Implement poller and in-memory cache

MANIFEST.in

@@ -1,4 +1,4 @@
 include requirements.txt
 include README.md
 include requirements-test.txt
-include VERSION.txt
+include LICENSE

eppo_client/__init__.py

@@ -0,0 +1,71 @@
+from typing import Optional
+from eppo_client.client import EppoClient
+from eppo_client.config import Config
+from eppo_client.configuration_requestor import (
+    ExperimentConfigurationDto,
+    ExperimentConfigurationRequestor,
+)
+from eppo_client.configuration_store import ConfigurationStore
+from eppo_client.constants import MAX_CACHE_ENTRIES
+from eppo_client.http_client import HttpClient, SdkParams
+from eppo_client.read_write_lock import ReadWriteLock
+
+__version__ = "0.0.1"
+
+__client: Optional[EppoClient] = None
+__lock = ReadWriteLock()
+
+
+def init(config: Config) -> EppoClient:
+    """Initializes a global Eppo client instance
+
+    This method should be called once on application startup.
+    If invoked more than once, it will re-initialize the global client instance.
+    Use the :func:`eppo_client.get_instance()` method to access the client instance.
+
+    :param config: client configuration containing the API Key
+    :type config: Config
+    """
+    config._validate()
+    sdk_params = SdkParams(
+        apiKey=config.api_key, sdkName="python", sdkVersion=__version__
+    )
+    http_client = HttpClient(base_url=config.base_url, sdk_params=sdk_params)
+    config_store: ConfigurationStore[ExperimentConfigurationDto] = ConfigurationStore(
+        max_size=MAX_CACHE_ENTRIES
+    )
+    config_requestor = ExperimentConfigurationRequestor(
+        http_client=http_client, config_store=config_store
+    )
+    global __client
+    global __lock
+    try:
+        __lock.acquire_write()
+        if __client:
+            # if a client was already initialized, stop the background processes of the old client
+            __client._shutdown()
+        __client = EppoClient(config_requestor=config_requestor)
+        return __client
+    finally:
+        __lock.release_write()
+
+
+def get_instance() -> EppoClient:
+    """Used to access an initialized client instance
+
+    Use this method to get a client instance for assigning variants.
+    This method may only be called after invocation of :func:`eppo_client.init()`, otherwise it throws an exception.
+
+    :return: a shared client instance
+    :rtype: EppoClient
+    """
+    global __client
+    global __lock
+    try:
+        __lock.acquire_read()
+        if __client:
+            return __client
+        else:
+            raise Exception("init() must be called before get_instance()")
+    finally:
+        __lock.release_read()

eppo_client/client.py

@@ -3,13 +3,21 @@
     ExperimentConfigurationDto,
     ExperimentConfigurationRequestor,
 )
+from eppo_client.constants import POLL_INTERVAL_MILLIS, POLL_JITTER_MILLIS
+from eppo_client.poller import Poller
 from eppo_client.shard import get_shard, is_in_shard_range
 from eppo_client.validation import validate_not_blank
 
 
 class EppoClient:
-    def __init__(self, config_requestor: ExperimentConfigurationRequestor) -> None:
+    def __init__(self, config_requestor: ExperimentConfigurationRequestor):
         self.__config_requestor = config_requestor
+        self.__poller = Poller(
+            interval_millis=POLL_INTERVAL_MILLIS,
+            jitter_millis=POLL_JITTER_MILLIS,
+            callback=config_requestor.fetch_and_store_configurations,
+        )
+        self.__poller.start()
 
     def assign(self, subject: str, experiment_key: str) -> Optional[str]:
         """Maps a subject to a variation for a given experiment
@@ -24,7 +32,7 @@ def assign(self, subject: str, experiment_key: str) -> Optional[str]:
         if (
             experiment_config is None
             or not experiment_config.enabled
-            or not self.is_in_experiment_sample(
+            or not self._is_in_experiment_sample(
                 subject, experiment_key, experiment_config
             )
         ):
@@ -42,7 +50,13 @@ def assign(self, subject: str, experiment_key: str) -> Optional[str]:
             None,
         )
 
