) {
String host = null;
int port = -1;
// ......
Set<URL> urls = writableMetadataService.getExportedServiceURLs();
if (CollectionUtils.isNotEmpty(urls)) {
ApplicationModel applicationModel =
serviceInstance.getApplicationModel();
String preferredProtocol =
applicationModel.getCurrentConfig().getProtocol();
// The default value of preferredProtocol is dubbo.
if (preferredProtocol != null) {
for (URL exportedURL : urls) {
// The protocol of exportedURL is tri
// because of tri != dubbo, the host and port will not be
assigned values.
if (preferredProtocol.equals(exportedURL.getProtocol())) {
host = exportedURL.getHost();
port = exportedURL.getPort();
break;
}
}
} else {
URL url = urls.iterator().next();
host = url.getHost();
port = url.getPort();
}
// If not the port of instace > 0, the application instance will not
be registered to RegistryCenter(like Nacos, Zookeeper).
// As a result, the consumer can't find the avaliabe providers when
our settging is registry-mode=instance
if (serviceInstance instanceof DefaultServiceInstance) {
DefaultServiceInstance instance = (DefaultServiceInstance)
serviceInstance;
instance.setHost(host);
instance.setPort(port);
}
}
}
```
For the moment, if you want both consumer and provider to force using
`Application Discovery` in tri protocol, just change one line config of
provider as follows:
```yaml
dubbo:
application:
metadata-type: remote
register-mode: instance
protocol: tri <==== HERE, set preferredProtocol as tri
```
### The possible solution
change the default `application-protocol` as `TRIPLE` in the future
> Why not now? Because most users are still trying to migrate from 2.0 to 3.0
```java
// => org.apache.dubbo.config.ApplicationConfig#checkDefault
protected void checkDefault() {
super.checkDefault();
if (protocol == null) {
protocol = DUBBO; // <======= HERE
}
// ......
}
```
In `ServiceInstanceHostPortCustomizer`, If the default protocol not exists,
just try to get the first protocol and log an error log.
Related to #8666
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