feat!: make set_provider non-blocking, add set_provider_and_wait (#595)

jonathannorris · toddbaert · web-flow · commit cbacef0093c0 · 2026-05-29T15:09:23.000-04:00
* feat!: make set_provider non-blocking, add set_provider_and_wait

Signed-off-by: Jonathan Norris &lt;jonathan.norris@dynatrace.com&gt;

* fix: ruff format signature collapse in api.py

Signed-off-by: Jonathan Norris &lt;jonathan.norris@dynatrace.com&gt;

* fix: use threading.Event in error event test to avoid flaky busy-wait

Signed-off-by: Jonathan Norris &lt;jonathan.norris@dynatrace.com&gt;

* fixup: pr feedback and additional checks

Signed-off-by: Todd Baert &lt;todd.baert@dynatrace.com&gt;

* fix: check active registration in stale-init guard, not _provider_status

Signed-off-by: Jonathan Norris &lt;jonathan.norris@dynatrace.com&gt;

* fixup: edge shutdown race

Signed-off-by: Todd Baert &lt;todd.baert@dynatrace.com&gt;

---------

Signed-off-by: Jonathan Norris &lt;jonathan.norris@dynatrace.com&gt;
Signed-off-by: Todd Baert &lt;todd.baert@dynatrace.com&gt;
Co-authored-by: Todd Baert &lt;todd.baert@dynatrace.com&gt;
diff --git a/README.md b/README.md
@@ -130,6 +130,15 @@ api.set_provider(NoOpProvider())
 open_feature_client = api.get_client()
 ```
 
+`set_provider()` is non-blocking: it registers the provider immediately and runs initialization in a background thread.
+Flag evaluations during the initialization window return the default value with a `PROVIDER_NOT_READY` error code.
+Use `set_provider_and_wait()` if you need to ensure the provider is ready before proceeding:
+
+```python
+# blocks until the provider is initialized (or raises on failure)
+api.set_provider_and_wait(NoOpProvider())
+```
+
 In some situations, it may be beneficial to register multiple providers in the same application.
 This is possible using [domains](#domains), which is covered in more detail below.
 
diff --git a/openfeature/api.py b/openfeature/api.py
@@ -33,6 +33,7 @@
     "remove_handler",
     "set_evaluation_context",
     "set_provider",
+    "set_provider_and_wait",
     "set_transaction_context",
     "set_transaction_context_propagator",
     "shutdown",
@@ -52,6 +53,13 @@ def set_provider(provider: FeatureProvider, domain: str | None = None) -> None:
         provider_registry.set_provider(domain, provider)
 
 
+def set_provider_and_wait(provider: FeatureProvider, domain: str | None = None) -> None:
+    if domain is None:
+        provider_registry.set_default_provider(provider, wait_for_init=True)
+    else:
+        provider_registry.set_provider(domain, provider, wait_for_init=True)
+
+
 def clear_providers() -> None:
     provider_registry.clear_providers()
     _event_support.clear()
diff --git a/openfeature/provider/_registry.py b/openfeature/provider/_registry.py
@@ -1,3 +1,5 @@
+import threading
+
 from openfeature._event_support import run_handlers_for_provider
 from openfeature.evaluation_context import EvaluationContext, get_evaluation_context
 from openfeature.event import (
@@ -13,77 +15,144 @@ class ProviderRegistry:
     _default_provider: FeatureProvider
     _providers: dict[str, FeatureProvider]
     _provider_status: dict[FeatureProvider, ProviderStatus]
+    _lock: threading.RLock
 
     def __init__(self) -> None:
+        self._lock = threading.RLock()
         self._default_provider = NoOpProvider()
         self._providers = {}
         self._provider_status = {
             self._default_provider: ProviderStatus.READY,
         }
 
-    def set_provider(self, domain: str, provider: FeatureProvider) -> None:
+    def set_provider(
+        self, domain: str, provider: FeatureProvider, wait_for_init: bool = False
+    ) -> None:
         if provider is None:
             raise GeneralError(error_message="No provider")
         if domain is None:
             raise GeneralError(error_message="No domain")
-        providers = self._providers
-        if domain in providers:
-            old_provider = providers[domain]
-            del providers[domain]
-            if (
-                old_provider != self._default_provider
-                and old_provider not in providers.values()
-            ):
-                self._shutdown_provider(old_provider)
-        if provider != self._default_provider and provider not in providers.values():
-            self._initialize_provider(provider)
-        providers[domain] = provider
+
+        old_provider: FeatureProvider | None = None
+        needs_init = False
+        with self._lock:
+            old_provider = self._providers.get(domain)
+            self._providers[domain] = provider
+            already_bound = provider is self._default_provider or any(
+                p is provider for d, p in self._providers.items() if d != domain
+            )
+            if not already_bound:
+                needs_init = True
+                self._provider_status[provider] = ProviderStatus.NOT_READY
+
+        if needs_init:
+            self._initialize_provider(provider, wait_for_init=wait_for_init)
+
+        # old-provider shutdown is always async so a hanging shutdown() cannot