-    def is_in_experiment_sample(
+    def _shutdown(self):
+        """Stops all background processes used by the client
+        Do not use the client after calling this method.
+        """
+        self.__poller.stop()
+
+    def _is_in_experiment_sample(
         self,
         subject: str,
         experiment_key: str,

eppo_client/configuration_requestor.py

@@ -1,8 +1,13 @@
-from typing import List, Optional
+import logging
+from typing import Dict, List, Optional, cast
 from eppo_client.base_model import SdkBaseModel
+from eppo_client.configuration_store import ConfigurationStore
+from eppo_client.http_client import HttpClient, HttpRequestError
 
 from eppo_client.shard import ShardRange
 
+logger = logging.getLogger(__name__)
+
 
 class VariationDto(SdkBaseModel):
     name: str
@@ -14,12 +19,40 @@ class ExperimentConfigurationDto(SdkBaseModel):
     percent_exposure: float
     enabled: bool
     variations: List[VariationDto]
-    name: str
+    name: Optional[str]
+
+
+RAC_ENDPOINT = "/randomized_assignment/config"
 
 
 class ExperimentConfigurationRequestor:
+    def __init__(
+        self,
+        http_client: HttpClient,
+        config_store: ConfigurationStore[ExperimentConfigurationDto],
+    ):
+        self.__http_client = http_client
+        self.__config_store = config_store
+
     def get_configuration(
         self, experiment_key: str
     ) -> Optional[ExperimentConfigurationDto]:
-        # TODO: implement this method
-        return None
+        if self.__http_client.is_unauthorized():
+            raise ValueError("Unauthorized: please check your API key")
+        return self.__config_store.get_configuration(experiment_key)
+
+    def fetch_and_store_configurations(self) -> Dict[str, ExperimentConfigurationDto]:
+        try:
+            configs = cast(
+                dict, self.__http_client.get(RAC_ENDPOINT).get("experiments", {})
+            )
+            for exp_key, exp_config in configs.items():
+                configs[exp_key] = ExperimentConfigurationDto(**exp_config)
+            self.__config_store.set_configurations(configs)
+            return configs
+        except HttpRequestError as e:
+            logger.error("Error retrieving assignment configurations: " + str(e))
+            if e.is_recoverable():
+                return {}
+            else:
+                raise e  # caught by the polling task; causes assignment polling to stop

eppo_client/configuration_store.py

@@ -0,0 +1,29 @@
+from typing import Dict, Optional, TypeVar, Generic
+from cachetools import LRUCache
+
+from eppo_client.read_write_lock import ReadWriteLock
+
+T = TypeVar("T")
+
+
+class ConfigurationStore(Generic[T]):
+    def __init__(self, max_size: int):
+        self.__cache: LRUCache = LRUCache(maxsize=max_size)
+        self.__lock = ReadWriteLock()
+
+    def get_configuration(self, key: str) -> Optional[T]:
+        try:
+            self.__lock.acquire_read()
+            return self.__cache[key]
+        except KeyError:
+            return None  # key does not exist
+        finally:
+            self.__lock.release_read()
+
+    def set_configurations(self, configs: Dict[str, T]):
+        try:
+            self.__lock.acquire_write()
+            for key, config in configs.items():
+                self.__cache[key] = config
+        finally:
+            self.__lock.release_write()

eppo_client/constants.py

@@ -0,0 +1,8 @@
+# configuration cache
+MAX_CACHE_ENTRIES = 1000  # arbitrary; the caching library requires a max limit
+
+# poller
+SECOND_MILLIS = 1000
+MINUTE_MILLIS = 60 * SECOND_MILLIS
+POLL_JITTER_MILLIS = 30 * SECOND_MILLIS
+POLL_INTERVAL_MILLIS = 5 * MINUTE_MILLIS

