Servlet transports block Tomcat thread

Abierto

#859 abierto el 7/3/2026 1 comentario 1 reacción 0 asignados Ver en GitHub

Nadie ha tomado este issue todavía.

Evaluación

Dificultad: 4/5
Tiempo estimado: 3-5 días
Aptitud para principiantes: 48/100

Tipo de issue: Error
Claridad: Bastante claro
Estado de actividad: Tranquilo
Stack tecnológico: java
Área: api, backend

Línea de trabajo

Comienza con HttpServletStreamableServerTransportProvider, HttpServletSseServerTransportProvider y HttpServletStatelessServerTransport, rastreando dónde las solicitudes entrantes llaman a .block() y cómo se utiliza AsyncContext. Compara las rutas de SyncToolSpecification y AsyncToolSpecification y, después, verifica que los hilos de las solicitudes se liberan mientras la operación reactiva continúa y se completa correctamente.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

needs confirmation

Bug Description

All Servlet-based transports in the SDK (e.g., HttpServletStreamableServerTransportProvider, HttpServletSseServerTransportProvider, HttpServletStatelessServerTransport) call .block() on the Tomcat request thread when processing incoming HTTP requests.

Although AsyncContext is used to hold connections for SSE/streaming, the Tomcat thread remains parked until tool execution completes. As a result, the reactive/async architecture does not provide any real thread efficiency: the request thread stays blocked for the entire duration of the tool call.

With SyncToolSpecification and immediateExecution=false, the tool handler is executed on a separate thread (typically from the boundedElastic pool). This prevents blocking logic from running on the Tomcat thread itself, but the request thread still remains blocked waiting for completion.

With AsyncToolSpecification(via McpAsyncServer), handlers are expected to return fully asynchronous operations (e.g., non-blocking I/O). However, because the transport calls .block(), the HTTP request handling remains synchronous. The handler is not automatically scheduled on another thread unless the developer explicitly does so.

As a result, both SyncToolSpecification and AsyncToolSpecification ultimately block the Tomcat request thread until completion.

I’m not sure if this behavior is intentional. Based on the design of McpAsyncServer, I would expect the transport layer to release the Tomcat thread immediately and allow the reactive pipeline to complete asynchronously. (AsyncContext + subscribe).