[router] Refactor router and policy traits with dependency injection (#7987)

Co-authored-by: Jin Pan <jpan236@wisc.edu>
Co-authored-by: Keru Yang <rukeyang@gmail.com>
Co-authored-by: Yingyi Huang <yingyihuang2000@outlook.com>
Co-authored-by: Philip Zhu <phlipzhux@gmail.com>

sgl-router/src/policies/round_robin.rs

@@ -0,0 +1,136 @@
+//! Round-robin load balancing policy
+
+use super::{get_healthy_worker_indices, LoadBalancingPolicy};
+use crate::core::Worker;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+/// Round-robin selection policy
+///
+/// Selects workers in sequential order, cycling through all healthy workers.
+#[derive(Debug, Default)]
+pub struct RoundRobinPolicy {
+    counter: AtomicUsize,
+}
+
+impl RoundRobinPolicy {
+    pub fn new() -> Self {
+        Self {
+            counter: AtomicUsize::new(0),
+        }
+    }
+}
+
+impl LoadBalancingPolicy for RoundRobinPolicy {
+    fn select_worker(
+        &self,
+        workers: &[Box<dyn Worker>],
+        _request_text: Option<&str>,
+    ) -> Option<usize> {
+        let healthy_indices = get_healthy_worker_indices(workers);
+
+        if healthy_indices.is_empty() {
+            return None;
+        }
+
+        // Get and increment counter atomically
+        let count = self.counter.fetch_add(1, Ordering::Relaxed);
+        let selected_idx = count % healthy_indices.len();
+
+        Some(healthy_indices[selected_idx])
+    }
+
+    fn name(&self) -> &'static str {
+        "round_robin"
+    }
+
+    fn reset(&self) {
+        self.counter.store(0, Ordering::Relaxed);
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::core::{BasicWorker, WorkerType};
+
+    #[test]
+    fn test_round_robin_selection() {
+        let policy = RoundRobinPolicy::new();
+        let workers: Vec<Box<dyn Worker>> = vec![
+            Box::new(BasicWorker::new(
+                "http://w1:8000".to_string(),
+                WorkerType::Regular,
+            )),
+            Box::new(BasicWorker::new(
+                "http://w2:8000".to_string(),
+                WorkerType::Regular,
+            )),
+            Box::new(BasicWorker::new(
+                "http://w3:8000".to_string(),
+                WorkerType::Regular,
+            )),
+        ];
+
+        // Should select workers in order: 0, 1, 2, 0, 1, 2, ...
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+        assert_eq!(policy.select_worker(&workers, None), Some(1));
+        assert_eq!(policy.select_worker(&workers, None), Some(2));
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+        assert_eq!(policy.select_worker(&workers, None), Some(1));
+    }
+
+    #[test]
+    fn test_round_robin_with_unhealthy_workers() {
+        let policy = RoundRobinPolicy::new();
+        let workers: Vec<Box<dyn Worker>> = vec![
+            Box::new(BasicWorker::new(
+                "http://w1:8000".to_string(),
+                WorkerType::Regular,
+            )),
+            Box::new(BasicWorker::new(
+                "http://w2:8000".to_string(),
+                WorkerType::Regular,
+            )),
+            Box::new(BasicWorker::new(
+                "http://w3:8000".to_string(),
+                WorkerType::Regular,
+            )),
+        ];
+
+        // Mark middle worker as unhealthy
+        workers[1].set_healthy(false);
+
+        // Should skip unhealthy worker: 0, 2, 0, 2, ...
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+        assert_eq!(policy.select_worker(&workers, None), Some(2));
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+        assert_eq!(policy.select_worker(&workers, None), Some(2));
+    }
+
+    #[test]
+    fn test_round_robin_reset() {
+        let policy = RoundRobinPolicy::new();
+        let workers: Vec<Box<dyn Worker>> = vec![
+            Box::new(BasicWorker::new(
+                "http://w1:8000".to_string(),
+                WorkerType::Regular,
+            )),
+            Box::new(BasicWorker::new(
+                "http://w2:8000".to_string(),
+                WorkerType::Regular,
+            )),
+        ];
+
+        // Advance the counter
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+        assert_eq!(policy.select_worker(&workers, None), Some(1));
+
+        // Reset should start from beginning
+        policy.reset();
+        assert_eq!(policy.select_worker(&workers, None), Some(0));
+    }
+}