Rename reducer to event handler.

README.md

@@ -50,13 +50,14 @@ from esser.entities import Entity
 from esser.registry import register
 from items import commands
 from items import receivers
-from items.reducer import ItemReducer
+from items.event_handler import ItemEventHandler
 
 
 @register
 class Item(Entity):
-
-    reducer = ItemReducer()
+
+    # set event handler to aggregate state
+    event_handler = ItemEventHandler()
     created = commands.CreateItem()
     price_updated = commands.UpdatePrice()
 
@@ -87,14 +88,14 @@ class UpdatePrice(BaseCommand):
     }
 ```
 
-#### Add reducer to fold event stream
+#### Add event handler to fold event stream
 
-`items/reducer.py`
+`items/event_handler.py`
 
 ```python
-from esser.reducer import BaseReducer
+from esser.event_handler import BaseEventHandler
 
-class ItemReducer(BaseReducer):
+class ItemEventHandler(BaseEventHandler):
 
     def on_item_created(self, aggregate, next_event):
         return self.on_created(aggregate, next_event)

docs/source/api.rst

@@ -15,6 +15,7 @@ Entities / Aggregates
     :members:
     :undoc-members:
 
+
 Repositories
 ==================
 
@@ -32,7 +33,8 @@ DynamoDB
     :members:
     :undoc-members:
 
-Events
+
+Commands
 -----------------------
 
 .. autoclass:: esser.commands.BaseCommand
@@ -48,10 +50,10 @@ Events
     :undoc-members:
 
 
-Reducers
+Event Handlers
 ---------------------
 
-.. autoclass:: esser.reducer.BaseReducer
+.. autoclass:: esser.event_handler.BaseEventHandler
     :members:
     :undoc-members:
 

esser/entities.py

@@ -3,6 +3,7 @@
 from esser.commands import BaseCommand
 from esser.repositories.dynamodb import DynamoDBRepository
 from esser.utils import cached_property
+from esser.signals import event_pre_save
 
 
 class Entity(object):
@@ -46,12 +47,12 @@ def get_initial_state(self):
     def get_state_at(self, version):
         sequence = self.repository.get_events(version)
         initial_state = self.get_last_snapshot() or self.get_initial_state()
-        return reduce(self.reducer.reduce, sequence, initial_state)
+        return reduce(self.event_handler.apply, sequence, initial_state)
 
     def get_current_state(self):
         sequence = self.repository.get_all_events()
         initial_state = self.get_last_snapshot() or self.get_initial_state()
-        return reduce(self.reducer.reduce, sequence, initial_state)
+        return reduce(self.event_handler.apply, sequence, initial_state)
 
     current_state = property(get_current_state)
     cached_current_state = cached_property(get_current_state)

esser/reducer.py → esser/event_handler.py

@@ -1,7 +1,9 @@
 import re
 from esser.exceptions import AggregateDeleted
 
-re_camel_case = re.compile(r'(((?<=[a-z])[A-Z])|([A-Z](?![A-Z]|$)))')
+re_camel_case = re.compile(
+    r'(((?<=[a-z])[A-Z])|([A-Z](?![A-Z]|$)))'
+)
 
 
 def camel_case_to_spaces(value):
@@ -12,9 +14,22 @@ def camel_case_to_spaces(value):
     return re_camel_case.sub(r'_\1', value).strip('_').lower()
 
 
-class BaseReducer(object):
+class BaseEventHandler(object):
+    """
+    Responsible for building up the read state from all events
+    """
+
+    def apply(self, aggregate, next_event):
+        """Called by reduce(), this method should find the
+        corresponding handler based on the event name
+
+        Args:
+            aggregate (dict): current state of an aggregate
+            next_event (esser.repositories.base.Event): next event to apply
 
-    def reduce(self, aggregate, next_event):
+        Returns:
+            dict: aggregate state
+        """
         event_name = camel_case_to_spaces(next_event.event_type)
         self.update_version(aggregate, next_event)
         handler = getattr(self, 'on_%s' % event_name, None)

esser/exceptions.py

@@ -1,16 +1,20 @@
 
-class IntegrityError(Exception):
-    def __init__(self, errors, message='Duplicate hash and range keys'):
-        message = '%s: %s' % (message, errors)
-        super(IntegrityError, self).__init__(message)
+class EsserException(Exception):
+
+    def __init__(self, errors, message=None):
+        message = '%s: %s' % (message or self.message, errors)
+        super(EsserException, self).__init__(message)
         self.errors = errors
 
 
-class EventValidationException(Exception):
-    def __init__(self, errors, message='Event validation error'):
-        message = '%s: %s' % (message, errors)
-        super(EventValidationException, self).__init__(message)
-        self.errors = errors
+class IntegrityError(EsserException):
+
+    message = 'Duplicate hash and range keys'
+
+
+class EventValidationException(EsserException):
+
+    message = 'Event validation error'
 
 
 class AggregateDoesNotExist(Exception):

examples/basket/aggregate.py

@@ -1,10 +1,10 @@
 from esser.entities import Entity
-from esser.reducer import BaseReducer
+from esser.event_handler import BaseEventHandler
 from basket import commands
 from items.aggregate import Item
 
 
-class BasketReducer(BaseReducer):
+class BasketEventHandler(BaseEventHandler):
 
     def on_basket_created(self, aggregate, next_event):
         return self.on_created(aggregate, next_event)
@@ -23,7 +23,7 @@ class Basket(Entity):
     """
     Basket is an Aggregate root
     """
-    reducer = BasketReducer()
+    event_handler = BasketEventHandler()
     created = commands.CreateBasket()
     item_added = commands.AddItem()
     item_added_with_validation = commands.AddItemWithExistanceValidation()

examples/items/aggregate.py

@@ -1,11 +1,11 @@
 from esser.entities import Entity
-from esser.reducer import BaseReducer
+from esser.event_handler import BaseEventHandler
 from esser.registry import register
 from items import commands
 from items import receivers
 
 
-class ItemReducer(BaseReducer):
+class ItemEventHandler(BaseEventHandler):
 
     def on_item_created(self, aggregate, next_event):
         return self.on_created(aggregate, next_event)
@@ -25,7 +25,7 @@ def on_colors_added(self, aggregate, next_event):
 @register
 class Item(Entity):
 
-    reducer = ItemReducer()
+    event_handler = ItemEventHandler()
     price_updated = commands.UpdatePrice()
     colors_added = commands.AddColors()
     created = commands.CreateItem()