Merge pull request #661 from jphein/perf/graph-cache

Re-reviewed: both requested changes addressed (threading.Lock on cache globals, col-ignoring documented). Merging.

mempalace/palace_graph.py

@@ -18,6 +18,8 @@ No external graph DB needed — built from ChromaDB metadata.
 import hashlib
 import json
 import os
+import threading
+import time
 from collections import Counter, defaultdict
 from datetime import datetime, timezone
 
@@ -25,6 +27,23 @@ from .config import MempalaceConfig
 from .palace import get_collection as _get_palace_collection
 from .palace import mine_lock
 
+# Module-level graph cache with TTL and write-invalidation.
+# Warm cache serves build_graph() in O(1); invalidate_graph_cache() clears on writes.
+_graph_cache_lock = threading.Lock()
+_graph_cache_nodes = None
+_graph_cache_edges = None
+_graph_cache_time = 0.0
+_GRAPH_CACHE_TTL = 60.0  # seconds — graph changes less often than metadata
+
+
+def invalidate_graph_cache():
+    """Clear the graph cache. Called from mcp_server.py on writes."""
+    global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
+    with _graph_cache_lock:
+        _graph_cache_nodes = None
+        _graph_cache_edges = None
+        _graph_cache_time = 0.0
+
 
 def _get_collection(config=None):
     config = config or MempalaceConfig()
@@ -42,10 +61,25 @@ def build_graph(col=None, config=None):
     """
     Build the palace graph from ChromaDB metadata.
 
+    Returns cached result if fresh (within TTL). Cache is invalidated
+    on writes via invalidate_graph_cache(). Thread-safe via _graph_cache_lock.
+
+    Note: warm cache ignores ``col`` and ``config`` arguments — this is
+    intentional for the MCP server's single-palace use case. Callers
+    switching collections should call ``invalidate_graph_cache()`` first.
+
     Returns:
         nodes: dict of {room: {wings: set, halls: set, count: int}}
         edges: list of {room, wing_a, wing_b, hall} — one per tunnel crossing
     """
+    global _graph_cache_nodes, _graph_cache_edges, _graph_cache_time
+    now = time.time()
+    # NOTE: warm cache ignores col/config args — intentional for the MCP server's
+    # single-palace use case. Callers switching collections must invalidate first.
+    with _graph_cache_lock:
+        if _graph_cache_nodes is not None and (now - _graph_cache_time) < _GRAPH_CACHE_TTL:
+            return _graph_cache_nodes, _graph_cache_edges
+
     if col is None:
         col = _get_collection(config)
     if not col:
@@ -101,6 +135,14 @@ def build_graph(col=None, config=None):
             "dates": sorted(data["dates"])[-5:] if data["dates"] else [],
         }
 
+    # Only cache non-empty graphs so new data is picked up immediately
+    # when the palace is first populated.
+    if nodes:
+        with _graph_cache_lock:
+            _graph_cache_nodes = nodes
+            _graph_cache_edges = edges
+            _graph_cache_time = time.time()
+
     return nodes, edges