feat: optional session store (resumabillity support)

crates/rmcp/Cargo.toml

@@ -303,3 +303,12 @@ required-features = [
 ]
 path = "tests/test_streamable_http_stale_session.rs"
 
+[[test]]
+name = "test_streamable_http_session_store"
+required-features = [
+  "client",
+  "server",
+  "transport-streamable-http-client-reqwest",
+  "transport-streamable-http-server",
+]
+path = "tests/test_streamable_http_session_store.rs"

crates/rmcp/src/transport/streamable_http_server.rs

@@ -1,6 +1,6 @@
 pub mod session;
 #[cfg(all(feature = "transport-streamable-http-server", not(feature = "local")))]
 pub mod tower;
-pub use session::{SessionId, SessionManager};
+pub use session::{RestoreOutcome, SessionId, SessionManager, SessionRestoreMarker};
 #[cfg(all(feature = "transport-streamable-http-server", not(feature = "local")))]
 pub use tower::{StreamableHttpServerConfig, StreamableHttpService};

crates/rmcp/src/transport/streamable_http_server/session.rs

@@ -30,6 +30,39 @@ use crate::{
 
 pub mod local;
 pub mod never;
+pub mod store;
+
+pub use store::{SessionState, SessionStore, SessionStoreError};
+
+/// Extension marker inserted into the `initialize` request extensions during a
+/// session restore replay. Handlers can check for its presence to distinguish a
+/// cross-instance restore from a genuine client-initiated `initialize` request.
+///
+/// ```rust,ignore
+/// if req.extensions().get::<SessionRestoreMarker>().is_some() {
+///     // this is a restore replay, not a fresh client connection
+/// }
+/// ```
+#[derive(Debug, Clone)]
+pub struct SessionRestoreMarker {
+    pub id: SessionId,
+}
+
+/// The outcome of a [`SessionManager::restore_session`] call.
+#[derive(Debug)]
+pub enum RestoreOutcome<T> {
+    /// The session was just re-created from external state; the caller must
+    /// spawn an MCP handler against the returned transport and replay the
+    /// `initialize` handshake.
+    Restored(T),
+    /// The session was already present in memory (e.g. a concurrent request
+    /// already restored it). The caller should proceed as if `has_session`
+    /// had returned `true` — no further action is required.
+    AlreadyPresent,
+    /// This session manager does not support external-store restore.
+    /// The caller should fall through to the normal 404 response.
+    NotSupported,
+}
 
 /// Controls how MCP sessions are created, validated, and closed.
 ///
@@ -98,4 +131,22 @@ pub trait SessionManager: Send + Sync + 'static {
     ) -> impl Future<
         Output = Result<impl Stream<Item = ServerSseMessage> + Send + Sync + 'static, Self::Error>,
     > + Send;
+
+    /// Attempt to restore a previously-known session from external state,
+    /// creating a fresh in-memory session worker with the given `id`.
+    ///
+    /// See [`RestoreOutcome`] for the three possible results:
+    /// - [`RestoreOutcome::Restored`] — session re-created; caller must spawn
+    ///   an MCP handler and replay the `initialize` handshake.
+    /// - [`RestoreOutcome::AlreadyPresent`] — session is already in memory
+    ///   (e.g. a concurrent request restored it first); caller proceeds
+    ///   normally.
+    /// - [`RestoreOutcome::NotSupported`] (default) — this session manager
+    ///   does not support external-store restore; caller returns 404.
+    fn restore_session(
+        &self,
+        _id: SessionId,
+    ) -> impl Future<Output = Result<RestoreOutcome<Self::Transport>, Self::Error>> + Send {
+        futures::future::ready(Ok(RestoreOutcome::NotSupported))
+    }
 }

crates/rmcp/src/transport/streamable_http_server/session/local.rs

@@ -128,6 +128,20 @@ impl SessionManager for LocalSessionManager {
         handle.push_message(message, None).await?;
         Ok(())
     }
+
+    async fn restore_session(
+        &self,
+        id: SessionId,
+    ) -> Result<RestoreOutcome<Self::Transport>, Self::Error> {
+        let mut sessions = self.sessions.write().await;
+        if sessions.contains_key(&id) {
+            // A concurrent request already restored this session.
+            return Ok(RestoreOutcome::AlreadyPresent);
+        }
+        let (handle, worker) = create_local_session(id.clone(), self.session_config.clone());
+        sessions.insert(id, handle);
+        Ok(RestoreOutcome::Restored(WorkerTransport::spawn(worker)))
+    }
 }
 
 /// `<index>/request_id>`
@@ -179,7 +193,7 @@ impl std::str::FromStr for EventId {
     }
 }
 
-use super::{ServerSseMessage, SessionManager};
+use super::{RestoreOutcome, ServerSseMessage, SessionManager};
 
 struct CachedTx {
     tx: Sender<ServerSseMessage>,

crates/rmcp/src/transport/streamable_http_server/session/store.rs

@@ -0,0 +1,62 @@
+use crate::model::InitializeRequestParams;
+
+/// State persisted to an external store for cross-instance session recovery.
+///
+/// When a client reconnects to a different server instance, the new instance
+/// loads this state to transparently replay the `initialize` handshake without
+/// the client needing to re-initialize.
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct SessionState {
+    /// Parameters from the client's original `initialize` request.
+    pub initialize_params: InitializeRequestParams,
+}
+
+/// Type alias for boxed session store errors.
+pub type SessionStoreError = Box<dyn std::error::Error + Send + Sync + 'static>;
+
+/// Pluggable external session store for cross-instance recovery.
+///
+/// Implement this trait to back sessions with Redis, a database, or any
+/// key-value store. The simplest usage is to set
+/// `StreamableHttpServerConfig::session_store` to an `Arc<impl SessionStore>`.
+///
+/// # Example (in-memory, for testing)
+///
+/// ```rust,ignore
+/// use std::{collections::HashMap, sync::Arc};
+/// use tokio::sync::RwLock;
+/// use rmcp::transport::streamable_http_server::session::store::{
+///     SessionState, SessionStore, SessionStoreError,
+/// };
+///
+/// #[derive(Default)]
+/// struct InMemoryStore(Arc<RwLock<HashMap<String, SessionState>>>);
+///
+/// #[async_trait::async_trait]
+/// impl SessionStore for InMemoryStore {
+///     async fn load(&self, id: &str) -> Result<Option<SessionState>, SessionStoreError> {
+///         Ok(self.0.read().await.get(id).cloned())
+///     }
+///     async fn store(&self, id: &str, state: &SessionState) -> Result<(), SessionStoreError> {
+///         self.0.write().await.insert(id.to_owned(), state.clone());
+///         Ok(())
+///     }
+///     async fn delete(&self, id: &str) -> Result<(), SessionStoreError> {
+///         self.0.write().await.remove(id);
+///         Ok(())
+///     }
+/// }
+/// ```
+#[async_trait::async_trait]
+pub trait SessionStore: Send + Sync + 'static {
+    /// Load session state for the given `session_id`.
+    ///
+    /// Returns `Ok(None)` when no entry exists (i.e. session is unknown to the store).
+    async fn load(&self, session_id: &str) -> Result<Option<SessionState>, SessionStoreError>;
+
+    /// Persist session state for the given `session_id`.
+    async fn store(&self, session_id: &str, state: &SessionState) -> Result<(), SessionStoreError>;
+
+    /// Remove session state for the given `session_id`.
+    async fn delete(&self, session_id: &str) -> Result<(), SessionStoreError>;
+}