fix: use AliasChoices for REDIS_URL env var fallback (#35)

REDIS_URL was being ignored because config was read in two places:
- decorator.py checked REDIS_URL via os.environ.get()
- RedisBackendConfig used pydantic-settings with CACHEKIT_ prefix

The connection pool always read from RedisBackendConfig.from_env()
which only checked CACHEKIT_REDIS_URL, falling back to localhost:6379.

Fix uses pydantic-settings AliasChoices for proper env var priority:
CACHEKIT_REDIS_URL > REDIS_URL > localhost:6379 (default)

This is the declarative pydantic-settings approach vs manual os.environ.

Author: 27Bslash6

src/cachekit/backends/redis/config.py

@@ -7,7 +7,7 @@
 
 from __future__ import annotations
 
-from pydantic import Field
+from pydantic import AliasChoices, Field
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -56,7 +56,8 @@ class RedisBackendConfig(BaseSettings):
 
     redis_url: str = Field(
         default="redis://localhost:6379",
-        description="Redis connection URL",
+        validation_alias=AliasChoices("CACHEKIT_REDIS_URL", "REDIS_URL"),
+        description="Redis connection URL (env: CACHEKIT_REDIS_URL or REDIS_URL)",
     )
     connection_pool_size: int = Field(
         default=10,
@@ -76,7 +77,11 @@ class RedisBackendConfig(BaseSettings):
     def from_env(cls) -> RedisBackendConfig:
         """Create Redis configuration from environment variables.
 
-        Reads CACHEKIT_REDIS_URL, CACHEKIT_CONNECTION_POOL_SIZE, etc.
+        Priority (via AliasChoices): CACHEKIT_REDIS_URL > REDIS_URL > default
+
+        This allows standard 12-factor app conventions (REDIS_URL) while
+        supporting namespaced configuration (CACHEKIT_REDIS_URL) for
+        multi-service deployments.
 
         Returns:
             RedisBackendConfig instance loaded from environment
@@ -86,8 +91,11 @@ def from_env(cls) -> RedisBackendConfig:
 
             .. code-block:: bash
 
+                # Option 1: Namespaced (takes priority)
                 export CACHEKIT_REDIS_URL="redis://localhost:6379"
-                export CACHEKIT_CONNECTION_POOL_SIZE=20
+
+                # Option 2: Standard 12-factor convention
+                export REDIS_URL="redis://localhost:6379"
 
             .. code-block:: python
 

src/cachekit/config/decorator.py

@@ -107,9 +107,10 @@ def _resolve_backend(explicit_backend: object = _UNSET) -> BaseBackend | None:
     if _default_backend is not None:
         return _default_backend
 
-    # Tier 3: Auto-create from REDIS_URL env var
-    redis_url = os.environ.get("REDIS_URL")
-    if redis_url:
+    # Tier 3: Auto-create from env var (CACHEKIT_REDIS_URL > REDIS_URL)
+    # Check if either env var is set as signal to create Redis backend
+    # Actual URL resolution handled by RedisBackendConfig via AliasChoices
+    if os.environ.get("CACHEKIT_REDIS_URL") or os.environ.get("REDIS_URL"):
         # Lazy import to avoid circular dependency
         from cachekit.backends.provider import CacheClientProvider
         from cachekit.backends.redis import RedisBackend
@@ -118,7 +119,7 @@ def _resolve_backend(explicit_backend: object = _UNSET) -> BaseBackend | None:
         # Inject client_provider explicitly (follows Dependency Injection Principle)
         container = DIContainer()
         client_provider = container.get(CacheClientProvider)
-        return RedisBackend(redis_url, client_provider=client_provider)
+        return RedisBackend(client_provider=client_provider)
 
     # No backend configured - fail fast with helpful message
     raise ConfigurationError(

tests/unit/test_config_env_fallback.py

@@ -93,3 +93,52 @@ def test_backend_and_cache_configs_independent(self, monkeypatch):
         # Verify no cross-contamination
         assert not hasattr(cache_config, "redis_url")
         assert hasattr(cache_config, "default_ttl")
+
+
+class TestRedisUrlAliasChoicesPriority:
+    """Test AliasChoices priority for redis_url field.
+
+    The redis_url field uses AliasChoices to support both:
+    - CACHEKIT_REDIS_URL (namespaced, takes priority)
+    - REDIS_URL (standard 12-factor convention, fallback)
+    """
+
+    def test_only_redis_url_set(self, monkeypatch):
+        """REDIS_URL alone should work (12-factor convention)."""
+        monkeypatch.delenv("CACHEKIT_REDIS_URL", raising=False)
+        monkeypatch.setenv("REDIS_URL", "redis://generic:6379")
+
+        config = RedisBackendConfig()
+        assert config.redis_url == "redis://generic:6379"
+
+    def test_only_redis_url_via_from_env(self, monkeypatch):
+        """REDIS_URL works via from_env() factory method."""
+        monkeypatch.delenv("CACHEKIT_REDIS_URL", raising=False)
+        monkeypatch.setenv("REDIS_URL", "redis://factory:6379")
+
+        config = RedisBackendConfig.from_env()
+        assert config.redis_url == "redis://factory:6379"
+
+    def test_only_cachekit_redis_url_set(self, monkeypatch):
+        """CACHEKIT_REDIS_URL alone should work (namespaced)."""
+        monkeypatch.delenv("REDIS_URL", raising=False)
+        monkeypatch.setenv("CACHEKIT_REDIS_URL", "redis://cachekit:6379")
+
+        config = RedisBackendConfig()
+        assert config.redis_url == "redis://cachekit:6379"
+
+    def test_both_set_cachekit_wins(self, monkeypatch):
+        """CACHEKIT_REDIS_URL takes priority over REDIS_URL."""
+        monkeypatch.setenv("CACHEKIT_REDIS_URL", "redis://cachekit-priority:6379")
+        monkeypatch.setenv("REDIS_URL", "redis://generic-fallback:6379")
+
+        config = RedisBackendConfig()
+        assert config.redis_url == "redis://cachekit-priority:6379"
+
+    def test_neither_set_uses_default(self, monkeypatch):
+        """Falls back to localhost:6379 when neither env var is set."""
+        monkeypatch.delenv("CACHEKIT_REDIS_URL", raising=False)
+        monkeypatch.delenv("REDIS_URL", raising=False)
+
+        config = RedisBackendConfig()
+        assert config.redis_url == "redis://localhost:6379"