feat: flag tracking event listener mode

Author: dsun0720

README.md

@@ -140,6 +140,40 @@ if client.initialize:
 
 > Note that if evaluation called before Go SDK client initialized, you set the wrong flag key/user for the evaluation or the related feature flag is not found, SDK will return the default value you set. The `fbclient.common_types.EvalDetail` will explain the details of the latest evaluation including error raison.
 
+### Flag Tracking
+
+`fbclient.client.FBClient.flag_tracker` registers a listener to be notified of feature flag changes in general.
+
+Note that a flag value change listener is bound to a specific user and flag key.
+
+The flag value change listener will be notified whenever the SDK receives any change to any feature flag's configuration,
+or to a user segment that is referenced by a feature flag. To register a flag value change listener, use `add_flag_value_may_changed_listener` or `add_flag_value_changed_listener`
+
+When you track a flag change using `add_flag_value_may_changed_listener`, this does not necessarily mean the flag's value has changed for any particular flag, only that some part of the flag configuration was changed so that it *_MAY_* return a different value than it previously returned for some user.
+
+If you want to track a flag whose value *_MUST_* be changed, `add_flag_value_changed_listener` will register a listener that will be notified if and only if the flag value changes.
+
+Change notices only work if the SDK is actually connecting to FeatBit feature flag center.
+If the SDK is in offline mode, then it cannot know when there is a change, because flags are read on an as-needed basis.
+
+```python
+if client.initialize:
+    #  flag value may be changed
+    client.flag_tracker.add_flag_value_may_changed_listener(flag_key, user, callback_fn)
+    #  flag value must be changed
+    client.flag_tracker.add_flag_value_changed_listener(flag_key, user, callback_fn)
+
+```
+`flag_key`: the key of the feature flag to track
+
+`user`: the user to evaluate the flag value
+
+`callback_fn`: the function to be called for the flag value change
+* the first argument is the flag key
+* the second argument is the latest flag value
+
+
+
 ### Offline Mode
 
 In some situations, you might want to stop making remote calls to FeatBit. Here is how:

fbclient/client.py

@@ -12,7 +12,9 @@
                                 REASON_USER_NOT_SPECIFIED, Evaluator)
 from fbclient.event_processor import DefaultEventProcessor, NullEventProcessor
 from fbclient.event_types import FlagEvent, Metric, MetricEvent, UserEvent
+from fbclient.flag_change_notification import FlagTracker
 from fbclient.interfaces import DataUpdateStatusProvider
+from fbclient.notice_broadcaster import NoticeBroadcater
 from fbclient.status import DataUpdateStatusProviderImpl
 from fbclient.streaming import Streaming, _data_to_dict
 from fbclient.update_processor import NullUpdateProcessor
@@ -71,6 +73,9 @@ def __init__(self, config: Config, start_wait: float = 15.):
         else:
             self._config.validate()
 
+        self._broadcaster = NoticeBroadcater()
+        self._flag_tracker = FlagTracker(self._broadcaster, self.variation)
+
         # init components
         # event processor
         self._event_processor = self._build_event_processor(config)
@@ -84,8 +89,7 @@ def __init__(self, config: Config, start_wait: float = 15.):
         self._update_status_provider = DataUpdateStatusProviderImpl(config.data_storage)
         # update processor
         update_processor_ready = threading.Event()
-        self._update_processor = self._build_update_processor(config, self._update_status_provider,
-                                                              update_processor_ready)
+        self._update_processor = self._build_update_processor(config, self._broadcaster, self._update_status_provider, update_processor_ready)
         self._update_processor.start()
 
         if start_wait > 0:
@@ -111,7 +115,7 @@ def _build_event_processor(self, config: Config):
 
         return DefaultEventProcessor(config, DefaultSender('insight', config, max_size=10))
 
-    def _build_update_processor(self, config: Config, update_status_provider, update_processor_event):
+    def _build_update_processor(self, config: Config, broadcaster: NoticeBroadcater, update_status_provider, update_processor_event):
         if config.update_processor_imp:
             log.debug("Using user-specified update processor: %s" % str(config.update_processor_imp))
             return config.update_processor_imp(config, update_status_provider, update_processor_event)
@@ -120,7 +124,7 @@ def _build_update_processor(self, config: Config, update_status_provider, update
             log.debug("Offline mode, SDK disable streaming data updating")
             return NullUpdateProcessor(config, update_status_provider, update_processor_event)
 
