Add claim check recipe.

Based on temporalio/ai-cookbook#9

Author: robholland

cookbook/claim-check-pattern-python.mdx

@@ -0,0 +1,383 @@
+---
+title: Claim Check Pattern with Temporal
+description: Use the Claim Check pattern to efficiently handle large payloads by storing them externally and passing only keys through Temporal.
+tags: [foundations, python, patterns]
+source: https://github.com/temporalio/ai-cookbook/tree/main/foundations/claim_check_pattern_python
+---
+
+The Claim Check pattern enables efficient handling of large payloads by storing
+them externally and passing only keys through Temporal workflows and activities.
+
+This pattern is particularly useful when:
+- Working with large files, images, or datasets
+- Processing bulk data operations
+- Handling payloads that exceed Temporal's size limits
+- Improving performance by reducing payload serialization overhead
+
+## How the Claim Check Pattern Works
+
+The Claim Check pattern implements a `PayloadCodec` that:
+
+1. **Encode**: Replaces large payloads with unique keys and stores the original
+   data in external storage (Redis, S3, etc.)
+2. **Decode**: Retrieves the original payload using the key when needed
+
+This allows Temporal workflows to operate with small, lightweight keys instead
+of large payloads, while maintaining transparent access to the full data through
+automatic encoding/decoding.
+
+## Claim Check Codec Implementation
+
+The `ClaimCheckCodec` class implements the `PayloadCodec` interface to handle
+the encoding and decoding of payloads.
+
+```python
+import uuid
+import redis.asyncio as redis
+from typing import Iterable, List
+
+from temporalio.api.common.v1 import Payload
+from temporalio.converter import PayloadCodec
+
+class ClaimCheckCodec(PayloadCodec):
+    def __init__(self, redis_host: str = "localhost", redis_port: int = 6379):
+        self.redis_client = redis.Redis(host=redis_host, port=redis_port)
+
+    async def encode(self, payloads: Iterable[Payload]) -> List[Payload]:
+        """Replace large payloads with keys and store original data in Redis."""
+        out: List[Payload] = []
+        for payload in payloads:
+            encoded = await self.encode_payload(payload)
+            out.append(encoded)
+        return out
+
+    async def decode(self, payloads: Iterable[Payload]) -> List[Payload]:
+        """Retrieve original payloads from Redis using stored keys."""
+        out: List[Payload] = []
+        for payload in payloads:
+            if payload.metadata.get("temporal.io/claim-check-codec", b"").decode() != "v1":
+                # Not a claim-checked payload, pass through unchanged
+                out.append(payload)
+                continue
+
+            redis_key = payload.data.decode("utf-8")
+            stored_data = await self.redis_client.get(redis_key)
+            if stored_data is None:
+                raise ValueError(f"Claim check key not found in Redis: {redis_key}")
+            
+            original_payload = Payload.FromString(stored_data)
+            out.append(original_payload)
+        return out
+
+    async def encode_payload(self, payload: Payload) -> Payload:
+        """Store payload in Redis and return a key-based payload."""
+        key = str(uuid.uuid4())
+        serialized_data = payload.SerializeToString()
+        
+        # Store the original payload data in Redis
+        await self.redis_client.set(key, serialized_data)
+        
+        # Return a lightweight payload containing only the key
+        return Payload(
+            metadata={
+                "encoding": b"claim-checked",
+                "temporal.io/claim-check-codec": b"v1",
+            },
+            data=key.encode("utf-8"),
+        )
+```
+
+## Claim Check Plugin
+
+The `ClaimCheckPlugin` integrates the codec with Temporal's client configuration.
+
+```python
+import os
+from temporalio.client import Plugin, ClientConfig
+from temporalio.converter import DataConverter
+
+from claim_check_codec import ClaimCheckCodec
+
+class ClaimCheckPlugin(Plugin):
+    def __init__(self):
+        self.redis_host = os.getenv("REDIS_HOST", "localhost")
+        self.redis_port = int(os.getenv("REDIS_PORT", "6379"))
+
+    def get_data_converter(self, config: ClientConfig) -> DataConverter:
+        """Configure the data converter with claim check codec."""
