rpc server: add health/readiness endpoint (#4802)

Previous attempt paritytech/substrate#14314

Close #4443 

Ideally, we should move /health and /health/readiness to the prometheus
server but because it's was quite easy to implement on the RPC server
and that RPC server already exposes /health.

Manual tests on a polkadot node syncing:

```bash
➜ polkadot-sdk (na-fix-4443) ✗ curl -v localhost:9944/health
* Host localhost:9944 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:9944...
* connect to ::1 port 9944 from ::1 port 55024 failed: Connection refused
*   Trying 127.0.0.1:9944...
* Connected to localhost (127.0.0.1) port 9944
> GET /health HTTP/1.1
> Host: localhost:9944
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 200 OK
< content-type: application/json; charset=utf-8
< content-length: 53
< date: Fri, 14 Jun 2024 16:12:23 GMT
<
* Connection #0 to host localhost left intact
{"peers":0,"isSyncing":false,"shouldHavePeers":false}%
➜ polkadot-sdk (na-fix-4443) ✗ curl -v localhost:9944/health/readiness
* Host localhost:9944 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:9944...
* connect to ::1 port 9944 from ::1 port 54328 failed: Connection refused
*   Trying 127.0.0.1:9944...
* Connected to localhost (127.0.0.1) port 9944
> GET /health/readiness HTTP/1.1
> Host: localhost:9944
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< content-type: application/json; charset=utf-8
< content-length: 0
< date: Fri, 14 Jun 2024 16:12:36 GMT
<
* Connection #0 to host localhost left intact
```

//cc @BulatSaif you may be interested in this..

---------

Co-authored-by: Bastian Köcher <[email protected]>

prdoc/pr_4802.prdoc

@@ -0,0 +1,16 @@
+# Schema: Polkadot SDK PRDoc Schema (prdoc) v1.0.0
+# See doc at https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/prdoc/schema_user.json
+
+title: Add `health/readiness endpoint` to the rpc server
+
+doc:
+  - audience: Node Operator
+    description: |
+      Add `/health/readiness endpoint` to the rpc server which returns HTTP status code 200 if the chain is synced
+      and can connect to the rest of the network otherwise status code 500 is returned.
+      The endpoint can be reached by performing a HTTP GET request to the
+      endpoint such as `$ curl <endpoint>/health/readiness`
+
+crates:
+  - name: sc-rpc-server
+    bump: patch

substrate/client/rpc-servers/Cargo.toml

@@ -25,6 +25,7 @@ ip_network = "0.4.1"
 jsonrpsee = { version = "0.22", features = ["server"] }
 log = { workspace = true, default-features = true }
 prometheus-endpoint = { package = "substrate-prometheus-endpoint", path = "../../utils/prometheus" }
+serde = { workspace = true }
 serde_json = { workspace = true, default-features = true }
 tokio = { version = "1.22.0", features = ["parking_lot"] }
 tower = { version = "0.4.13", features = ["util"] }

substrate/client/rpc-servers/src/lib.rs

@@ -32,12 +32,10 @@ use hyper::{
 	service::{make_service_fn, service_fn},
 };
 use jsonrpsee::{
-	server::{
-		middleware::http::ProxyGetRequestLayer, stop_channel, ws, PingConfig, StopHandle,
-		TowerServiceBuilder,
-	},
+	server::{stop_channel, ws, PingConfig, StopHandle, TowerServiceBuilder},
 	Methods, RpcModule,
 };
+use middleware::NodeHealthProxyLayer;
 use tokio::net::TcpListener;
 use tower::Service;
 use utils::{build_rpc_api, format_cors, get_proxy_ip, host_filtering, try_into_cors};
@@ -132,8 +130,8 @@ where
 
 	let http_middleware = tower::ServiceBuilder::new()
 		.option_layer(host_filter)
-		// Proxy `GET /health` requests to internal `system_health` method.
-		.layer(ProxyGetRequestLayer::new("/health", "system_health")?)
+		// Proxy `GET /health, /health/readiness` requests to the internal `system_health` method.
+		.layer(NodeHealthProxyLayer::default())
 		.layer(try_into_cors(cors)?);
 
 	let mut builder = jsonrpsee::server::Server::builder()

substrate/client/rpc-servers/src/middleware/mod.rs

@@ -32,9 +32,11 @@ use jsonrpsee::{
 };
 
 mod metrics;
+mod node_health;
 mod rate_limit;
 
 pub use metrics::*;
+pub use node_health::*;
 pub use rate_limit::*;
 
 const MAX_JITTER: Duration = Duration::from_millis(50);

