java-sdk/docs/CLIENT.SSE.TRANSPORT.md

SSE Server Transport Implementation for MCP

Overview

The SseClientTransport class implements the Model Context Protocol (MCP) HTTP with SSE transport specification, providing a bidirectional communication channel between clients and servers. This implementation is part of the Spring AI MCP Core library and follows the MCP HTTP with SSE Transport Specification.

Key Features

• Bidirectional Communication:

  • Inbound: Receives messages through SSE connection
  • Outbound: Sends messages via HTTP POST requests

• Error Handling:

  • Automatic reconnection for transient failures
  • Graceful shutdown capabilities
  • Comprehensive error reporting

• Message Flow:

  1. Client establishes SSE connection to /sse endpoint
  2. Server sends 'endpoint' event with message URI
  3. Client uses provided URI for outbound messages

Technical Details

Transport Architecture

The transport consists of two main processing pipelines:

1. Inbound Processing:

   • Establishes SSE connection to /sse endpoint
   • Handles two types of events:
     • message: JSON-RPC protocol messages
     • endpoint: Server-provided URI for outbound messages
   • Implements automatic retry logic for connection failures

2. Outbound Processing:

   • Sends messages via HTTP POST to server-provided endpoint
   • Uses dedicated scheduler for sequential message processing
   • Handles JSON serialization and HTTP communication

Key Components

• WebClient: Handles HTTP communications for both SSE and POST requests
• ObjectMapper: Manages JSON serialization/deserialization
• Scheduler: Ensures sequential outbound message processing
• Message Endpoint Sink: Manages server-provided endpoint URI

Usage Example

// Create WebClient builder with base URL
WebClient.Builder webClientBuilder = WebClient.builder()
    .baseUrl("http://localhost:3001");

// Initialize transport
SseClientTransport transport = new SseClientTransport(webClientBuilder);

// Create a JSON-RPC request
JSONRPCRequest request = new JSONRPCRequest(
    McpSchema.JSONRPC_VERSION,
    "method-name",
    "request-id",
    Map.of("key", "value")
);

// Send message
transport.sendMessage(request)
    .subscribe(
        success -> System.out.println("Message sent successfully"),
        error -> System.err.println("Error sending message: " + error)
    );

// Graceful shutdown when done
transport.closeGracefully()
    .subscribe();

Implementation Details

Message Processing

1. Inbound Messages:

   • Received through SSE connection
   • Automatically deserialized from JSON
   • Routed to appropriate handlers based on event type

2. Outbound Messages:

   • Serialized to JSON
   • Sent via HTTP POST
   • Processed sequentially on dedicated thread

Error Handling

• Connection Failures:

  • Automatic retry for transient failures
  • Configurable retry policies
  • Graceful handling of permanent failures

• Message Processing Errors:

  • Comprehensive error reporting
  • Clean failure handling
  • Maintains system stability

Shutdown Process

The transport implements a graceful shutdown mechanism that:

1. Marks transport as closing
2. Disposes of all subscriptions
3. Cleans up schedulers
4. Completes message endpoint sink

Technical Specifications

• Protocol: HTTP with Server-Sent Events (SSE)
• Message Format: JSON-RPC 2.0
• Content Type: application/json for outbound, text/event-stream for inbound
• Endpoints:
  • SSE Connection: /sse
  • Message Endpoint: Dynamically provided by server

Dependencies

• Spring WebFlux for reactive HTTP client
• Jackson for JSON processing
• Project Reactor for reactive streams
• SLF4J for logging

Thread Safety

The implementation is thread-safe and designed for concurrent use:

• Inbound processing uses SSE event loop
• Outbound processing uses dedicated single-threaded scheduler
• State management uses thread-safe constructs

Testing

The implementation includes comprehensive tests covering:

• Message processing
• Error handling
• Connection management
• Shutdown procedures
• Edge cases and failure scenarios

For detailed test examples, see SseClientTransportTests.java.