Python: [Purview] Add Caching and background processing in Python Purview Middleware (#1844)

* [PythonPurview] Add Caching and background processing

* [PythonPurview] Updates based on comments

python/samples/getting_started/purview_agent/README.md

@@ -2,9 +2,14 @@
 
 This getting-started sample shows how to attach Microsoft Purview policy evaluation to an Agent Framework `ChatAgent` using the **middleware** approach.
 
+**What this sample demonstrates:**
 1. Configure an Azure OpenAI chat client
 2. Add Purview policy enforcement middleware (`PurviewPolicyMiddleware`)
-3. Run a short conversation and observe prompt / response blocking behavior
+3. Add Purview policy enforcement at the chat client level (`PurviewChatPolicyMiddleware`)
+4. Implement a custom cache provider for advanced caching scenarios
+5. Run conversations and observe prompt / response blocking behavior
+
+**Note:** Caching is **automatic** and enabled by default with sensible defaults (30-minute TTL, 200MB max size).
 
 ---
 ## 1. Setup
@@ -20,8 +25,6 @@ This getting-started sample shows how to attach Microsoft Purview policy evaluat
 | `PURVIEW_CERT_PATH` | Yes (when cert auth on) | Path to your .pfx certificate |
 | `PURVIEW_CERT_PASSWORD` | Optional | Password for encrypted certs |
 
-*A demo default exists in code for illustration only—always set your own value.
-
 ### 2. Auth Modes Supported
 
 #### A. Interactive Browser Authentication (default)
@@ -42,7 +45,7 @@ $env:PURVIEW_CERT_PATH = "C:\path\to\cert.pfx"
 $env:PURVIEW_CERT_PASSWORD = "optional-password"
 ```
 
-Certificate steps (summary): create / register app, generate certificate, upload public key, export .pfx with private key, grant required Graph / Purview permissions.
+Certificate steps (summary): create / register entra app, generate certificate, upload public key, export .pfx with private key, grant required Graph / Purview permissions.
 
 ---
 
@@ -61,18 +64,39 @@ If interactive auth is used, a browser window will appear the first time.
 
 ## 4. How It Works
 
+The sample demonstrates three different scenarios:
+
+### A. Agent Middleware (`run_with_agent_middleware`)
 1. Builds an Azure OpenAI chat client (using the environment endpoint / deployment)
 2. Chooses credential mode (certificate vs interactive)
 3. Creates `PurviewPolicyMiddleware` with `PurviewSettings`
 4. Injects middleware into the agent at construction
 5. Sends two user messages sequentially
 6. Prints results (or policy block messages)
+7. Uses default caching automatically
 
+### B. Chat Client Middleware (`run_with_chat_middleware`)
+1. Creates a chat client with `PurviewChatPolicyMiddleware` attached directly
+2. Policy evaluation happens at the chat client level rather than agent level
+3. Demonstrates an alternative integration point for Purview policies
+4. Uses default caching automatically
+
+### C. Custom Cache Provider (`run_with_custom_cache_provider`)
+1. Implements the `CacheProvider` protocol with a custom class (`SimpleDictCacheProvider`)
+2. Shows how to add custom logging and metrics to cache operations
+3. The custom provider must implement three async methods:
+   - `async def get(self, key: str) -> Any | None`
+   - `async def set(self, key: str, value: Any, ttl_seconds: int | None = None) -> None`
+   - `async def remove(self, key: str) -> None`
+
+**Policy Behavior:**
 Prompt blocks set a system-level message: `Prompt blocked by policy` and terminate the run early. Response blocks rewrite the output to `Response blocked by policy`.
 
 ---
 
-## 5. Code Snippet (Middleware Injection)
+## 5. Code Snippets
+
+### Agent Middleware Injection
 
 ```python
 agent = ChatAgent(
@@ -80,9 +104,41 @@ agent = ChatAgent(
 	instructions="You are good at telling jokes.",
 	name="Joker",
 	middleware=[
-		PurviewPolicyMiddleware(credential, PurviewSettings(app_name="Sample App", default_user_id="<guid>"))
+		PurviewPolicyMiddleware(credential, PurviewSettings(app_name="Sample App"))
 	],
 )
 ```
 
+### Custom Cache Provider Implementation
+
+This is only needed if you want to integrate with external caching systems.
+
+```python
+class SimpleDictCacheProvider:
+    """Custom cache provider that implements the CacheProvider protocol."""
+    
+    def __init__(self) -> None:
+        self._cache: dict[str, Any] = {}
+    
+    async def get(self, key: str) -> Any | None:
+        """Get a value from the cache."""
+        return self._cache.get(key)
+    
+    async def set(self, key: str, value: Any, ttl_seconds: int | None = None) -> None:
+        """Set a value in the cache."""
+        self._cache[key] = value
+    
+    async def remove(self, key: str) -> None:
+        """Remove a value from the cache."""
+        self._cache.pop(key, None)
+
+# Use the custom cache provider
+custom_cache = SimpleDictCacheProvider()
+middleware = PurviewPolicyMiddleware(
+    credential,
+    PurviewSettings(app_name="Sample App"),
+    cache_provider=custom_cache,
+)
+```
+
 ---