feat: add distributed SSE event store

Author: DaleSeo

crates/rmcp/Cargo.toml

@@ -414,6 +414,16 @@ required-features = [
 ]
 path = "tests/test_streamable_http_session_store.rs"
 
+[[test]]
+name = "test_streamable_http_event_store"
+required-features = [
+  "client",
+  "server",
+  "transport-streamable-http-client-reqwest",
+  "transport-streamable-http-server",
+]
+path = "tests/test_streamable_http_event_store.rs"
+
 [[test]]
 name = "test_streamable_http_connection_reuse"
 required-features = [

crates/rmcp/src/transport/common/server_side_http.rs

@@ -119,6 +119,15 @@ impl ServerSseMessage {
             retry: Some(retry),
         }
     }
+
+    /// Create a retry hint without changing the client's last event ID.
+    pub fn retry(retry: Duration) -> Self {
+        Self {
+            event_id: None,
+            message: None,
+            retry: Some(retry),
+        }
+    }
 }
 
 pub(crate) fn sse_stream_response(

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

@@ -32,7 +32,10 @@ pub mod local;
 pub mod never;
 pub mod store;
 
-pub use store::{SessionState, SessionStore, SessionStoreError};
+pub use store::{
+    EventId, EventStore, EventStoreError, EventStream, SessionState, SessionStore,
+    SessionStoreError, StreamId,
+};
 
 /// Extension marker inserted into the `initialize` request extensions during a
 /// session restore replay. Handlers can check for its presence to distinguish a
@@ -151,4 +154,9 @@ pub trait SessionManager: Send + Sync + 'static {
     ) -> impl Future<Output = Result<RestoreOutcome<Self::Transport>, Self::Error>> + Send {
         futures::future::ready(Ok(RestoreOutcome::NotSupported))
     }
+
+    /// Return whether event IDs are assigned by an external event store.
+    fn uses_external_event_store(&self) -> bool {
+        false
+    }
 }