Use asgiref local storage for backend cache (fixes #11)

CHANGELOG.md

@@ -1,3 +1,9 @@
+## 3.6.0
+
+* Switched to [uv](https://docs.astral.sh/uv/) for development
+* Ensure backend cache is thread-safe (#11)
+
+
 ## 3.5.0
 
 * Removed `TriggerType.snapshot` so that delete triggers will now record snapshots.

pyproject.toml

@@ -21,6 +21,7 @@ classifiers = [
 
 [dependency-groups]
 dev = [
+    "django",
     "psycopg[binary]",
 ]
 

src/history/backends/__init__.py

@@ -1,12 +1,13 @@
+from asgiref.local import Local
 from django.conf import settings
 from django.db import DEFAULT_DB_ALIAS
 
-backend_cache = {}
+backend_cache = Local()
 
 
 def get_backend(alias=DEFAULT_DB_ALIAS, cls=None, cache=True):
-    if cache and alias in backend_cache:
-        return backend_cache[alias]
+    if cache and hasattr(backend_cache, alias):
+        return getattr(backend_cache, alias)
 
     if cls:
         backend = cls(alias)
@@ -24,7 +25,7 @@ def get_backend(alias=DEFAULT_DB_ALIAS, cls=None, cache=True):
             raise ValueError("Unsupported database engine: {}".format(engine))
 
     if cache:
-        backend_cache[alias] = backend
+        setattr(backend_cache, alias, backend)
 
     return backend