+        # block set_provider.
+        if old_provider is not None and old_provider is not provider:
+            self._shutdown_if_unused(old_provider)
 
     def get_provider(self, domain: str | None) -> FeatureProvider:
         if domain is None:
             return self._default_provider
         return self._providers.get(domain, self._default_provider)
 
-    def set_default_provider(self, provider: FeatureProvider) -> None:
+    def set_default_provider(
+        self, provider: FeatureProvider, wait_for_init: bool = False
+    ) -> None:
         if provider is None:
             raise GeneralError(error_message="No provider")
-        if (
-            self._default_provider
-            and self._default_provider not in self._providers.values()
-        ):
-            self._shutdown_provider(self._default_provider)
-        self._default_provider = provider
 
-        if self._default_provider not in self._providers.values():
-            self._initialize_provider(provider)
+        old_provider: FeatureProvider | None = None
+        needs_init = False
+        with self._lock:
+            old_provider = self._default_provider
+            self._default_provider = provider
+            if (
+                provider is not old_provider
+                and provider not in self._providers.values()
+            ):
+                needs_init = True
+                self._provider_status[provider] = ProviderStatus.NOT_READY
+
+        if needs_init:
+            self._initialize_provider(provider, wait_for_init=wait_for_init)
+
+        if old_provider is not None and old_provider is not provider:
+            self._shutdown_if_unused(old_provider)
 
     def get_default_provider(self) -> FeatureProvider:
         return self._default_provider
 
     def clear_providers(self) -> None:
         self.shutdown()
-        self._providers.clear()
-        self._default_provider = NoOpProvider()
-        self._provider_status = {
-            self._default_provider: ProviderStatus.READY,
-        }
+        with self._lock:
+            self._providers.clear()
+            self._default_provider = NoOpProvider()
+            self._provider_status = {
+                self._default_provider: ProviderStatus.READY,
+            }
 
     def shutdown(self) -> None:
-        for provider in {self._default_provider, *self._providers.values()}:
+        with self._lock:
+            providers = {self._default_provider, *self._providers.values()}
+
+        for provider in providers:
             self._shutdown_provider(provider)
 
     def _get_evaluation_context(self) -> EvaluationContext:
         return get_evaluation_context()
 
-    def _initialize_provider(self, provider: FeatureProvider) -> None:
+    def _initialize_provider(
+        self, provider: FeatureProvider, wait_for_init: bool
+    ) -> None:
         provider.attach(self.dispatch_event)
+        if not hasattr(provider, "initialize"):
+            # nothing async to do; dispatch READY synchronously.
+            self.dispatch_event(
+                provider, ProviderEvent.PROVIDER_READY, ProviderEventDetails()
+            )
+            return
+        if wait_for_init:
+            self._run_initialize(provider, raise_on_error=True)
+            return
+
+        thread = threading.Thread(
+            target=self._run_initialize,
+            args=(provider,),
+            kwargs={"raise_on_error": False},
+            daemon=True,
+        )
+        thread.start()
+
+    def _run_initialize(
+        self, provider: FeatureProvider, raise_on_error: bool = False
+    ) -> None:
         try:
-            if hasattr(provider, "initialize"):
-                provider.initialize(self._get_evaluation_context())
+            provider.initialize(self._get_evaluation_context())
+            # stale init: provider was replaced/shut down during initialize(); drop event.
+            # Check active registration, not _provider_status, since replaced providers
+            # remain in _provider_status until async shutdown pops them.
+            with self._lock:
+                if (
+                    provider is not self._default_provider
+                    and provider not in self._providers.values()
+                ):
+                    return
             self.dispatch_event(
                 provider, ProviderEvent.PROVIDER_READY, ProviderEventDetails()
             )
         except Exception as err:
+            # stale init: provider was replaced/shut down during initialize(); drop event.
+            # Check active registration, not _provider_status, since replaced providers
+            # remain in _provider_status until async shutdown pops them.
+            with self._lock:
+                if (
+                    provider is not self._default_provider
+                    and provider not in self._providers.values()
+                ):
+                    return
             error_code = (
                 err.error_code
                 if isinstance(err, OpenFeatureError)
