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

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;