substrate/client/rpc-servers/src/middleware/node_health.rs

@@ -0,0 +1,199 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Middleware for handling `/health` and `/health/readiness` endpoints.
+
+use std::{
+	error::Error,
+	future::Future,
+	pin::Pin,
+	task::{Context, Poll},
+};
+
+use futures::future::FutureExt;
+use http::{HeaderValue, Method, StatusCode, Uri};
+use hyper::Body;
+use jsonrpsee::types::{Response as RpcResponse, ResponseSuccess as RpcResponseSuccess};
+use tower::Service;
+
+const RPC_SYSTEM_HEALTH_CALL: &str = r#"{"jsonrpc":"2.0","method":"system_health","id":0}"#;
+const HEADER_VALUE_JSON: HeaderValue = HeaderValue::from_static("application/json; charset=utf-8");
+
+/// Layer that applies [`NodeHealthProxy`] which
+/// proxies `/health` and `/health/readiness` endpoints.
+#[derive(Debug, Clone, Default)]
+pub struct NodeHealthProxyLayer;
+
+impl<S> tower::Layer<S> for NodeHealthProxyLayer {
+	type Service = NodeHealthProxy<S>;
+
+	fn layer(&self, service: S) -> Self::Service {
+		NodeHealthProxy::new(service)
+	}
+}
+
+/// Middleware that proxies `/health` and `/health/readiness` endpoints.
+pub struct NodeHealthProxy<S>(S);
+
+impl<S> NodeHealthProxy<S> {
+	/// Creates a new [`NodeHealthProxy`].
+	pub fn new(service: S) -> Self {
+		Self(service)
+	}
+}
+
+impl<S> tower::Service<http::Request<Body>> for NodeHealthProxy<S>
+where
+	S: Service<http::Request<Body>, Response = http::Response<Body>>,
+	S::Response: 'static,
+	S::Error: Into<Box<dyn Error + Send + Sync>> + 'static,
+	S::Future: Send + 'static,
+{
+	type Response = S::Response;
+	type Error = Box<dyn Error + Send + Sync + 'static>;
+	type Future =
+		Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>> + Send + 'static>>;
+
+	fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+		self.0.poll_ready(cx).map_err(Into::into)
+	}
+
+	fn call(&mut self, mut req: http::Request<Body>) -> Self::Future {
+		let maybe_intercept = InterceptRequest::from_http(&req);
+
+		// Modify the request and proxy it to `system_health`
+		if let InterceptRequest::Health | InterceptRequest::Readiness = maybe_intercept {
+			// RPC methods are accessed with `POST`.
+			*req.method_mut() = Method::POST;
+			// Precautionary remove the URI.
+			*req.uri_mut() = Uri::from_static("/");
+
+			// Requests must have the following headers:
+			req.headers_mut().insert(http::header::CONTENT_TYPE, HEADER_VALUE_JSON);
+			req.headers_mut().insert(http::header::ACCEPT, HEADER_VALUE_JSON);
+
+			// Adjust the body to reflect the method call.
+			req = req.map(|_| Body::from(RPC_SYSTEM_HEALTH_CALL));
+		}
+
+		// Call the inner service and get a future that resolves to the response.
+		let fut = self.0.call(req);
+
+		async move {
+			let res = fut.await.map_err(|err| err.into())?;
+
+			Ok(match maybe_intercept {
+				InterceptRequest::Deny =>
+					http_response(StatusCode::METHOD_NOT_ALLOWED, Body::empty()),
+				InterceptRequest::No => res,
+				InterceptRequest::Health => {
+					let health = parse_rpc_response(res.into_body()).await?;
+					http_ok_response(serde_json::to_string(&health)?)
+				},
+				InterceptRequest::Readiness => {
+					let health = parse_rpc_response(res.into_body()).await?;
+					if (!health.is_syncing && health.peers > 0) || !health.should_have_peers {
+						http_ok_response(Body::empty())
+					} else {
+						http_internal_error()
+					}
+				},
+			})
+		}
+		.boxed()
+	}
+}
+
+// NOTE: This is duplicated here to avoid dependency to the `RPC API`.
+#[derive(Clone, Debug, PartialEq, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct Health {
+	/// Number of connected peers
+	pub peers: usize,
+	/// Is the node syncing
+	pub is_syncing: bool,
+	/// Should this node have any peers
+	///
+	/// Might be false for local chains or when running without discovery.
+	pub should_have_peers: bool,
+}
+
+fn http_ok_response<S: Into<hyper::Body>>(body: S) -> hyper::Response<hyper::Body> {
+	http_response(StatusCode::OK, body)
+}
+
+fn http_response<S: Into<hyper::Body>>(
+	status_code: StatusCode,
+	body: S,
+) -> hyper::Response<hyper::Body> {
+	hyper::Response::builder()
+		.status(status_code)
+		.header(http::header::CONTENT_TYPE, HEADER_VALUE_JSON)
+		.body(body.into())
+		.expect("Header is valid; qed")
+}
+
+fn http_internal_error() -> hyper::Response<hyper::Body> {
+	http_response(hyper::StatusCode::INTERNAL_SERVER_ERROR, Body::empty())
+}
+
+async fn parse_rpc_response(body: Body) -> Result<Health, Box<dyn Error + Send + Sync + 'static>> {
+	let bytes = hyper::body::to_bytes(body).await?;
+
+	let raw_rp = serde_json::from_slice::<RpcResponse<Health>>(&bytes)?;
+	let rp = RpcResponseSuccess::<Health>::try_from(raw_rp)?;
+
+	Ok(rp.result)
+}
+
+/// Whether the request should be treated as ordinary RPC call or be modified.
+enum InterceptRequest {
+	/// Proxy `/health` to `system_health`.
+	Health,
+	/// Checks if node has at least one peer and is not doing major syncing.
+	///
+	/// Returns HTTP status code 200 on success otherwise HTTP status code 500 is returned.
+	Readiness,
+	/// Treat as a ordinary RPC call and don't modify the request or response.
+	No,
+	/// Deny health or readiness calls that is not HTTP GET request.
+	///
+	/// Returns HTTP status code 405.
+	Deny,
+}
+
+impl InterceptRequest {
+	fn from_http(req: &http::Request<Body>) -> InterceptRequest {
+		match req.uri().path() {
+			"/health" =>
+				if req.method() == http::Method::GET {
+					InterceptRequest::Health
+				} else {
+					InterceptRequest::Deny
+				},
+			"/health/readiness" =>
+				if req.method() == http::Method::GET {
+					InterceptRequest::Readiness
+				} else {
+					InterceptRequest::Deny
+				},
+			// Forward all other requests to the RPC server.
+			_ => InterceptRequest::No,
+		}
+	}
+}