eppo_client/http_client.py

@@ -0,0 +1,68 @@
+from typing import Any
+from requests.exceptions import Timeout
+from requests.adapters import HTTPAdapter, Retry
+from http import HTTPStatus
+
+import requests
+
+from eppo_client.base_model import SdkBaseModel
+
+
+class SdkParams(SdkBaseModel):
+    # attributes are camelCase because that's what the backend endpoint expects
+    apiKey: str
+    sdkName: str
+    sdkVersion: str
+
+
+class HttpRequestError(Exception):
+    def __init__(self, message: str, status_code: int):
+        self.status_code = status_code
+        super().__init__(message)
+
+    def is_recoverable(self) -> bool:
+        if self.status_code >= 400 and self.status_code < 500:
+            return (
+                self.status_code == HTTPStatus.TOO_MANY_REQUESTS
+                or self.status_code == HTTPStatus.REQUEST_TIMEOUT
+            )
+        return True
+
+
+REQUEST_TIMEOUT_SECONDS = 2
+# Retry reference: https://urllib3.readthedocs.io/en/latest/reference/urllib3.util.html#module-urllib3.util.retry
+# This applies only to failed DNS lookups and connection timeouts,
+# never to requests where data has made it to the server.
+MAX_RETRIES = Retry(total=3, backoff_factor=1)
+
+
+class HttpClient:
+    def __init__(self, base_url: str, sdk_params: SdkParams):
+        self.__base_url = base_url
+        self.__sdk_params = sdk_params
+        self.__session = requests.Session()
+        self.__session.mount("https://", HTTPAdapter(max_retries=MAX_RETRIES))
+        self.__is_unauthorized = False
+
+    def is_unauthorized(self) -> bool:
+        return self.__is_unauthorized
+
+    def get(self, resource: str) -> Any:
+        try:
+            response = self.__session.get(
+                self.__base_url + resource,
+                params=self.__sdk_params.dict(),
+                timeout=REQUEST_TIMEOUT_SECONDS,
+            )
+            self.__is_unauthorized = response.status_code == HTTPStatus.UNAUTHORIZED
+            if response.status_code != HTTPStatus.OK:
+                raise self._get_http_error(response.status_code, resource)
+            return response.json()
+        except Timeout:
+            raise self._get_http_error(HTTPStatus.REQUEST_TIMEOUT, resource)
+
+    def _get_http_error(self, status_code: int, resource: str) -> HttpRequestError:
+        return HttpRequestError(
+            "HTTP {} error while requesting resource {}".format(status_code, resource),
+            status_code=status_code,
+        )

eppo_client/poller.py

@@ -0,0 +1,38 @@
+import logging
+from multiprocessing import Event
+from random import randrange
+from threading import Thread
+from typing import Callable
+
+logger = logging.getLogger(__name__)
+
+
+class Poller:
+    def __init__(self, interval_millis: int, jitter_millis: int, callback: Callable):
+        self.__jitter_millis = jitter_millis
+        self.__interval = interval_millis
+        self.__stop_event = Event()
+        self.__callback = callback
+        self.__thread = Thread(target=self.poll, daemon=True)
+
+    def start(self):
+        self.__thread.start()
+
+    def stop(self):
+        self.__stop_event.set()
+
+    def is_stopped(self):
+        return self.__stop_event.is_set()
+
+    def poll(self):
+        while not self.is_stopped():
+            try:
+                self.__callback()
+            except Exception as e:
+                logger.error("Unexpected error running poll task: " + str(e))
+                break
+            self._wait_for_interval()
+
+    def _wait_for_interval(self):
+        interval_with_jitter = self.__interval - randrange(0, self.__jitter_millis)
+        self.__stop_event.wait(interval_with_jitter / 1000)

eppo_client/read_write_lock.py

@@ -0,0 +1,42 @@
+import threading
+
+# Copied from: https://www.oreilly.com/library/view/python-cookbook/0596001673/ch06s04.html
+
+
+class ReadWriteLock:
+    """A lock object that allows many simultaneous "read locks", but
+    only one "write lock." """
+
+    def __init__(self):
+        self._read_ready = threading.Condition(threading.Lock())
+        self._readers = 0
+
+    def acquire_read(self):
+        """Acquire a read lock. Blocks only if a thread has
+        acquired the write lock."""
+        self._read_ready.acquire()
+        try:
+            self._readers += 1
+        finally:
+            self._read_ready.release()
+
+    def release_read(self):
+        """Release a read lock."""
+        self._read_ready.acquire()
+        try:
+            self._readers -= 1
+            if not self._readers:
+                self._read_ready.notifyAll()
+        finally:
+            self._read_ready.release()
+
+    def acquire_write(self):
+        """Acquire a write lock. Blocks until there are no
+        acquired read or write locks."""
+        self._read_ready.acquire()
+        while self._readers > 0:
+            self._read_ready.wait()
+
+    def release_write(self):
+        """Release a write lock."""
+        self._read_ready.release()

requirements-test.txt

@@ -1,4 +1,5 @@
 tox
 pytest
 mypy
-google-cloud-storage
+google-cloud-storage
+httpretty

requirements.txt

@@ -1 +1,5 @@
-pydantic
+pydantic
+requests
+cachetools
+types-cachetools
+types-requests