Entities engine improvements

- Added cache support to prevent duplicate EntityUpdateEvents
- The cache is smartly pre-populated and kept up-to-date, so it's
  possible to trigger events as soon as the entities are published by
  the plugin (not only when the records are flushed to the internal db)

platypush/entities/_engine.py

@@ -1,11 +1,11 @@
 from logging import getLogger
 from queue import Queue, Empty
-from threading import Thread, Event
+from threading import Thread, Event, RLock
 from time import time
-from typing import Iterable, List
+from typing import Iterable, List, Optional
 
 from sqlalchemy import and_, or_
-from sqlalchemy.orm import Session
+from sqlalchemy.orm import Session, make_transient
 
 from platypush.context import get_bus
 from platypush.message.event.entities import EntityUpdateEvent
@@ -23,6 +23,98 @@ class EntitiesEngine(Thread):
         self.logger = getLogger(name=obj_name)
         self._queue = Queue()
         self._should_stop = Event()
+        self._entities_cache_lock = RLock()
+        self._entities_cache = {
+            'by_id': {},
+            'by_external_id_and_plugin': {},
+            'by_name_and_plugin': {},
+        }
+
+    def _get_db(self):
+        from platypush.context import get_plugin
+
+        db = get_plugin('db')
+        assert db
+        return db
+
+    def _get_cached_entity(self, entity: Entity) -> Optional[dict]:
+        if entity.id:
+            e = self._entities_cache['by_id'].get(entity.id)
+            if e:
+                return e
+
+        if entity.external_id and entity.plugin:
+            e = self._entities_cache['by_external_id_and_plugin'].get(
+                (entity.external_id, entity.plugin)
+            )
+            if e:
+                return e
+
+        if entity.name and entity.plugin:
+            e = self._entities_cache['by_name_and_plugin'].get(
+                (entity.name, entity.plugin)
+            )
+            if e:
+                return e
+
+    @staticmethod
+    def _cache_repr(entity: Entity) -> dict:
+        repr_ = entity.to_json()
+        repr_.pop('data', None)
+        repr_.pop('meta', None)
+        repr_.pop('created_at', None)
+        repr_.pop('updated_at', None)
+        return repr_
+
+    def _cache_entities(self, *entities: Entity, overwrite_cache=False):
+        for entity in entities:
+            e = self._cache_repr(entity)
+            if not overwrite_cache:
+                existing_entity = self._entities_cache['by_id'].get(entity.id)
+                if existing_entity:
+                    for k, v in existing_entity.items():
+                        if e.get(k) is None:
+                            e[k] = v
+
+            if entity.id:
+                self._entities_cache['by_id'][entity.id] = e
+            if entity.external_id and entity.plugin:
+                self._entities_cache['by_external_id_and_plugin'][
+                    (entity.external_id, entity.plugin)
+                ] = e
+            if entity.name and entity.plugin:
+                self._entities_cache['by_name_and_plugin'][
+                    (entity.name, entity.plugin)
+                ] = e
+
+    def _entity_has_changes(self, new_entity: Entity) -> bool:
+        with self._entities_cache_lock:
+            cached_entity = self._get_cached_entity(new_entity)
+            if cached_entity:
+                if cached_entity.get('id'):
+                    new_entity.id = cached_entity['id']
+                if cached_entity == self._cache_repr(new_entity):
+                    return False
+
+            if new_entity.id:
+                self._cache_entities(new_entity)
+
+            return True
+
+    def _init_entities_cache(self):
+        with self._get_db().get_session() as session:
+            entities = session.query(Entity).all()
+            for entity in entities:
+                make_transient(entity)
+
+        with self._entities_cache_lock:
+            self._cache_entities(*entities, overwrite_cache=True)
+
+        self.logger.info('Entities cache initialized')
+
+    def _process_event(self, entity: Entity):
+        if self._entity_has_changes(entity):
+            get_bus().post(EntityUpdateEvent(entity=entity))
 
     def post(self, *entities: Entity):
         for entity in entities:
@@ -38,6 +130,7 @@ class EntitiesEngine(Thread):
     def run(self):
         super().run()
         self.logger.info('Started entities engine')
+        self._init_entities_cache()
 
         while not self.should_stop:
             msgs = []
@@ -53,6 +146,9 @@ class EntitiesEngine(Thread):
 
                 if msg:
                     msgs.append(msg)
+                    # Trigger an EntityUpdateEvent if there has
+                    # been a change on the entity state
+                    self._process_event(msg)
 
             if not msgs or self.should_stop:
                 continue
@@ -126,13 +222,12 @@ class EntitiesEngine(Thread):
         return new_entities
 
     def _process_entities(self, *entities: Entity):
-        from platypush.context import get_plugin
+        with self._get_db().get_session() as session:
-
-        with get_plugin('db').get_session() as session:  # type: ignore
             existing_entities = self._get_if_exist(session, entities)
             entities = self._merge_entities(entities, existing_entities)  # type: ignore
             session.add_all(entities)
             session.commit()
 
+        with self._entities_cache_lock:
             for entity in entities:
-            get_bus().post(EntityUpdateEvent(entity=entity))
+                self._cache_entities(entity, overwrite_cache=True)