+        default_converter_class = config["data_converter"].payload_converter_class
+        claim_check_codec = ClaimCheckCodec(self.redis_host, self.redis_port)
+        
+        return DataConverter(
+            payload_converter_class=default_converter_class,
+            payload_codec=claim_check_codec
+        )
+
+    def configure_client(self, config: ClientConfig) -> ClientConfig:
+        """Apply the claim check configuration to the client."""
+        config["data_converter"] = self.get_data_converter(config)
+        return super().configure_client(config)
+```
+
+## Example: Large Data Processing Workflow
+
+This example demonstrates processing large datasets using the Claim Check pattern.
+
+```python
+from temporalio import workflow, activity
+from dataclasses import dataclass
+from typing import List
+import json
+
+@dataclass
+class LargeDataset:
+    """Represents a large dataset that would benefit from claim check pattern."""
+    data: List[dict]
+    metadata: dict
+
+@dataclass
+class ProcessingResult:
+    """Result of processing the large dataset."""
+    processed_count: int
+    summary: dict
+    errors: List[str]
+
+@activity.defn
+async def process_large_dataset(dataset: LargeDataset) -> ProcessingResult:
+    """Process a large dataset - this would normally cause payload size issues.
+    
+    This activity demonstrates how the claim check pattern allows processing
+    of large datasets without hitting Temporal's payload size limits.
+    
+    Note: For production workloads processing very large datasets, consider adding
+    activity heartbeats to prevent timeout issues during long-running processing.
+    
+    Args:
+        dataset: Large dataset to process
+        
+    Returns:
+        ProcessingResult with processing statistics and any errors
+    """
+    processed_count = 0
+    errors = []
+    summary = {"total_items": len(dataset.data)}
+    
+    for item in dataset.data:
+        try:
+            # Simulate processing each item
+            if "value" in item:
+                item["processed_value"] = item["value"] * 2
+                processed_count += 1
+            elif "text" in item:
+                # Simulate text processing
+                item["word_count"] = len(item["text"].split())
+                processed_count += 1
+        except Exception as e:
+            errors.append(f"Error processing item {item.get('id', 'unknown')}: {str(e)}")
+    
+    summary["processed_items"] = processed_count
+    summary["error_count"] = len(errors)
+    
+    return ProcessingResult(
+        processed_count=processed_count,
+        summary=summary,
+        errors=errors
+    )
+
+@workflow.defn
+class LargeDataProcessingWorkflow:
+    @workflow.run
+    async def run(self, dataset: LargeDataset) -> ProcessingResult:
+        """Process large dataset using claim check pattern."""
+        result = await workflow.execute_activity(
+            process_large_dataset,
+            dataset,
+            start_to_close_timeout=timedelta(minutes=10),
+            summary="Process large dataset"
+        )
+        return result
+```
+
+## Configuration
+
+Set environment variables to configure the Redis connection:
+
+```bash
+# Configure Redis connection (optional - defaults to localhost:6379)
+export REDIS_HOST=localhost
+export REDIS_PORT=6379
+```
+
+## Prerequisites
+
+- **Redis Server**: Required for external storage of large payloads
+
+## Running the Example
+
+1. Start Redis server:
+```bash
+redis-server
+```
+
+2. Start the Temporal Dev Server:
+```bash
+temporal server start-dev
+```
+
+3. Run the worker:
+```bash
+uv run python -m worker
+```
+
+4. Start execution:
+```bash
+uv run python -m start_workflow
+```
+
+## Codec Server for Web UI
+
+When using the Claim Check pattern, the Temporal Web UI will show encoded Redis
+keys instead of the actual payload data. This makes debugging and monitoring
+difficult since you can't see what data is being passed through your workflows.
+
+### The Problem
+
+Without a codec server, the Web UI displays raw claim check keys like:
+```
+abc123-def4-5678-9abc-def012345678
+```
+
+This provides no context about what data is stored or how to access it, making
+workflow debugging and monitoring challenging.
