Add Layer 4 caching to reduce AI load

Implemented 5-minute short-term caching for relationship inference:

**store.rs**:
- Added relationship_cache SQLite table
- save_relationship_cache(), get_cached_relationship()
- save_all_relationships_cache(), get_cached_all_relationships()
- clear_relationship_cache() - called on memory create/update/delete
- Cache duration: 5 minutes (configurable constant)

**relationship.rs**:
- Modified infer_all_relationships() to use cache
- Added get_relationship() function with caching support
- Cache hit: return immediately
- Cache miss: compute, save to cache, return

**base.rs**:
- Updated tool_get_relationship() to use cached version
- Reduced load from O(n) scan to O(1) cache lookup

**Benefits**:
- Reduces AI load when frequently querying relationships
- Automatic cache invalidation on data changes
- Scales better with growing memory count
- No user-facing changes

**Documentation**:
- Updated ARCHITECTURE.md with caching strategy details

This addresses scalability concerns for Layer 4 as memory data grows.

docs/ARCHITECTURE.md

@@ -362,10 +362,16 @@ if user.extraversion < 0.5 {
 
 ### Design Philosophy
 
-**推測のみ、保存なし**:
+**推測ベース + 短期キャッシング**:
 - 毎回Layer 1-3.5から計算
-- キャッシュなし（シンプルさ優先）
+- 5分間の短期キャッシュで負荷軽減
-- 後でキャッシング追加可能
+- メモリ更新時にキャッシュ無効化
+
+**キャッシング戦略**:
+- SQLiteテーブル（`relationship_cache`）に保存
+- 個別エンティティ: `get_relationship(entity_id)`
+- 全体リスト: `list_relationships()`
+- メモリ作成/更新/削除時に自動クリア
 
 **独立性**:
 - Layer 1-3.5に依存

src/core/mod.rs

@@ -9,5 +9,5 @@ pub use analysis::UserAnalysis;
 pub use error::{MemoryError, Result};
 pub use memory::Memory;
 pub use profile::{UserProfile, TraitScore};
-pub use relationship::{RelationshipInference, infer_all_relationships};
+pub use relationship::{RelationshipInference, infer_all_relationships, get_relationship};
 pub use store::MemoryStore;

src/core/relationship.rs

@@ -184,6 +184,11 @@ impl RelationshipInference {
 pub fn infer_all_relationships(
     store: &MemoryStore,
 ) -> Result<Vec<RelationshipInference>> {
+    // Check cache first
+    if let Some(cached) = store.get_cached_all_relationships()? {
+        return Ok(cached);
+    }
+
     // Get all memories
     let memories = store.list()?;
 
@@ -219,9 +224,41 @@ pub fn infer_all_relationships(
             .unwrap_or(std::cmp::Ordering::Equal)
     });
 
+    // Cache the result
+    store.save_all_relationships_cache(&relationships)?;
+
     Ok(relationships)
 }
 
+/// Get relationship inference for a specific entity (with caching)
+pub fn get_relationship(
+    store: &MemoryStore,
+    entity_id: &str,
+) -> Result<RelationshipInference> {
+    // Check cache first
+    if let Some(cached) = store.get_cached_relationship(entity_id)? {
+        return Ok(cached);
+    }
+
+    // Get all memories
+    let memories = store.list()?;
+
+    // Get user profile
+    let user_profile = store.get_profile()?;
+
+    // Infer relationship
+    let relationship = RelationshipInference::infer(
+        entity_id.to_string(),
+        &memories,
+        &user_profile,
+    );
+
+    // Cache it
+    store.save_relationship_cache(entity_id, &relationship)?;
+
+    Ok(relationship)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;

src/core/store.rs

@@ -102,6 +102,17 @@ impl MemoryStore {
             [],
         )?;
 
+        // Create relationship_cache table (Layer 4 - relationship inference cache)
+        // entity_id = "" for all_relationships cache
+        conn.execute(
+            "CREATE TABLE IF NOT EXISTS relationship_cache (
+                entity_id TEXT PRIMARY KEY,
+                data TEXT NOT NULL,
+                cached_at TEXT NOT NULL
+            )",
+            [],
+        )?;
+
         Ok(Self { conn })
     }
 
@@ -137,6 +148,10 @@ impl MemoryStore {
                 memory.updated_at.to_rfc3339(),
             ],
         )?;
+
+        // Clear relationship cache since memory data changed
+        self.clear_relationship_cache()?;
+
         Ok(())
     }
 
@@ -204,6 +219,9 @@ impl MemoryStore {
             return Err(MemoryError::NotFound(memory.id.clone()));
         }
 
+        // Clear relationship cache since memory data changed
+        self.clear_relationship_cache()?;
+
         Ok(())
     }
 
@@ -217,6 +235,9 @@ impl MemoryStore {
             return Err(MemoryError::NotFound(id.to_string()));
         }
 
+        // Clear relationship cache since memory data changed
+        self.clear_relationship_cache()?;
+
         Ok(())
     }
 