-        return Streaming(config, update_status_provider, update_processor_event)
+        return Streaming(config, broadcaster, update_status_provider, update_processor_event)
 
     @property
     def initialize(self) -> bool:
@@ -136,6 +140,15 @@ def initialize(self) -> bool:
     def update_status_provider(self) -> DataUpdateStatusProvider:
         return self._update_status_provider
 
+    @property
+    def flag_tracker(self) -> FlagTracker:
+        """
+        Returns an object for tracking changes in feature flag configurations.
+        The :class:`FlagTracker` contains methods for requesting notifications about feature flag changes using
+        an event listener model.
+        """
+        return self._flag_tracker
+
     def stop(self):
         """Releases all threads and network connections used by SDK.
 
@@ -145,6 +158,7 @@ def stop(self):
         self._data_storage.stop()
         self._update_processor.stop()
         self._event_processor.stop()
+        self._broadcaster.stop()
 
     def __enter__(self):
         return self

fbclient/evaluator.py

@@ -146,7 +146,7 @@ def _match_default_user_variation(self, flag: dict, user: FBUser) -> Optional[_E
 
     def _match_any_rule(self, user: FBUser, rule: dict) -> bool:
         # conditions cannot be empty
-        return all(self._process_condition(user, condiction) for condiction in rule['conditions'])
+        return all(self._process_condition(user, condition) for condition in rule['conditions'])
 
     def _process_condition(self, user: FBUser, condition: dict) -> bool:
         op = condition['op']

