platypush/plugins/variable/__init__.py

@@ -1,11 +1,45 @@
-from typing import Collection, Dict, Iterable, Optional, Union
+from functools import wraps
+from threading import Event, RLock
+from typing import Any, Callable, Collection, Dict, Iterable, Optional, Union
 from typing_extensions import override
 
-from platypush.entities import EntityManager
+from platypush.entities import EntityManager, get_entities_engine
 from platypush.entities.variables import Variable
 from platypush.plugins import Plugin, action
 
 
+# pylint: disable=protected-access
+def ensure_initialized(f: Callable[..., Any]):
+    """
+    Ensures that the entities engine has been initialized before
+    reading/writing the db.
+
+    It also performs lazy initialization of the variables in the local cache.
+    """
+
+    @wraps(f)
+    def wrapper(*args, **kwargs):
+        self: VariablePlugin = args[0]
+
+        if not self._initialized.is_set():
+            with self._init_lock:
+                get_entities_engine(timeout=20)
+
+                if not self._initialized.is_set():
+                    self._initialized.set()
+                    with self._db.get_session() as session:
+                        self._db_vars.update(
+                            {  # type: ignore
+                                str(var.name): var.value
+                                for var in session.query(Variable).all()
+                            }
+                        )
+
+        return f(*args, **kwargs)
+
+    return wrapper
+
+
 class VariablePlugin(Plugin, EntityManager):
     """
     This plugin allows you to manipulate context variables that can be
@@ -17,18 +51,15 @@ class VariablePlugin(Plugin, EntityManager):
     def __init__(self, **kwargs):
         super().__init__(**kwargs)
 
-        db = self._db
         self._db_vars: Dict[str, Optional[str]] = {}
         """ Local cache for db variables. """
-
-        with db.get_session() as session:
-            self._db_vars.update(
-                {  # type: ignore
-                    str(var.name): var.value for var in session.query(Variable).all()
-                }
-            )
+        self._initialized = Event()
+        """ Lazy initialization event for the _db_vars map. """
+        self._init_lock = RLock()
+        """ Lock for the _db_vars map initialization. """
 
     @action
+    @ensure_initialized
     def get(self, name: Optional[str] = None, default_value=None):
         """
         Get the value of a variable by name from the local db.
@@ -45,6 +76,7 @@ class VariablePlugin(Plugin, EntityManager):
         )
 
     @action
+    @ensure_initialized
     def set(self, **kwargs):
         """
         Set a variable or a set of variables on the local db.
@@ -57,6 +89,7 @@ class VariablePlugin(Plugin, EntityManager):
         return kwargs
 
     @action
+    @ensure_initialized
     def delete(self, name: str):
         """
         Delete a variable from the database.
@@ -76,6 +109,7 @@ class VariablePlugin(Plugin, EntityManager):
         return True
 
     @action
+    @ensure_initialized
     def unset(self, name: str):
         """
         Unset a variable by name if it's set on the local db.
@@ -89,6 +123,7 @@ class VariablePlugin(Plugin, EntityManager):
         return self.set(**{name: None})
 
     @action
+    @ensure_initialized
     def mget(self, name: str):
         """
         Get the value of a variable by name from Redis.
@@ -100,6 +135,7 @@ class VariablePlugin(Plugin, EntityManager):
         return self._redis.mget([name])
 
     @action
+    @ensure_initialized
     def mset(self, **kwargs):
         """
         Set a variable or a set of variables on Redis.
@@ -112,6 +148,7 @@ class VariablePlugin(Plugin, EntityManager):
         return kwargs
 
     @action
+    @ensure_initialized
     def munset(self, name: str):
         """
         Unset a Redis variable by name if it's set
@@ -122,6 +159,7 @@ class VariablePlugin(Plugin, EntityManager):
         return self._redis.delete(name)
 
     @action
+    @ensure_initialized
     def expire(self, name: str, expire: int):
         """
         Set a variable expiration on Redis
@@ -157,6 +195,7 @@ class VariablePlugin(Plugin, EntityManager):
 
     @override
     @action
+    @ensure_initialized
     def status(self, *_, **__):
         variables = {
             name: value for name, value in self._db_vars.items() if value is not None