@@ -464,6 +485,123 @@ impl MemoryStore {
 
         Ok(profile)
     }
+
+    // ========== Layer 4: Relationship Cache Methods ==========
+
+    /// Cache duration in minutes
+    const RELATIONSHIP_CACHE_DURATION_MINUTES: i64 = 5;
+
+    /// Save relationship inference to cache
+    pub fn save_relationship_cache(
+        &self,
+        entity_id: &str,
+        relationship: &super::relationship::RelationshipInference,
+    ) -> Result<()> {
+        let data = serde_json::to_string(relationship)?;
+        let cached_at = Utc::now().to_rfc3339();
+
+        self.conn.execute(
+            "INSERT OR REPLACE INTO relationship_cache (entity_id, data, cached_at) VALUES (?1, ?2, ?3)",
+            params![entity_id, data, cached_at],
+        )?;
+
+        Ok(())
+    }
+
+    /// Get cached relationship inference
+    pub fn get_cached_relationship(
+        &self,
+        entity_id: &str,
+    ) -> Result<Option<super::relationship::RelationshipInference>> {
+        let mut stmt = self
+            .conn
+            .prepare("SELECT data, cached_at FROM relationship_cache WHERE entity_id = ?1")?;
+
+        let result = stmt.query_row([entity_id], |row| {
+            let data: String = row.get(0)?;
+            let cached_at: String = row.get(1)?;
+            Ok((data, cached_at))
+        });
+
+        match result {
+            Ok((data, cached_at_str)) => {
+                // Check if cache is still valid (within 5 minutes)
+                let cached_at = DateTime::parse_from_rfc3339(&cached_at_str)
+                    .map_err(|e| MemoryError::Parse(e.to_string()))?
+                    .with_timezone(&Utc);
+
+                let age_minutes = (Utc::now() - cached_at).num_minutes();
+
+                if age_minutes < Self::RELATIONSHIP_CACHE_DURATION_MINUTES {
+                    let relationship: super::relationship::RelationshipInference =
+                        serde_json::from_str(&data)?;
+                    Ok(Some(relationship))
+                } else {
+                    // Cache expired
+                    Ok(None)
+                }
+            }
+            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+            Err(e) => Err(e.into()),
+        }
+    }
+
+    /// Save all relationships list to cache (use empty string as entity_id)
+    pub fn save_all_relationships_cache(
+        &self,
+        relationships: &[super::relationship::RelationshipInference],
+    ) -> Result<()> {
+        let data = serde_json::to_string(relationships)?;
+        let cached_at = Utc::now().to_rfc3339();
+
+        self.conn.execute(
+            "INSERT OR REPLACE INTO relationship_cache (entity_id, data, cached_at) VALUES ('', ?1, ?2)",
+            params![data, cached_at],
+        )?;
+
+        Ok(())
+    }
+
+    /// Get cached all relationships list
+    pub fn get_cached_all_relationships(
+        &self,
+    ) -> Result<Option<Vec<super::relationship::RelationshipInference>>> {
+        let mut stmt = self
+            .conn
+            .prepare("SELECT data, cached_at FROM relationship_cache WHERE entity_id = ''")?;
+
+        let result = stmt.query_row([], |row| {
+            let data: String = row.get(0)?;
+            let cached_at: String = row.get(1)?;
+            Ok((data, cached_at))
+        });
+
+        match result {
+            Ok((data, cached_at_str)) => {
+                let cached_at = DateTime::parse_from_rfc3339(&cached_at_str)
+                    .map_err(|e| MemoryError::Parse(e.to_string()))?
+                    .with_timezone(&Utc);
+
+                let age_minutes = (Utc::now() - cached_at).num_minutes();
+
+                if age_minutes < Self::RELATIONSHIP_CACHE_DURATION_MINUTES {
+                    let relationships: Vec<super::relationship::RelationshipInference> =
+                        serde_json::from_str(&data)?;
+                    Ok(Some(relationships))
+                } else {
+                    Ok(None)
+                }
+            }
+            Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+            Err(e) => Err(e.into()),
+        }
+    }
+
+    /// Clear all relationship caches (call when memories are modified)
+    pub fn clear_relationship_cache(&self) -> Result<()> {
+        self.conn.execute("DELETE FROM relationship_cache", [])?;
+        Ok(())
+    }
 }
 
 #[cfg(test)]

src/mcp/base.rs

@@ -2,7 +2,7 @@ use anyhow::Result;
 use serde_json::{json, Value};
 use std::io::{self, BufRead, Write};
 
-use crate::core::{Memory, MemoryStore, UserAnalysis, RelationshipInference, infer_all_relationships};
+use crate::core::{Memory, MemoryStore, UserAnalysis, RelationshipInference, infer_all_relationships, get_relationship};
 
 pub struct BaseMCPServer {
     store: MemoryStore,
@@ -581,31 +581,9 @@ impl BaseMCPServer {
             });
         }
 
-        // Get memories and user profile
+        // Get relationship (with caching)
-        let memories = match self.store.list() {
+        match get_relationship(&self.store, entity_id) {
-            Ok(m) => m,
+            Ok(relationship) => json!({
-            Err(e) => return json!({
-                "success": false,
-                "error": format!("Failed to get memories: {}", e)
-            }),
-        };
-
-        let user_profile = match self.store.get_profile() {
-            Ok(p) => p,
-            Err(e) => return json!({
-                "success": false,
-                "error": format!("Failed to get profile: {}", e)
-            }),
-        };
-
-        // Infer relationship
-        let relationship = RelationshipInference::infer(
-            entity_id.to_string(),
-            &memories,
-            &user_profile,
-        );
-
-        json!({
                 "success": true,
                 "relationship": {
                     "entity_id": relationship.entity_id,
@@ -617,7 +595,12 @@ impl BaseMCPServer {
                     "confidence": relationship.confidence,
                     "inferred_at": relationship.inferred_at
                 }
-        })
+            }),
+            Err(e) => json!({
+                "success": false,
+                "error": format!("Failed to get relationship: {}", e)
+            }),
+        }
     }
 
     fn tool_list_relationships(&self, arguments: &Value) -> Value {