fbclient/flag_change_notification.py

@@ -0,0 +1,186 @@
+
+from abc import ABC, abstractmethod
+from typing import Any, Callable
+from fbclient.common_types import FBUser
+from fbclient.interfaces import Notice
+from fbclient.notice_broadcaster import NoticeBroadcater
+
+FLAG_CHANGE_NOTICE_TYPE = 'flag_change_notice'
+
+
+class FlagChangedNotice(Notice):
+    def __init__(self, flag_key: str):
+        self.__flag_key = flag_key
+
+    @property
+    def notice_type(self) -> str:
+        return FLAG_CHANGE_NOTICE_TYPE
+
+    @property
+    def flag_key(self) -> str:
+        return self.__flag_key
+
+
+class FlagChangedListener(ABC):
+    """
+    A notice listener that is notified when a feature flag's configuration has changed.
+
+    This is an abstract class. You need to implement your own listener by overriding the :func:`on_flag_change` method.
+
+    """
+    @abstractmethod
+    def on_flag_change(self, notice: FlagChangedNotice):
+        pass
+
+
+class FlagValueChangedListener(FlagChangedListener):
+    def __init__(self,
+                 flag_key: str,
+                 user: dict,
+                 evaluate_fn: Callable[[str, dict, Any], Any],
+                 flag_value_changed_fn: Callable[[str, Any], None]):
+        self.__flag_key = flag_key
+        self.__user = user
+        self.__evaluate_fn = evaluate_fn
+        self.__fn = flag_value_changed_fn
+        # record the flag value when the listener is created
+        self.__prvious_flag_value = self.__evaluate_fn(self.__flag_key, self.__user, None)
+
+    def on_flag_change(self, notice: FlagChangedNotice):
+        if notice.flag_key == self.__flag_key:
+            prev_flag_value = self.__prvious_flag_value
+            curr_flag_value = self.__evaluate_fn(self.__flag_key, self.__user, None)
+            if prev_flag_value != curr_flag_value:
+                self.__fn(self.__flag_key, curr_flag_value)
+                self.__prvious_flag_value = curr_flag_value
+
+
+class FlagValueMayChangedListener(FlagChangedListener):
+    def __init__(self,
+                 flag_key: str,
+                 user: dict,
+                 evaluate_fn: Callable[[str, dict, Any], Any],
+                 flag_value_changed_fn: Callable[[str, Any], None]):
+        self.__flag_key = flag_key
+        self.__user = user
+        self.__evaluate_fn = evaluate_fn
+        self.__fn = flag_value_changed_fn
+
+    def on_flag_change(self, notice: FlagChangedNotice):
+        if notice.flag_key == self.__flag_key:
+            curr_flag_value = self.__evaluate_fn(self.__flag_key, self.__user, None)
+            self.__fn(self.__flag_key, curr_flag_value)
+
+
+class FlagTracker:
+    """
+    A registry to register the flag change listeners in order to track changes in feature flag configurations.
+
+    The registered listerners only work if the SDK is actually connecting to FeatBit feature flag center.
+    If the SDK is only in offline mode then it cannot know when there is a change, because flags are read on an as-needed basis.
+
+    Application code never needs to initialize or extend this class directly.
+    """
+
+    def __init__(self,
+                 flag_change_broadcaster: NoticeBroadcater,
+                 evaluate_fn: Callable[[str, dict, Any], Any]):
+        """
+        :param flag_change_broadcaster: The broadcaster that broadcasts the flag change notices
+        :param evaluate_fn: The function to evaluate the flag value
+        """
+        self.__broadcater = flag_change_broadcaster
+        self.__evaluate_fn = evaluate_fn
+
+    def add_flag_value_changed_listener(self,
+                                        flag_key: str,
+                                        user: dict,
+                                        flag_value_changed_fn: Callable[[str, Any], None]) -> FlagValueChangedListener:
+        """
+        Registers a listener to be notified of a change in a specific feature flag's value for a specific FeatBit user.
+
+        The listener will be notified whenever the SDK receives any change to any feature flag's configuration,
+        or to a user segment that is referenced by a feature flag.
+
+        When you call this method, it first immediately evaluates the feature flag. It then uses :class:`FlagChangeListener` to start listening for feature flag configuration
+        changes, and whenever the specified feature flag changes, it re-evaluates the flag for the same user. It then calls :class:`FlagValueChangeListener`
+        if and only if the resulting value has changed.
+
+        :param flag_key: The key of the feature flag to track
+        :param user: The user to evaluate the flag value
+        :param flag_value_changed_fn: The function to be called only when this flag value changes
+            * the first argument is the flag key
+            * the second argument is the latest flag value, this value must be different from the previous value
+
+        :return: A listener object that can be used to remove it later on.
+        """
+
+        # check flag key
+        if not isinstance(flag_key, str) or not flag_key:
+            raise ValueError('flag_key must be a non-empty string')
+        # check user
+        FBUser.from_dict(user)
+        # check flag_value_changed_fn
+        if not isinstance(flag_value_changed_fn, Callable) or not flag_value_changed_fn:
+            raise ValueError('flag_value_changed_fn must be a callable function')
+
+        listener = FlagValueChangedListener(flag_key, user, self.__evaluate_fn, flag_value_changed_fn)
+        self.add_flag_changed_listener(listener)
+        return listener
+
+    def add_flag_value_may_changed_listener(self,
+                                            flag_key: str,
+                                            user: dict,
+                                            flag_value_changed_fn: Callable[[str, Any], None]) -> FlagValueMayChangedListener:
+        """
+        Registers a listener to be notified of a change in a specific feature flag's value for a specific FeatBit user.
+
+        The listener will be notified whenever the SDK receives any change to any feature flag's configuration,
+        or to a user segment that is referenced by a feature flag.
+
+        Note that this does not necessarily mean the flag's value has changed for any particular flag,
+        only that some part of the flag configuration was changed so that it may return a different value than it previously returned for some user.
+
+        If you want to track flag value changes,use :func:`add_flag_value_changed_listener instead.
+
+        :param flag_key: The key of the feature flag to track
+        :param user: The user to evaluate the flag value
+        :param flag_value_changed_fn: The function to be called only if any changes to a specific flag
+            * the first argument is the flag key
+            * the second argument is the latest flag value, this value may be same as the previous value
+
+        :return: A listener object that can be used to remove it later on.
+
+        """
+
+        # check flag key
+        if not isinstance(flag_key, str) or not flag_key:
+            raise ValueError('flag_key must be a non-empty string')
+        # check user
+        FBUser.from_dict(user)
+        # check flag_value_changed_fn
+        if not isinstance(flag_value_changed_fn, Callable) or not flag_value_changed_fn:
+            raise ValueError('flag_value_changed_fn must be a callable function')
+
+        listener = FlagValueMayChangedListener(flag_key, user, self.__evaluate_fn, flag_value_changed_fn)
+        self.add_flag_changed_listener(listener)
+        return listener
+
+    def add_flag_changed_listener(self, listener: FlagChangedListener):
+        """
+        Registers a listener to be notified of feature flag changes in general.
+
+        The listener will be notified whenever the SDK receives any change to any feature flag's configuration,
+        or to a user segment that is referenced by a feature flag.
+
+        :param listener: The listener to be registered. The :class:`FlagChangedListner` is an abstract class. You need to implement your own listener.
+        """
+        self.__broadcater.add_listener(FLAG_CHANGE_NOTICE_TYPE, listener.on_flag_change)  # type: ignore
+
+    def remove_flag_change_notifier(self, listener: FlagChangedListener):
+        """
+        Unregisters a listener so that it will no longer be notified of feature flag changes.
+
+        :param listener: The listener to be unregistered. The listener must be the same object that was passed to :func:`add_flag_changed_listner` or :func:`add_flag_value_changed_listerner`
+        """
+        self.__broadcater.remove_listener(FLAG_CHANGE_NOTICE_TYPE, listener.on_flag_change)  # type: ignore