+
+### Our Solution: Lightweight Codec Server
+
+We've designed a codec server that provides helpful information without the
+risks of reading large payload data:
+
+#### Design Principles
+
+1. **No Data Reading**: The codec server never reads the actual payload data during decode operations
+2. **On-Demand Access**: Full data is available via a separate endpoint when needed
+3. **Simple & Safe**: Just provides the Redis key and a link - no assumptions about data content
+4. **Performance First**: Zero impact on Web UI performance
+
+#### What It Shows
+
+Instead of raw keys, the Web UI displays:
+```
+"Claim check data (key: abc123-def4-5678-9abc-def012345678) - View at: http://localhost:8081/view/abc123-def4-5678-9abc-def012345678"
+```
+
+This gives you:
+- **Clear identification**: You know this is claim check data
+- **Redis key**: The actual key used for storage
+- **Direct access**: Click the URL to view the full payload data
+
+### Running the Codec Server
+
+1. Start the codec server:
+```bash
+uv run python -m codec_server
+```
+
+2. Configure the Temporal Web UI to use the codec server. For `temporal server
+   start-dev`, see the [Temporal documentation on configuring codec
+   servers](https://docs.temporal.io/production-deployment/data-encryption#set-your-codec-server-endpoints-with-web-ui-and-cli)
+   for the appropriate configuration method.
+
+3. Access the Temporal Web UI and you'll see helpful summaries instead of raw keys.
+
+### Why This Approach?
+
+#### Avoiding Common Pitfalls
+
+**❌ What we DON'T do:**
+- Parse or analyze payload data (could be huge or malformed)
+- Attempt to summarize content (assumes data structure)
+- Read data during decode operations (performance impact)
+
+**✅ What we DO:**
+- Provide the Redis key for manual inspection
+- Offer a direct link to view full data when needed
+- Keep the Web UI responsive with minimal information
+
+#### Benefits
+
+- **Performance**: No Redis calls during Web UI operations
+- **Safety**: No risk of parsing problematic data
+- **Flexibility**: Works with any data type or size
+- **Debugging**: Full data accessible when needed via `/view/{key}` endpoint
+
+### Configuration Details
+
+The codec server implements the Temporal codec server protocol with two endpoints:
+
+- **`/decode`**: Returns helpful text with Redis key and view URL
+- **`/view/{key}`**: Serves the raw payload data for inspection
+
+When you click the view URL, you'll see the complete payload data as stored in
+Redis, formatted appropriately for text or binary content.
+
+## Considerations
+
+### Performance Trade-offs
+
+- **Benefits**: Reduces payload size, improves workflow performance, enables handling of large data
+- **Costs**: Additional network calls to external storage, potential latency increase
+
+### Storage Backend Options
+
+While this example uses Redis, production systems typically use:
+- **AWS S3**: For AWS environments
+- **Google Cloud Storage**: For GCP environments  
+- **Azure Blob Storage**: For Azure environments
+- **Redis**: For development and testing
+
+### Activity Heartbeats
+
+For production workloads processing very large datasets, consider implementing
+activity heartbeats to prevent timeout issues:
+
+```python
+@activity.defn
+async def process_large_dataset(dataset: LargeDataset) -> ProcessingResult:
+    total_items = len(dataset.data)
+    
+    for i, item in enumerate(dataset.data):
+        # Send heartbeat every 100 items to prevent timeout
+        if i % 100 == 0:
+            await activity.heartbeat(f"Processed {i}/{total_items} items")
+        
+        # Process item...
+```
+
+This ensures Temporal knows the activity is still making progress during
+long-running operations.
+
+### Error Handling
+
+The codec includes error handling for:
+- Missing keys in storage
+- Storage connection failures
+- Serialization/deserialization errors
+
+### Cleanup
+
+Consider implementing cleanup strategies for stored data:
+- TTL (Time To Live) for automatic expiration
+- Manual cleanup workflows
+- Lifecycle policies for cloud storage
+
+## Best Practices
+
+1. **Enable globally**: The claim check pattern applies to all payloads when
+   enabled, so consider the performance impact across your entire system
+2. **Monitor storage**: Track storage usage and costs since all payloads will be
+   stored externally
+3. **Implement cleanup**: Prevent storage bloat with appropriate cleanup
+   strategies
+4. **Test thoroughly**: Verify the pattern works correctly with your specific
+   data types and doesn't introduce unexpected latency
+5. **Consider alternatives**: Evaluate if data compression or other
+   optimizations might be sufficient before implementing claim check