MCP Server can not support cluster

[yu199195]

Hi the this code ：

        if (this.isClosing) {
            return ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE)
                    .body("Server is shutting down");
        }

        if (request.param("sessionId").isEmpty()) {
            return ServerResponse.badRequest()
                    .body(new McpError("Session ID missing in message endpoint"));
        }

        String sessionId = request.param("sessionId").get();
        McpServerSession session = sessions.get(sessionId);

        if (session == null) {
            return ServerResponse.status(HttpStatus.NOT_FOUND)
                    .body(new McpError("Session not found: " + sessionId));
        }

when mcp server is cluster model. McpServerSession session = sessions.get(sessionId); is null

[lambochen]

When deploying in a cluster, I connect through the domain name. Since SSE is a long-lived, stateful protocol, the intermediate network proxy layer (such as Nginx) is configured with routing rules to ensure that the same client request is routed to the same MCP server, for example, IP Hash.

[lambochen]

'Streamable HTTP Transport' can solve or alleviate this problem

[yu199195]

@lambochen thanks～

[chensi2017]

java什么时候才能支持streamable http呢

[lvchengwei0901]

这个当前有解决方案吗？

[chemicL]

On one hand, this issue will not be solved for the legacy SSE transport type due to its' design. We can address the problem for Streamable HTTP in two ways:

• use the Stateless transport variants (losing the bi-directional nature)
• implement a persistent session storage

There is a drawback with the current Streamable HTTP specification though, which assumes the client first initializes with the server. In case of multiple replicas of the server, the replicas that have not seen the client might just fail, that's implementation specific, not addressed by the MCP Specification. There is a current effort to implement a Stateless-by-default Streamable HTTP specification, which would help resolve this.

I intend to keep the issue open as implementing the session storage will be crucial for the upcoming spec changes and we could perhaps relax the initialization requirement to make scalability with sessions possible.