@@ -97,12 +166,42 @@ def _initialize_provider(self, provider: FeatureProvider) -> None:
                     error_code=error_code,
                 ),
             )
-
-    def _shutdown_provider(self, provider: FeatureProvider) -> None:
+            if raise_on_error:
+                raise
+
+    def _shutdown_if_unused(self, provider: FeatureProvider) -> None:
+        # only shut down if no longer referenced. shutdown runs on a daemon
+        # thread so a hanging shutdown() cannot block the caller.
+        with self._lock:
+            if provider is self._default_provider:
+                return
+            if provider in self._providers.values():
+                return
+
+        thread = threading.Thread(
+            target=self._shutdown_provider,
+            args=(provider,),
+            kwargs={"abort_if_re_registered": True},
+            daemon=True,
+        )
+        thread.start()
+
+    def _shutdown_provider(
+        self, provider: FeatureProvider, abort_if_re_registered: bool = False
+    ) -> None:
         try:
             if hasattr(provider, "shutdown"):
                 provider.shutdown()
-            del self._provider_status[provider]
+            # if provider is being re-registered, leave its status and event wiring intact
+            if abort_if_re_registered:
+                with self._lock:
+                    if (
+                        provider is self._default_provider
+                        or provider in self._providers.values()
+                    ):
+                        return
+            with self._lock:
+                self._provider_status.pop(provider, None)
         except Exception as err:
             self.dispatch_event(
                 provider,
@@ -132,17 +231,18 @@ def _update_provider_status(
         event: ProviderEvent,
         details: ProviderEventDetails,
     ) -> None:
-        if event == ProviderEvent.PROVIDER_READY:
-            self._provider_status[provider] = ProviderStatus.READY
-        elif event == ProviderEvent.PROVIDER_STALE:
-            self._provider_status[provider] = ProviderStatus.STALE
-        elif event == ProviderEvent.PROVIDER_ERROR:
-            status = (
-                ProviderStatus.FATAL
-                if details.error_code == ErrorCode.PROVIDER_FATAL
-                else ProviderStatus.ERROR
-            )
-            self._provider_status[provider] = status
+        with self._lock:
+            if event == ProviderEvent.PROVIDER_READY:
+                self._provider_status[provider] = ProviderStatus.READY
+            elif event == ProviderEvent.PROVIDER_STALE:
+                self._provider_status[provider] = ProviderStatus.STALE
+            elif event == ProviderEvent.PROVIDER_ERROR:
+                status = (
+                    ProviderStatus.FATAL
+                    if details.error_code == ErrorCode.PROVIDER_FATAL
+                    else ProviderStatus.ERROR
+                )
+                self._provider_status[provider] = status
 
 
 provider_registry = ProviderRegistry()
diff --git a/tests/conftest.py b/tests/conftest.py
@@ -15,5 +15,5 @@ def clear_providers():
 
 @pytest.fixture()
 def no_op_provider_client():
-    api.set_provider(NoOpProvider())
+    api.set_provider_and_wait(NoOpProvider())
     return api.get_client()
diff --git a/tests/features/steps/metadata_steps.py b/tests/features/steps/metadata_steps.py
@@ -1,13 +1,13 @@
 from behave import given, then
 
-from openfeature.api import get_client, set_provider
+from openfeature.api import get_client, set_provider_and_wait
 from openfeature.provider.in_memory_provider import InMemoryProvider
 from tests.features.data import IN_MEMORY_FLAGS
 
 
 @given("a stable provider")
 def step_impl_stable_provider(context):
-    set_provider(InMemoryProvider(IN_MEMORY_FLAGS))
+    set_provider_and_wait(InMemoryProvider(IN_MEMORY_FLAGS))
     context.client = get_client()
 
 
diff --git a/tests/features/steps/steps.py b/tests/features/steps/steps.py
@@ -4,7 +4,7 @@
 
 from behave import given, then, when
 
-from openfeature.api import get_client, set_provider
+from openfeature.api import get_client, set_provider_and_wait
 from openfeature.client import OpenFeatureClient
 from openfeature.evaluation_context import EvaluationContext
 from openfeature.exception import ErrorCode
@@ -28,13 +28,13 @@ def step_impl_resolved_should_be(context, flag_type, key, expected_reason):
 
 @given("a provider is registered with cache disabled")
 def step_impl_provider_without_cache(context):
-    set_provider(InMemoryProvider(IN_MEMORY_FLAGS))
+    set_provider_and_wait(InMemoryProvider(IN_MEMORY_FLAGS))
     context.client = get_client()
 
 
 @given("a provider is registered")
 def step_impl_provider(context):
-    set_provider(InMemoryProvider(IN_MEMORY_FLAGS))
+    set_provider_and_wait(InMemoryProvider(IN_MEMORY_FLAGS))
     context.client = get_client()
 
 
diff --git a/tests/provider/test_registry.py b/tests/provider/test_registry.py
diff --git a/tests/test_api.py b/tests/test_api.py
