feat: add LRU eviction tests and last_accessed tracking

- Add touch_model() in handlers to update last_accessed timestamp
  on every proxied request, driving LRU eviction ordering
- 5 integration tests: LRU eviction, pinned model protection,
  nothing-to-evict case, lifecycle_cycles increment, and
  last_accessed update verification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/src/handlers.rs

@@ -9,6 +9,7 @@ use axum::extract::State;
 use axum::http::HeaderMap;
 use axum::response::{IntoResponse, Json, Response};
 use axum::routing::{get, post};
+use chrono::Utc;
 use cortex_core::node::{CortexModelEntry, ModelLocation};
 use serde_json::{Value, json};
 use std::sync::Arc;
@@ -39,6 +40,8 @@ async fn chat_completions(
         Err(e) => return error_response(404, &e.to_string()),
     };
 
+    touch_model(&fleet, &route.node_name, &model_id).await;
+
     match proxy::forward_request(
         &fleet.http_client,
         &route,
@@ -69,6 +72,8 @@ async fn completions(
         Err(e) => return error_response(404, &e.to_string()),
     };
 
+    touch_model(&fleet, &route.node_name, &model_id).await;
+
     match proxy::forward_request(&fleet.http_client, &route, "/v1/completions", headers, body).await
     {
         Ok(resp) => resp,
@@ -190,6 +195,16 @@ async fn health(State(fleet): State<Arc<CortexState>>) -> Json<Value> {
 
 // ── Helpers ──────────────────────────────────────────────────────────
 
+/// Update `last_accessed` timestamp for a model on a node (drives LRU eviction).
+async fn touch_model(fleet: &CortexState, node_name: &str, model_id: &str) {
+    let mut nodes = fleet.nodes.write().await;
+    if let Some(node) = nodes.get_mut(node_name)
+        && let Some(entry) = node.models.get_mut(model_id)
+    {
+        entry.last_accessed = Some(Utc::now());
+    }
+}
+
 fn extract_model(body: &[u8]) -> Option<String> {
     let v: Value = serde_json::from_slice(body).ok()?;
     v.get("model")?.as_str().map(|s| s.to_string())

