Model affinity for consistent model selection in agentic loops (#827)

crates/brightstaff/src/router/llm.rs

@@ -1,4 +1,4 @@
-use std::{collections::HashMap, sync::Arc};
+use std::{collections::HashMap, sync::Arc, time::Duration, time::Instant};
 
 use common::{
     configuration::TopLevelRoutingPreference,
@@ -9,6 +9,7 @@ use super::router_model::{ModelUsagePreference, RoutingPreference};
 use hermesllm::apis::openai::Message;
 use hyper::header;
 use thiserror::Error;
+use tokio::sync::RwLock;
 use tracing::{debug, info};
 
 use super::http::{self, post_and_extract_content};
@@ -17,6 +18,17 @@ use super::router_model::RouterModel;
 
 use crate::router::router_model_v1;
 
+const DEFAULT_SESSION_TTL_SECONDS: u64 = 600;
+const DEFAULT_SESSION_MAX_ENTRIES: usize = 10_000;
+const MAX_SESSION_MAX_ENTRIES: usize = 10_000;
+
+#[derive(Clone, Debug)]
+pub struct CachedRoute {
+    pub model_name: String,
+    pub route_name: Option<String>,
+    pub cached_at: Instant,
+}
+
 pub struct RouterService {
     router_url: String,
     client: reqwest::Client,
@@ -24,6 +36,9 @@ pub struct RouterService {
     routing_provider_name: String,
     top_level_preferences: HashMap<String, TopLevelRoutingPreference>,
     metrics_service: Option<Arc<ModelMetricsService>>,
+    session_cache: RwLock<HashMap<String, CachedRoute>>,
+    session_ttl: Duration,
+    session_max_entries: usize,
 }
 
 #[derive(Debug, Error)]
@@ -44,6 +59,8 @@ impl RouterService {
         router_url: String,
         routing_model_name: String,
         routing_provider_name: String,
+        session_ttl_seconds: Option<u64>,
+        session_max_entries: Option<usize>,
     ) -> Self {
         let top_level_preferences: HashMap<String, TopLevelRoutingPreference> = top_level_prefs
             .map_or_else(HashMap::new, |prefs| {
@@ -74,6 +91,12 @@ impl RouterService {
             router_model_v1::MAX_TOKEN_LEN,
         ));
 
+        let session_ttl =
+            Duration::from_secs(session_ttl_seconds.unwrap_or(DEFAULT_SESSION_TTL_SECONDS));
+        let session_max_entries = session_max_entries
+            .unwrap_or(DEFAULT_SESSION_MAX_ENTRIES)
+            .min(MAX_SESSION_MAX_ENTRIES);
+
         RouterService {
             router_url,
             client: reqwest::Client::new(),
@@ -81,6 +104,64 @@ impl RouterService {
             routing_provider_name,
             top_level_preferences,
             metrics_service,
+            session_cache: RwLock::new(HashMap::new()),
+            session_ttl,
+            session_max_entries,
+        }
+    }
+
+    /// Look up a cached routing decision by session ID.
+    /// Returns None if not found or expired.
+    pub async fn get_cached_route(&self, session_id: &str) -> Option<CachedRoute> {
+        let cache = self.session_cache.read().await;
+        if let Some(entry) = cache.get(session_id) {
+            if entry.cached_at.elapsed() < self.session_ttl {
+                return Some(entry.clone());
+            }
+        }
+        None
+    }
+
+    /// Store a routing decision in the session cache.
+    /// If at max capacity, evicts the oldest entry.
+    pub async fn cache_route(
+        &self,
+        session_id: String,
+        model_name: String,
+        route_name: Option<String>,
+    ) {
+        let mut cache = self.session_cache.write().await;
+        if cache.len() >= self.session_max_entries && !cache.contains_key(&session_id) {
+            if let Some(oldest_key) = cache
+                .iter()
+                .min_by_key(|(_, v)| v.cached_at)
+                .map(|(k, _)| k.clone())
+            {
+                cache.remove(&oldest_key);
+            }
+        }
+        cache.insert(
+            session_id,
+            CachedRoute {
+                model_name,
+                route_name,
+                cached_at: Instant::now(),
+            },
+        );
+    }
+
+    /// Remove all expired entries from the session cache.
+    pub async fn cleanup_expired_sessions(&self) {
+        let mut cache = self.session_cache.write().await;
+        let before = cache.len();
+        cache.retain(|_, entry| entry.cached_at.elapsed() < self.session_ttl);
+        let removed = before - cache.len();
+        if removed > 0 {
+            info!(
+                removed = removed,
+                remaining = cache.len(),
+                "cleaned up expired session cache entries"
+            );
         }
     }
 
