[model-gateway] extract header extraction in policy and add (#16566)

sgl-model-gateway/src/policies/consistent_hashing.rs

@@ -18,16 +18,14 @@
 
 use std::sync::Arc;
 
-use http::header::HeaderName;
 use rand::Rng as _;
 
 use super::{LoadBalancingPolicy, SelectWorkerInfo};
-use crate::{core::Worker, observability::metrics::Metrics};
-
-/// Header for direct worker targeting by index (0-based)
-static HEADER_TARGET_WORKER: HeaderName = HeaderName::from_static("x-smg-target-worker");
-/// Header for consistent hash routing
-static HEADER_ROUTING_KEY: HeaderName = HeaderName::from_static("x-smg-routing-key");
+use crate::{
+    core::Worker,
+    observability::metrics::Metrics,
+    routers::header_utils::{extract_routing_key, extract_target_worker},
+};
 
 /// Execution branch for metrics
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -112,18 +110,8 @@ impl ConsistentHashingPolicy {
             return (None, Branch::NoHealthyWorkers);
         }
 
-        // Extract routing headers - to_str() is O(1), just validates ASCII, no allocation
-        let target_worker = info
-            .headers
-            .and_then(|h| h.get(&HEADER_TARGET_WORKER))
-            .and_then(|v| v.to_str().ok())
-            .filter(|s| !s.is_empty());
-
-        let routing_key = info
-            .headers
-            .and_then(|h| h.get(&HEADER_ROUTING_KEY))
-            .and_then(|v| v.to_str().ok())
-            .filter(|s| !s.is_empty());
+        let target_worker = extract_target_worker(info.headers);
+        let routing_key = extract_routing_key(info.headers);
 
         // Priority 1: X-SMG-Target-Worker - direct routing by worker index
         // O(1) parse + O(1) bounds check + O(1) health check

sgl-model-gateway/src/policies/manual.rs

@@ -16,17 +16,15 @@
 use std::{sync::Arc, time::Instant};
 
 use dashmap::{mapref::entry::Entry, DashMap};
-use http::header::HeaderName;
 use rand::Rng;
 use tracing::info;
 
 use super::{
     get_healthy_worker_indices, utils::PeriodicTask, LoadBalancingPolicy, SelectWorkerInfo,
 };
-use crate::{core::Worker, observability::metrics::Metrics};
-
-/// Header for routing key based sticky sessions
-static HEADER_ROUTING_KEY: HeaderName = HeaderName::from_static("x-smg-routing-key");
+use crate::{
+    core::Worker, observability::metrics::Metrics, routers::header_utils::extract_routing_key,
+};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum ExecutionBranch {
@@ -191,12 +189,7 @@ impl ManualPolicy {
             return (None, ExecutionBranch::NoHealthyWorkers);
         }
 
-        // Extract routing key from header
-        let routing_id = info
-            .headers
-            .and_then(|h| h.get(&HEADER_ROUTING_KEY))
-            .and_then(|v| v.to_str().ok())
-            .filter(|s| !s.is_empty());
+        let routing_id = extract_routing_key(info.headers);
 
         if let Some(routing_id) = routing_id {
             let (idx, branch) = self.select_by_routing_id(workers, routing_id, &healthy_indices);

sgl-model-gateway/src/routers/header_utils.rs

@@ -3,6 +3,25 @@ use axum::{
     extract::Request,
     http::{HeaderMap, HeaderValue},
 };
+use http::header::HeaderName;
+
+static HEADER_TARGET_WORKER: HeaderName = HeaderName::from_static("x-smg-target-worker");
+static HEADER_ROUTING_KEY: HeaderName = HeaderName::from_static("x-smg-routing-key");
+
+fn extract_header_value<'a>(headers: Option<&'a HeaderMap>, name: &HeaderName) -> Option<&'a str> {
+    headers
+        .and_then(|h| h.get(name))
+        .and_then(|v| v.to_str().ok())
+        .filter(|s| !s.is_empty())
+}
+
+pub fn extract_target_worker(headers: Option<&HeaderMap>) -> Option<&str> {
+    extract_header_value(headers, &HEADER_TARGET_WORKER)
+}
+
+pub fn extract_routing_key(headers: Option<&HeaderMap>) -> Option<&str> {
+    extract_header_value(headers, &HEADER_ROUTING_KEY)
+}
 
 /// Copy request headers to a Vec of name-value string pairs
 /// Used for forwarding headers to backend workers
@@ -203,6 +222,44 @@ pub fn should_forward_request_header(name: &str) -> bool {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_extract_header_value_returns_value() {
+        let mut headers = HeaderMap::new();
+        headers.insert("x-smg-routing-key", "test-key".parse().unwrap());
+        assert_eq!(extract_routing_key(Some(&headers)), Some("test-key"));
+    }
+
+    #[test]
+    fn test_extract_header_value_returns_none_for_missing() {
+        let headers = HeaderMap::new();
+        assert_eq!(extract_routing_key(Some(&headers)), None);
+    }
+
+    #[test]
+    fn test_extract_header_value_returns_none_for_empty() {
+        let mut headers = HeaderMap::new();
+        headers.insert("x-smg-routing-key", "".parse().unwrap());
+        assert_eq!(extract_routing_key(Some(&headers)), None);
+    }
+
+    #[test]
+    fn test_extract_header_value_returns_none_for_none_headers() {
+        assert_eq!(extract_routing_key(None), None);
+    }
+
+    #[test]
+    fn test_extract_target_worker() {
+        let mut headers = HeaderMap::new();
+        headers.insert("x-smg-target-worker", "2".parse().unwrap());
+        assert_eq!(extract_target_worker(Some(&headers)), Some("2"));
+    }
+
+    #[test]
+    fn test_extract_target_worker_missing() {
+        let headers = HeaderMap::new();
+        assert_eq!(extract_target_worker(Some(&headers)), None);
+    }
+
     #[test]
     fn test_should_forward_request_header_whitelist() {
         assert!(should_forward_request_header("authorization"));