crates/cortex-gateway/tests/eviction.rs

@@ -0,0 +1,275 @@
+mod common;
+
+use chrono::Utc;
+use cortex_core::config::{
+    EvictionSettings, EvictionStrategy, GatewayConfig, GatewaySettings, NodeConfig,
+};
+use cortex_core::node::{ModelEntry, ModelStatus};
+use cortex_gateway::state::CortexState;
+use serde_json::json;
+use std::sync::Arc;
+
+/// Spawn a mock backend that accepts `/v1/models/unload` and records the call.
+async fn spawn_eviction_mock() -> (String, Arc<tokio::sync::Mutex<Vec<String>>>) {
+    use axum::routing::{get, post};
+    use axum::{Json, Router};
+    use serde_json::Value;
+
+    let unloaded: Arc<tokio::sync::Mutex<Vec<String>>> = Arc::new(tokio::sync::Mutex::new(vec![]));
+    let unloaded_clone = Arc::clone(&unloaded);
+
+    let app = Router::new()
+        .route(
+            "/v1/models/unload",
+            post(move |Json(body): Json<Value>| {
+                let unloaded = Arc::clone(&unloaded_clone);
+                async move {
+                    let model_id = body
+                        .get("model_id")
+                        .and_then(|v| v.as_str())
+                        .unwrap_or("")
+                        .to_string();
+                    unloaded.lock().await.push(model_id);
+                    Json(json!({"status": "ok"}))
+                }
+            }),
+        )
+        .route(
+            "/v1/models",
+            get(|| async {
+                Json(json!({
+                    "object": "list",
+                    "data": []
+                }))
+            }),
+        );
+
+    let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+    let addr = listener.local_addr().unwrap();
+    tokio::spawn(async move {
+        axum::serve(listener, app).await.unwrap();
+    });
+
+    (format!("http://{addr}"), unloaded)
+}
+
+fn make_fleet(endpoint: &str, pinned: Vec<String>, defrag_after: u32) -> Arc<CortexState> {
+    let config = GatewayConfig {
+        gateway: GatewaySettings {
+            listen: "127.0.0.1:0".into(),
+            metrics_listen: "127.0.0.1:0".into(),
+        },
+        eviction: EvictionSettings {
+            strategy: EvictionStrategy::Lru,
+            defrag_after_cycles: defrag_after,
+        },
+        nodes: vec![NodeConfig {
+            name: "gpu-node".into(),
+            endpoint: endpoint.to_string(),
+            vram_mb: 24000,
+            pinned,
+        }],
+    };
+    Arc::new(CortexState::from_config(&config))
+}
+
+#[tokio::test]
+async fn test_evict_lru_model() {
+    let (mock_url, unloaded) = spawn_eviction_mock().await;
+    let fleet = make_fleet(&mock_url, vec![], 0);
+
+    // Seed two loaded models. "old-model" was accessed earlier than "new-model".
+    {
+        let mut nodes = fleet.nodes.write().await;
+        let node = nodes.get_mut("gpu-node").unwrap();
+        node.healthy = true;
+        node.models.insert(
+            "old-model".into(),
+            ModelEntry {
+                id: "old-model".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: Some(Utc::now() - chrono::Duration::hours(2)),
+                vram_estimate_mb: Some(8000),
+            },
+        );
+        node.models.insert(
+            "new-model".into(),
+            ModelEntry {
+                id: "new-model".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: Some(Utc::now()),
+                vram_estimate_mb: Some(8000),
+            },
+        );
+    }
+
+    let evicted = cortex_gateway::evictor::evict_lru_on_node(&fleet, "gpu-node")
+        .await
+        .expect("eviction should succeed");
+
+    // The older model should be evicted.
+    assert_eq!(evicted, Some("old-model".to_string()));
+
+    // Mock received the unload call.
+    let calls = unloaded.lock().await;
+    assert_eq!(calls.len(), 1);
+    assert_eq!(calls[0], "old-model");
+
+    // Local state updated.
+    let nodes = fleet.nodes.read().await;
+    let node = nodes.get("gpu-node").unwrap();
+    assert_eq!(
+        node.models.get("old-model").unwrap().status,
+        ModelStatus::Unloaded
+    );
+    assert_eq!(
+        node.models.get("new-model").unwrap().status,
+        ModelStatus::Loaded
+    );
+}
+
+#[tokio::test]
+async fn test_eviction_skips_pinned_models() {
+    let (mock_url, unloaded) = spawn_eviction_mock().await;
+    // Pin "old-model" so it can't be evicted.
+    let fleet = make_fleet(&mock_url, vec!["old-model".into()], 0);
+
+    {
+        let mut nodes = fleet.nodes.write().await;
+        let node = nodes.get_mut("gpu-node").unwrap();
+        node.healthy = true;
+        // old-model is pinned and older — normally it would be evicted.
+        node.models.insert(
+            "old-model".into(),
+            ModelEntry {
+                id: "old-model".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: Some(Utc::now() - chrono::Duration::hours(2)),
+                vram_estimate_mb: Some(8000),
+            },
+        );
+        node.models.insert(
+            "new-model".into(),
+            ModelEntry {
+                id: "new-model".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: Some(Utc::now()),
+                vram_estimate_mb: Some(8000),
+            },
+        );
+    }
+
+    let evicted = cortex_gateway::evictor::evict_lru_on_node(&fleet, "gpu-node")
+        .await
+        .expect("eviction should succeed");
+
+    // new-model is evicted instead because old-model is pinned.
+    assert_eq!(evicted, Some("new-model".to_string()));
+
+    let calls = unloaded.lock().await;
+    assert_eq!(calls[0], "new-model");
+}
+
+#[tokio::test]
+async fn test_eviction_nothing_to_evict() {
+    let (mock_url, unloaded) = spawn_eviction_mock().await;
+    // Pin the only model.
+    let fleet = make_fleet(&mock_url, vec!["only-model".into()], 0);
+
+    {
+        let mut nodes = fleet.nodes.write().await;
+        let node = nodes.get_mut("gpu-node").unwrap();
+        node.healthy = true;
+        node.models.insert(
+            "only-model".into(),
+            ModelEntry {
+                id: "only-model".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: None,
+                vram_estimate_mb: Some(8000),
+            },
+        );
+    }
+
+    let evicted = cortex_gateway::evictor::evict_lru_on_node(&fleet, "gpu-node")
+        .await
+        .expect("eviction should succeed");
+
+    assert_eq!(evicted, None);
+
+    // No unload call made.
+    let calls = unloaded.lock().await;
+    assert!(calls.is_empty());
+}
+
+#[tokio::test]
+async fn test_eviction_increments_lifecycle_cycles() {
+    let (mock_url, _) = spawn_eviction_mock().await;
+    let fleet = make_fleet(&mock_url, vec![], 0);
+
+    {
+        let mut nodes = fleet.nodes.write().await;
+        let node = nodes.get_mut("gpu-node").unwrap();
+        node.healthy = true;
+        node.lifecycle_cycles = 0;
+        node.models.insert(
+            "model-a".into(),
+            ModelEntry {
+                id: "model-a".into(),
+                status: ModelStatus::Loaded,
+                last_accessed: None,
+                vram_estimate_mb: None,
+            },
+        );
+    }
+
+    cortex_gateway::evictor::evict_lru_on_node(&fleet, "gpu-node")
+        .await
+        .expect("eviction should succeed");
+
+    let nodes = fleet.nodes.read().await;
+    assert_eq!(nodes.get("gpu-node").unwrap().lifecycle_cycles, 1);
+}
+
+#[tokio::test]
+async fn test_last_accessed_updated_on_request() {
+    let mock_url = common::spawn_mock_backend().await;
+    let (fleet, gw_url) = common::spawn_gateway_with_state(&mock_url).await;
+
+    // Verify last_accessed is None initially.
+    {
+        let nodes = fleet.nodes.read().await;
+        let node = nodes.get("mock-node").unwrap();
+        assert!(
+            node.models
+                .get("test-model")
+                .unwrap()
+                .last_accessed
+                .is_none()
+        );
+    }
+
+    // Make a request.
+    let client = reqwest::Client::new();
+    client
+        .post(format!("{gw_url}/v1/chat/completions"))
+        .header("content-type", "application/json")
+        .json(&json!({
+            "model": "test-model",
+            "messages": [{"role": "user", "content": "Hi"}]
+        }))
+        .send()
+        .await
+        .expect("request should succeed");
+
+    // Verify last_accessed is now set.
+    let nodes = fleet.nodes.read().await;
+    let node = nodes.get("mock-node").unwrap();
+    assert!(
+        node.models
+            .get("test-model")
+            .unwrap()
+            .last_accessed
+            .is_some()
+    );
+}