@@ -198,3 +279,102 @@ impl RouterService {
         Ok(result)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_router_service(ttl_seconds: u64, max_entries: usize) -> RouterService {
+        RouterService::new(
+            None,
+            None,
+            "http://localhost:12001/v1/chat/completions".to_string(),
+            "Arch-Router".to_string(),
+            "arch-router".to_string(),
+            Some(ttl_seconds),
+            Some(max_entries),
+        )
+    }
+
+    #[tokio::test]
+    async fn test_cache_miss_returns_none() {
+        let svc = make_router_service(600, 100);
+        assert!(svc.get_cached_route("unknown-session").await.is_none());
+    }
+
+    #[tokio::test]
+    async fn test_cache_hit_returns_cached_route() {
+        let svc = make_router_service(600, 100);
+        svc.cache_route(
+            "s1".to_string(),
+            "gpt-4o".to_string(),
+            Some("code".to_string()),
+        )
+        .await;
+
+        let cached = svc.get_cached_route("s1").await.unwrap();
+        assert_eq!(cached.model_name, "gpt-4o");
+        assert_eq!(cached.route_name, Some("code".to_string()));
+    }
+
+    #[tokio::test]
+    async fn test_cache_expired_entry_returns_none() {
+        let svc = make_router_service(0, 100);
+        svc.cache_route("s1".to_string(), "gpt-4o".to_string(), None)
+            .await;
+        assert!(svc.get_cached_route("s1").await.is_none());
+    }
+
+    #[tokio::test]
+    async fn test_cleanup_removes_expired() {
+        let svc = make_router_service(0, 100);
+        svc.cache_route("s1".to_string(), "gpt-4o".to_string(), None)
+            .await;
+        svc.cache_route("s2".to_string(), "claude".to_string(), None)
+            .await;
+
+        svc.cleanup_expired_sessions().await;
+
+        let cache = svc.session_cache.read().await;
+        assert!(cache.is_empty());
+    }
+
+    #[tokio::test]
+    async fn test_cache_evicts_oldest_when_full() {
+        let svc = make_router_service(600, 2);
+        svc.cache_route("s1".to_string(), "model-a".to_string(), None)
+            .await;
+        tokio::time::sleep(Duration::from_millis(10)).await;
+        svc.cache_route("s2".to_string(), "model-b".to_string(), None)
+            .await;
+
+        svc.cache_route("s3".to_string(), "model-c".to_string(), None)
+            .await;
+
+        let cache = svc.session_cache.read().await;
+        assert_eq!(cache.len(), 2);
+        assert!(!cache.contains_key("s1"));
+        assert!(cache.contains_key("s2"));
+        assert!(cache.contains_key("s3"));
+    }
+
+    #[tokio::test]
+    async fn test_cache_update_existing_session_does_not_evict() {
+        let svc = make_router_service(600, 2);
+        svc.cache_route("s1".to_string(), "model-a".to_string(), None)
+            .await;
+        svc.cache_route("s2".to_string(), "model-b".to_string(), None)
+            .await;
+
+        svc.cache_route(
+            "s1".to_string(),
+            "model-a-updated".to_string(),
+            Some("route".to_string()),
+        )
+        .await;
+
+        let cache = svc.session_cache.read().await;
+        assert_eq!(cache.len(), 2);
+        assert_eq!(cache.get("s1").unwrap().model_name, "model-a-updated");
+    }
+}