feat: implement consensus-based health check for lagging validators

- Add max_slot_lag config (default 50)
- Parse JSON-RPC response in health checks to extract slot number
- Calculate max slot across all backends
- Mark backends unhealthy if they lag behind max slot by > threshold

Author: Yuru Shao

src/config.rs

@@ -36,6 +36,7 @@ pub struct HealthCheckConfig {
     pub method: String,
     pub consecutive_failures_threshold: u32,
     pub consecutive_successes_threshold: u32,
+    pub max_slot_lag: u64,
 }
 
 impl Default for HealthCheckConfig {
@@ -46,6 +47,7 @@ impl Default for HealthCheckConfig {
             method: "getSlot".to_string(),
             consecutive_failures_threshold: 3,
             consecutive_successes_threshold: 2,
+            max_slot_lag: 50,
         }
     }
 }

src/health.rs

@@ -80,11 +80,14 @@ impl HealthState {
     }
 }
 
+/// Performs a health check against a backend.
+/// Returns `Ok(Some(slot))` if the method is `getSlot` or `getBlockHeight` and the response
+/// contains a numeric result. Returns `Ok(None)` for other methods. Returns `Err` on failure.
 async fn perform_health_check(
     client: &Client<HttpsConnector<HttpConnector>, Body>,
     backend: &Backend,
     health_config: &HealthCheckConfig,
-) -> Result<(), String> {
+) -> Result<Option<u64>, String> {
     // Build health check request
     let health_request = serde_json::json!({
         "jsonrpc": "2.0",
@@ -112,13 +115,34 @@ async fn perform_health_check(
 
     match result {
         Ok(Ok(response)) => {
-            if response.status().is_success() {
-                Ok(())
-            } else {
-                Err(format!(
+            if !response.status().is_success() {
+                return Err(format!(
                     "Health check returned status: {}",
                     response.status()
-                ))
+                ));
+            }
+
+            // Parse the response body to extract slot/block height
+            let method = health_config.method.as_str();
+            if method == "getSlot" || method == "getBlockHeight" {
+                let body_bytes = http_body_util::BodyExt::collect(response.into_body())
+                    .await
+                    .map_err(|e| format!("Failed to read response body: {}", e))?
+                    .to_bytes();
+
+                let json: serde_json::Value = serde_json::from_slice(&body_bytes)
+                    .map_err(|e| format!("Failed to parse response JSON: {}", e))?;
+
+                if let Some(slot) = json.get("result").and_then(|v| v.as_u64()) {
+                    Ok(Some(slot))
+                } else {
+                    Err(format!(
+                        "Health check response missing numeric 'result' field for method {}",
+                        method
+                    ))
+                }
+            } else {
+                Ok(None)
             }
         }
         Ok(Err(e)) => Err(format!("Health check request failed: {}", e)),
@@ -158,6 +182,15 @@ pub async fn health_check_loop(
 
         let results = future::join_all(check_futures).await;
 
+        // Collect slot numbers from successful checks to determine the max (consensus tip)
+        let max_slot: Option<u64> = results
+            .iter()
+            .filter_map(|(_, result)| match result {
+                Ok(Some(slot)) => Some(*slot),
+                _ => None,
+            })
+            .max();
+
         for (i, (label, check_result)) in results.into_iter().enumerate() {
             let backend = &current_state.backends[i];
 
@@ -169,23 +202,59 @@ pub async fn health_check_loop(
             let previous_healthy = current_status.healthy;
 
             match check_result {
-                Ok(_) => {
-                    current_status.consecutive_successes += 1;
-                    current_status.consecutive_failures = 0;
-                    current_status.last_error = None;
-
-                    // Mark healthy if threshold reached
-                    if current_status.consecutive_successes
-                        >= health_config.consecutive_successes_threshold
-                    {
-                        current_status.healthy = true;
+                Ok(slot_opt) => {
+                    // Check for slot lag against consensus
+                    let lagging = match (slot_opt, max_slot) {
+                        (Some(slot), Some(max)) if max > slot && (max - slot) > health_config.max_slot_lag => {
+                            true
+                        }
+                        _ => false,
+                    };
+
+                    if lagging {
+                        let slot = slot_opt.unwrap();
+                        let max = max_slot.unwrap();
+                        current_status.consecutive_failures += 1;
+                        current_status.consecutive_successes = 0;
+                        current_status.last_error = Some(format!(
+                            "Backend lagging: slot {} is {} behind max {}",
+                            slot,
+                            max - slot,
+                            max
+                        ));
+
+                        if current_status.consecutive_failures
+                            >= health_config.consecutive_failures_threshold
+                        {
+                            current_status.healthy = false;
+                        }
+
+                        tracing::warn!(
+                            "Backend {} is lagging: slot {} is {} behind consensus max {} (threshold: {})",
+                            label,
+                            slot,
+                            max - slot,
+                            max,
+                            health_config.max_slot_lag
+                        );
+                    } else {
+                        current_status.consecutive_successes += 1;
+                        current_status.consecutive_failures = 0;
+                        current_status.last_error = None;
+
+                        // Mark healthy if threshold reached
+                        if current_status.consecutive_successes
+                            >= health_config.consecutive_successes_threshold
+                        {
+                            current_status.healthy = true;
+                        }
+
+                        tracing::debug!(
+                            "Health check succeeded for backend {} (consecutive successes: {})",
+                            label,
+                            current_status.consecutive_successes
+                        );
                     }
-
-                    tracing::debug!(
-                        "Health check succeeded for backend {} (consecutive successes: {})",
-                        label,
-                        current_status.consecutive_successes
-                    );
                 }
                 Err(error) => {
                     current_status.consecutive_failures += 1;