RoutingMetadata not working with Spring Boot 4.0.0 RSocket Server - "No handler for destination ''" error

[Kotlin-GDE]

Description

When using ktor-client-rsocket (version 0.20.0) to connect to a Spring Boot 4.0.0 RSocket server, the routing metadata is not being properly recognized by the server, resulting in the error:

io.rsocket.kotlin.RSocketError$ApplicationError: No handler for destination ''

The empty string in the error message indicates that the routing metadata is either not being sent or not being decoded properly by the Spring Boot server.

Environment

Client:

• rsocket-kotlin version: 0.20.0
• ktor-client-rsocket version: 0.20.0
• Kotlin version: 2.2.21
• Ktor version: 3.3.3
• JVM: 24

Server:

• Spring Boot version: 4.0.0
• spring-boot-starter-rsocket
• Transport: WebSocket
• Endpoint: ws://localhost:9000/rsocket

Expected Behavior

The client should successfully send routing metadata that Spring Boot's @MessageMapping can recognize and route to the appropriate handler.

Actual Behavior

Spring Boot server receives an empty destination string, causing routing to fail with:

io.rsocket.kotlin.RSocketError$ApplicationError: No handler for destination ''

Code to Reproduce

Spring Boot Server (Working)

@Controller
class RSocketController {
    @MessageMapping("salam")
    suspend fun salam(@Payload(required = true) name: String?): String {
        delay(1000)
        val actualName = name ?: "unknown"
        return "Hello, $actualName!"
    }
}

application.yml:

spring:
  rsocket:
    server:
      port: 9000
      transport: websocket
      mapping-path: /rsocket

Client Code (Not Working)

Attempt 1: Using metadata() with RoutingMetadata

val rsocket = client.rSocket("ws://localhost:9000/rsocket")

val payload = buildPayload {
    data("halim")
    metadata(RoutingMetadata("salam"))
}

val response = rsocket.requestResponse(payload)
// Error: No handler for destination ''

Attempt 2: Using compositeMetadata with RoutingMetadata constructor

val payload = buildPayload {
    data("halim")
    compositeMetadata {
        add(RoutingMetadata(tags = listOf("salam")))
    }
}
// Error: No handler for destination ''

Attempt 3: Using compositeMetadata with RoutingMetadata builder

val payload = buildPayload {
    data("halim")
    compositeMetadata {
        add(RoutingMetadata {
            +"salam"  // unary plus operator
        })
    }
}
// Error: No handler for destination ''

Attempt 4: Using route() in builder

val payload = buildPayload {
    data("halim")
    compositeMetadata {
        add(RoutingMetadata {
            route("salam")
        })
    }
}
// Error: No handler for destination ''

All attempts result in the same error.

Workaround

Switching to Spring's RSocketRequester (Java/Reactor-based client) works perfectly:

val requester = RSocketRequester.builder()
    .websocket(URI.create("ws://localhost:9000/rsocket"))

val response = requester
    .route("salam")
    .data("halim")
    .retrieveMono(String::class.java)
    .awaitSingle()

// Works! Response: "Hello, halim!"

Analysis

This suggests that:

1. The routing metadata encoding in rsocket-kotlin 0.20.0 may not be compatible with Spring Boot 4.0.0's RSocket implementation
2. There might be a protocol version mismatch
3. The composite metadata format might not match what Spring Boot expects

Questions

1. Is rsocket-kotlin 0.20.0 tested against Spring Boot 4.0.0?
2. Are there any known compatibility issues with Spring Boot's RSocket implementation?
3. Is there a correct way to encode routing metadata that's compatible with Spring Boot?

Additional Context

• The WebSocket connection itself works (no connection errors)
• The error occurs during the request-response interaction
• The same Spring Boot server works perfectly with Spring's RSocketRequester
• This appears to be a metadata encoding/decoding compatibility issue

Related Issues

Similar issue reported in: https://stackoverflow.com/questions/68778681/rsocket-android-spring-boot-back-end-routing-error-no-handler-for-destination

[Kotlin-GDE]

cc @ktorio/ktor-team - Mentioning the Ktor team for visibility since this issue involves the ktor-client-rsocket integration and WebSocket transport compatibility with Spring Boot 4.0.0.

While this is primarily an rsocket-kotlin issue, the Ktor team might have insights on the WebSocket transport layer or metadata handling that could help resolve this compatibility issue.

[rocketraman]

This appears to work for me with Spring Boot 3.x:

metadata(
  CompositeMetadata(
    RoutingMetadata(route),
  )
)

[rocketraman]

I also did this in connectionConfig:

payloadMimeType =
  PayloadMimeType(
    data = WellKnownMimeType.ApplicationJson, 
    metadata = WellKnownMimeType.MessageRSocketCompositeMetadata
  )