Alanxtl opened a new issue, #3359:
URL: https://github.com/apache/dubbo-go/issues/3359
## Title
REST application-level service discovery requires importing Dubbo protocol
to fetch metadata
## Problem
A REST consumer using application-level service discovery fails unless it
also imports the Dubbo protocol extension:
```go
_ "dubbo.apache.org/dubbo-go/v3/protocol/dubbo"
```
Without this import, the consumer panics:
```text
panic: protocol for [dubbo] is not existing
```
This is surprising because the user-facing service protocol is REST. The
Dubbo protocol is only used internally by the service discovery path to call
`org.apache.dubbo.metadata.MetadataService`.
## Minimal Reproduction
There is currently no ready-made REST service-discovery sample in the
repository, so this was reproduced with a small local REST provider/consumer
sample.
### 1. Shared REST mapping
Create a REST service with one method:
- interface: `org.apache.dubbo.samples.rest.GreetingService`
- method: `GetGreeting`
- REST path: `POST /api/v1/users/{userID}/greeting`
- args:
- arg 0 -> path param `userID`
- arg 1 -> query param `name`
- arg 2 -> header `X-Trace-ID`
- arg 3 -> JSON body
Both provider and consumer install the same REST method config through:
```go
restconfig.SetRestProviderServiceConfigMap(...)
restconfig.SetRestConsumerServiceConfigMap(...)
```
### 2. Provider setup
The provider is configured as a REST service and uses application-level
registration:
```go
ins, err := dubbo.NewInstance(
dubbo.WithName("dubbo_rest_basic_server"),
dubbo.WithProtocol(
protocol.WithREST(),
protocol.WithIp("127.0.0.1"),
protocol.WithPort(20080),
),
dubbo.WithRegistry(
registry.WithZookeeper(),
registry.WithAddress("127.0.0.1:2181"),
registry.WithoutUseAsConfigCenter(),
registry.WithRegisterService(),
),
)
```
Then register the REST service:
```go
srv, _ := ins.NewServer()
err = srv.RegisterService(
&GreetingProvider{},
dubboserver.WithInterface("org.apache.dubbo.samples.rest.GreetingService"),
)
err = srv.Serve()
```
Required provider imports include REST protocol, registry protocol, service
discovery, metadata mapping/report, and ZooKeeper registry.
### 3. Consumer setup that fails
The consumer also uses application-level service discovery:
```go
ins, err := dubbo.NewInstance(
dubbo.WithName("dubbo_rest_basic_client"),
dubbo.WithRegistry(
registry.WithZookeeper(),
registry.WithAddress("127.0.0.1:2181"),
registry.WithoutUseAsConfigCenter(),
registry.WithRegisterService(),
),
)
cli, err := ins.NewClient(client.WithClientNoCheck())
conn, err := cli.Dial(
"org.apache.dubbo.samples.rest.GreetingService",
client.WithProtocol(constant.RESTProtocol),
)
```
The consumer imports REST protocol and service discovery related packages,
but intentionally does **not** import Dubbo protocol:
```go
_ "dubbo.apache.org/dubbo-go/v3/protocol/rest"
_ "dubbo.apache.org/dubbo-go/v3/registry/protocol"
_ "dubbo.apache.org/dubbo-go/v3/registry/servicediscovery"
_ "dubbo.apache.org/dubbo-go/v3/registry/directory"
_ "dubbo.apache.org/dubbo-go/v3/registry/zookeeper"
_ "dubbo.apache.org/dubbo-go/v3/metadata/mapping/metadata"
_ "dubbo.apache.org/dubbo-go/v3/metadata/report/zookeeper"
```
Run:
```bash
go run ./server
go run ./client
```
The consumer fails with:
```text
panic: protocol for [dubbo] is not existing
```
### 4. Workaround
Adding this import to the REST consumer fixes the issue:
```go
_ "dubbo.apache.org/dubbo-go/v3/protocol/dubbo"
```
After that, the REST call succeeds. The consumer discovers the provider
application, fetches metadata through:
```text
dubbo://127.0.0.1:<metadata-port>/org.apache.dubbo.metadata.MetadataService
```
and reconstructs a REST provider URL like:
```text
rest://127.0.0.1:20080/org.apache.dubbo.samples.rest.GreetingService
```
## Expected Behavior
A REST consumer should not need to manually import Dubbo protocol just
because it uses application-level service discovery.
If the metadata service internally uses Dubbo protocol, that dependency
should be hidden by the service discovery / metadata module, or the error
should clearly explain the required import.
## Actual Behavior
The REST consumer fails at runtime because service discovery tries to call
the metadata service through `dubbo://...`, but the Dubbo protocol extension is
not registered.
## Why This Matters
This leaks an internal metadata transport detail into user code. From the
user perspective, the configured service protocol is REST, the registry is
ZooKeeper/Nacos, and the registry type is application-level service discovery.
Requiring a Dubbo protocol import is non-obvious and easy to miss.
## Suggested Fix
One of the following would improve the behavior:
1. service discovery automatically registers the protocol needed for
metadata service calls;
2. provide a documented import bundle for application-level service
discovery dependencies;
3. return a clearer error such as: `metadata service uses dubbo protocol,
please import dubbo.apache.org/dubbo-go/v3/protocol/dubbo`.
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