fbclient/interfaces.py

@@ -265,3 +265,17 @@ def stop(self):
         Shuts down the connection to feature flag center
         """
         pass
+
+
+class Notice(ABC):
+    """
+    This is not an insight event to be sent to FeatBit Flag Center; it is a notice to notify the SDK that something has happened,
+    such as flag values updated
+    """
+    @property
+    @abstractmethod
+    def notice_type(self) -> str:
+        """
+        Returns the type of this notice
+        """
+        pass

fbclient/notice_broadcaster.py

@@ -0,0 +1,57 @@
+
+from queue import Empty, Queue
+from threading import Thread
+from typing import Callable
+from fbclient.interfaces import Notice
+
+from fbclient.utils import log
+
+
+class NoticeBroadcater:
+    def __init__(self):
+        self.__notice_queue = Queue()
+        self.__closed = False
+        self.__listeners = {}
+        self.__thread = Thread(daemon=True, target=self.__run)
+        log.debug('notice broadcaster starting...')
+        self.__thread.start()
+
+    def add_listener(self, notice_type: str, listener: Callable[[Notice], None]):
+        if isinstance(notice_type, str) and notice_type.strip() and listener is not None:
+            log.debug('add a listener for notice type %s' % notice_type)
+            if notice_type not in self.__listeners:
+                self.__listeners[notice_type] = []
+            self.__listeners[notice_type].append(listener)
+
+    def remove_listener(self, notice_type: str, listener: Callable[[Notice], None]):
+        if notice_type in self.__listeners and listener is not None:
+            log.debug('remove a listener for notice type %s' % notice_type)
+            notifiers = self.__listeners[notice_type]
+            if not notifiers:
+                del self.__listeners[notice_type]
+            else:
+                notifiers.remove(listener)
+
+    def broadcast(self, notice: Notice):
+        self.__notice_queue.put(notice)
+
+    def stop(self):
+        log.debug('notice broadcaster stopping...')
+        self.__closed = True
+        self.__thread.join()
+
+    def __run(self):
+        while not self.__closed:
+            try:
+                notice = self.__notice_queue.get(block=True, timeout=1)
+                self.__notice_process(notice)
+            except Empty:
+                pass
+
+    def __notice_process(self, notice: Notice):
+        if notice.notice_type in self.__listeners:
+            for listerner in self.__listeners[notice.notice_type]:
+                try:
+                    listerner(notice)
+                except Exception as e:
+                    log.exception('FB Python SDK: unexpected error in handle notice %s: %s' % (notice.notice_type, str(e)))

fbclient/status.py

@@ -36,6 +36,9 @@ def __handle_exception(self, error: Exception, error_type: str, message: str):
         log.exception('FB Python SDK: Data Storage error: %s, UpdateProcessor will attempt to receive the data' % str(error))
         self.update_state(State.interrupted_state(error_type, message))
 
+    def get_all(self, kind: Category) -> Mapping[str, dict]:
+        return self.__storage.get_all(kind)
+
     @property
     def initialized(self) -> bool:
         return self.__storage.initialized