Add an axum sse example (#12)

* add axum sse example

* fix: make the sse axum folder name correct

* feat: add common modules for Axum SSE example

This commit adds supporting modules for the Axum Server-Sent Events (SSE) example:
- Added `counter.rs` with a simple counter router implementation
- Added `jsonrpc_frame_codec.rs` for decoding JSON-RPC frames
- Created a `mod.rs` to expose these modules
- Removed the previous SSE example configuration from Cargo.toml

* move examples to root

* nit: remove extra newline here

Author: 4t145

crates/mcp-server/src/lib.rs

@@ -142,7 +142,8 @@ where
 
         tracing::info!("Server started");
         while let Some(msg_result) = transport.next().await {
-            let _span = tracing::span!(tracing::Level::INFO, "message_processing").entered();
+            let _span = tracing::span!(tracing::Level::INFO, "message_processing");
+            let _enter = _span.enter();
             match msg_result {
                 Ok(msg) => {
                     match msg {

examples/servers/Cargo.toml

@@ -17,6 +17,15 @@ tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tracing-appender = "0.2"
 futures = "0.3"
 
+[dev-dependencies]
+axum = { version = "0.8", features = ["macros"] }
+tokio-util = { version = "0.7", features = ["io", "codec"]}
+rand = { version = "0.8" }
+
 [[example]]
 name = "counter-server"
 path = "src/counter_server.rs"
+
+[[example]]
+name = "axum"
+path = "src/axum.rs"

examples/servers/src/axum.rs

@@ -0,0 +1,155 @@
+use axum::{
+    body::Body,
+    extract::{Query, State},
+    http::StatusCode,
+    response::sse::{Event, Sse},
+    routing::get,
+    Router,
+};
+use futures::{stream::Stream, StreamExt, TryStreamExt};
+use mcp_server::{ByteTransport, Server};
+use std::collections::HashMap;
+use tokio_util::codec::FramedRead;
+use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
+
+use anyhow::Result;
+use mcp_server::router::RouterService;
+use std::sync::Arc;
+use tokio::{
+    io::{self, AsyncWriteExt},
+    sync::Mutex,
+};
+use tracing_subscriber::{self};
+mod common;
+use common::counter;
+
+type C2SWriter = Arc<Mutex<io::WriteHalf<io::SimplexStream>>>;
+type SessionId = Arc<str>;
+
+const BIND_ADDRESS: &str = "127.0.0.1:8000";
+
+#[derive(Clone, Default)]
+pub struct App {
+    txs: Arc<tokio::sync::RwLock<HashMap<SessionId, C2SWriter>>>,
+}
+
+impl App {
+    pub fn new() -> Self {
+        Self {
+            txs: Default::default(),
+        }
+    }
+    pub fn router(&self) -> Router {
+        Router::new()
+            .route("/sse", get(sse_handler).post(post_event_handler))
+            .with_state(self.clone())
+    }
+}
+
+fn session_id() -> SessionId {
+    let id = format!("{:016x}", rand::random::<u128>());
+    Arc::from(id)
+}
+
+#[derive(Debug, serde::Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct PostEventQuery {
+    pub session_id: String,
+}
+
+async fn post_event_handler(
+    State(app): State<App>,
+    Query(PostEventQuery { session_id }): Query<PostEventQuery>,
+    body: Body,
+) -> Result<StatusCode, StatusCode> {
+    const BODY_BYTES_LIMIT: usize = 1 << 22;
+    let write_stream = {
+        let rg = app.txs.read().await;
+        rg.get(session_id.as_str())
+            .ok_or(StatusCode::NOT_FOUND)?
+            .clone()
+    };
+    let mut write_stream = write_stream.lock().await;
+    let mut body = body.into_data_stream();
+    if let (_, Some(size)) = body.size_hint() {
+        if size > BODY_BYTES_LIMIT {
+            return Err(StatusCode::PAYLOAD_TOO_LARGE);
+        }
+    }
+    // calculate the body size
+    let mut size = 0;
+    while let Some(chunk) = body.next().await {
+        let Ok(chunk) = chunk else {
+            return Err(StatusCode::BAD_REQUEST);
+        };
+        size += chunk.len();
+        if size > BODY_BYTES_LIMIT {
+            return Err(StatusCode::PAYLOAD_TOO_LARGE);
+        }
+        write_stream
+            .write_all(&chunk)
+            .await
+            .map_err(|_| StatusCode::INTERNAL_SERVER_ERROR)?;
+    }
+    write_stream
+        .write_u8(b'\n')
+        .await
+        .map_err(|_| StatusCode::INTERNAL_SERVER_ERROR)?;
+    Ok(StatusCode::ACCEPTED)
+}
+
+async fn sse_handler(State(app): State<App>) -> Sse<impl Stream<Item = Result<Event, io::Error>>> {
+    // it's 4KB
+    const BUFFER_SIZE: usize = 1 << 12;
+    let session = session_id();
+    tracing::info!(%session, "sse connection");
+    let (c2s_read, c2s_write) = tokio::io::simplex(BUFFER_SIZE);
+    let (s2c_read, s2c_write) = tokio::io::simplex(BUFFER_SIZE);
+    app.txs
+        .write()
+        .await
+        .insert(session.clone(), Arc::new(Mutex::new(c2s_write)));
+    {
+        let session = session.clone();
+        tokio::spawn(async move {
+            let router = RouterService(counter::CounterRouter::new());
+            let server = Server::new(router);
+            let bytes_transport = ByteTransport::new(c2s_read, s2c_write);
+            let _result = server
+                .run(bytes_transport)
+                .await
+                .inspect_err(|e| tracing::error!(?e, "server run error"));
+            app.txs.write().await.remove(&session);
+        });
+    }
+
+    let stream = futures::stream::once(futures::future::ok(
+        Event::default()
+            .event("endpoint")
+            .data(format!("?sessionId={session}")),
+    ))
+    .chain(
+        FramedRead::new(s2c_read, common::jsonrpc_frame_codec::JsonRpcFrameCodec)
+            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))
+            .and_then(move |bytes| match std::str::from_utf8(&bytes) {
+                Ok(message) => futures::future::ok(Event::default().event("message").data(message)),
+                Err(e) => futures::future::err(io::Error::new(io::ErrorKind::InvalidData, e)),
+            }),
+    );
+    Sse::new(stream)
+}
+
+#[tokio::main]
+async fn main() -> io::Result<()> {
+    tracing_subscriber::registry()
+        .with(
+            tracing_subscriber::EnvFilter::try_from_default_env()
+                .unwrap_or_else(|_| format!("info,{}=debug", env!("CARGO_CRATE_NAME")).into()),
+        )
+        .with(tracing_subscriber::fmt::layer())
+        .init();
+    let listener = tokio::net::TcpListener::bind(BIND_ADDRESS).await?;
+
+    tracing::debug!("listening on {}", listener.local_addr()?);
+    axum::serve(listener, App::new().router()).await
+}

examples/servers/src/common/counter.rs

@@ -0,0 +1,184 @@
+use std::{future::Future, pin::Pin, sync::Arc};
+
+use mcp_core::{
+    handler::{PromptError, ResourceError},
+    prompt::{Prompt, PromptArgument},
+    protocol::ServerCapabilities,
+    Content, Resource, Tool, ToolError,
+};
+use mcp_server::router::CapabilitiesBuilder;
+use serde_json::Value;
+use tokio::sync::Mutex;
+
+#[derive(Clone)]
+pub struct CounterRouter {
+    counter: Arc<Mutex<i32>>,
+}
+
+impl CounterRouter {
+    pub fn new() -> Self {
+        Self {
+            counter: Arc::new(Mutex::new(0)),
+        }
+    }
+
+    async fn increment(&self) -> Result<i32, ToolError> {
+        let mut counter = self.counter.lock().await;
+        *counter += 1;
+        Ok(*counter)
+    }
+
+    async fn decrement(&self) -> Result<i32, ToolError> {
+        let mut counter = self.counter.lock().await;
+        *counter -= 1;
+        Ok(*counter)
+    }
+
+    async fn get_value(&self) -> Result<i32, ToolError> {
+        let counter = self.counter.lock().await;
+        Ok(*counter)
+    }
+
+    fn _create_resource_text(&self, uri: &str, name: &str) -> Resource {
+        Resource::new(uri, Some("text/plain".to_string()), Some(name.to_string())).unwrap()
+    }
+}
+
+impl mcp_server::Router for CounterRouter {
+    fn name(&self) -> String {
+        "counter".to_string()
+    }
+
+    fn instructions(&self) -> String {
+        "This server provides a counter tool that can increment and decrement values. The counter starts at 0 and can be modified using the 'increment' and 'decrement' tools. Use 'get_value' to check the current count.".to_string()
+    }
+
+    fn capabilities(&self) -> ServerCapabilities {
+        CapabilitiesBuilder::new()
+            .with_tools(false)
+            .with_resources(false, false)
+            .with_prompts(false)
+            .build()
+    }
+
+    fn list_tools(&self) -> Vec<Tool> {
+        vec![
+            Tool::new(
+                "increment".to_string(),
+                "Increment the counter by 1".to_string(),
+                serde_json::json!({
+                    "type": "object",
+                    "properties": {},
+                    "required": []
+                }),
+            ),
+            Tool::new(
+                "decrement".to_string(),
+                "Decrement the counter by 1".to_string(),
+                serde_json::json!({
+                    "type": "object",
+                    "properties": {},
+                    "required": []
+                }),
+            ),
+            Tool::new(
+                "get_value".to_string(),
+                "Get the current counter value".to_string(),
+                serde_json::json!({
+                    "type": "object",
+                    "properties": {},
+                    "required": []
+                }),
+            ),
+        ]
+    }
+
+    fn call_tool(
+        &self,
+        tool_name: &str,
+        _arguments: Value,
+    ) -> Pin<Box<dyn Future<Output = Result<Vec<Content>, ToolError>> + Send + 'static>> {
+        let this = self.clone();
+        let tool_name = tool_name.to_string();
+
+        Box::pin(async move {
+            match tool_name.as_str() {
+                "increment" => {
+                    let value = this.increment().await?;
+                    Ok(vec![Content::text(value.to_string())])
+                }
+                "decrement" => {
+                    let value = this.decrement().await?;
+                    Ok(vec![Content::text(value.to_string())])
+                }
+                "get_value" => {
+                    let value = this.get_value().await?;
+                    Ok(vec![Content::text(value.to_string())])
+                }
+                _ => Err(ToolError::NotFound(format!("Tool {} not found", tool_name))),
+            }
+        })
+    }
+
+    fn list_resources(&self) -> Vec<Resource> {
+        vec![
+            self._create_resource_text("str:////Users/to/some/path/", "cwd"),
+            self._create_resource_text("memo://insights", "memo-name"),
+        ]
+    }
+
+    fn read_resource(
+        &self,
+        uri: &str,
+    ) -> Pin<Box<dyn Future<Output = Result<String, ResourceError>> + Send + 'static>> {
+        let uri = uri.to_string();
+        Box::pin(async move {
+            match uri.as_str() {
+                "str:////Users/to/some/path/" => {
+                    let cwd = "/Users/to/some/path/";
+                    Ok(cwd.to_string())
+                }
+                "memo://insights" => {
+                    let memo =
+                        "Business Intelligence Memo\n\nAnalysis has revealed 5 key insights ...";
+                    Ok(memo.to_string())
+                }
+                _ => Err(ResourceError::NotFound(format!(
+                    "Resource {} not found",
+                    uri
+                ))),
+            }
+        })
+    }
+
+    fn list_prompts(&self) -> Vec<Prompt> {
+        vec![Prompt::new(
+            "example_prompt",
+            Some("This is an example prompt that takes one required agrument, message"),
+            Some(vec![PromptArgument {
+                name: "message".to_string(),
+                description: Some("A message to put in the prompt".to_string()),
+                required: Some(true),
+            }]),
+        )]
+    }
+
+    fn get_prompt(
+        &self,
+        prompt_name: &str,
+    ) -> Pin<Box<dyn Future<Output = Result<String, PromptError>> + Send + 'static>> {
+        let prompt_name = prompt_name.to_string();
+        Box::pin(async move {
+            match prompt_name.as_str() {
+                "example_prompt" => {
+                    let prompt = "This is an example prompt with your message here: '{message}'";
+                    Ok(prompt.to_string())
+                }
+                _ => Err(PromptError::NotFound(format!(
+                    "Prompt {} not found",
+                    prompt_name
+                ))),
+            }
+        })
+    }
+}

examples/servers/src/common/jsonrpc_frame_codec.rs

@@ -0,0 +1,24 @@
+use tokio_util::codec::Decoder;
+
+#[derive(Default)]
+pub struct JsonRpcFrameCodec;
+impl Decoder for JsonRpcFrameCodec {
+    type Item = tokio_util::bytes::Bytes;
+    type Error = tokio::io::Error;
+    fn decode(
+        &mut self,
+        src: &mut tokio_util::bytes::BytesMut,
+    ) -> Result<Option<Self::Item>, Self::Error> {
+        if let Some(end) = src
+            .iter()
+            .enumerate()
+            .find_map(|(idx, &b)| (b == b'\n').then_some(idx))
+        {
+            let line = src.split_to(end);
+            let _char_next_line = src.split_to(1);
+            Ok(Some(line.freeze()))
+        } else {
+            Ok(None)
+        }
+    }
+}

examples/servers/src/common/mod.rs

@@ -0,0 +1,2 @@
+pub mod counter;
+pub mod jsonrpc_frame_codec;