AsyncRedisSaver Blob Error: Troubleshooting Aget_state_history

by Alex Johnson 63 views

Encountering errors while working with AsyncRedisSaver can be frustrating. One common issue is the AttributeError: 'Document' object has no attribute 'blob' that arises when calling the aget_state_history() function. This article delves into the root cause of this error and provides a detailed solution to resolve it, ensuring smooth operation of your LangGraph applications using Redis as a checkpointer backend.

Understanding the Problem: AttributeError with AsyncRedisSaver

When you're leveraging AsyncRedisSaver as the checkpointer backend within your LangGraph applications, the aget_state_history() function plays a crucial role in retrieving the history of states. However, you might encounter an AttributeError: 'Document' object has no attribute 'blob'. This error typically surfaces during the execution of aget_state_history() or any function that internally calls _abatch_load_pending_sends. Let's break down the error and understand its origin.

Tracing the Error

The error message usually appears within the traceback, pinpointing the location within the LangGraph library. A typical traceback might look like this:

File "/.venv/lib/python3.13/site-packages/langgraph/pregel/main.py", line 1409, in aget_state_history
 for checkpoint_tuple in [
 ^
 ...<4 lines>...
 ]:
 ^
File "/.venv/lib/python3.13/site-packages/langgraph/checkpoint/redis/aio.py", line 788, in alist
 pending_sends_map = await self._abatch_load_pending_sends(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 pending_sends_batch_keys
 ^^^^^^^^^^^^^^^^^^^^
 )
 ^
File "/.venv/lib/python3.13/site-packages/langgraph/checkpoint/redis/aio.py", line 1759, in _abatch_load_pending_sends
 results_map[batch_key] = [(d.type, d.blob) for d in sorted_docs]
 ^^^^^^
AttributeError: 'Document' object has no attribute 'blob'

This traceback indicates that the error occurs within the _abatch_load_pending_sends function in the langgraph/checkpoint/redis/aio.py file. Specifically, the issue arises when trying to access the blob attribute of a Document object.

Root Cause Analysis

The core of the problem lies in how the asynchronous version of the Redis saver (AsyncRedisSaver) handles the retrieval of pending sends compared to its synchronous counterpart. The asynchronous implementation was missing a crucial step in accessing the blob data. The Document object, which represents data retrieved from Redis, may not always have the blob attribute directly accessible. Instead, the blob data might be stored under a JSON path (e.g., $.blob) within the document.

The Solution: Applying Consistent Logic

To effectively resolve this AttributeError, the asynchronous version of the Redis saver needs to adopt the same logic used in the synchronous version. This involves handling both direct attribute access and JSON path access when retrieving the blob data.

Implementing the Fix

The proposed solution involves modifying the _abatch_load_pending_sends function in langgraph/checkpoint/redis/aio.py. The key is to ensure that the code attempts to retrieve the blob data using both direct attribute access (doc.blob) and JSON path access (doc.$.blob). Here’s the code snippet that implements this fix:

# langgraph/checkpoint/redis/__init__.py#L1253
 batch_query = FilterQuery(
 filter_expression=thread_filter
 & ns_filter
 & checkpoint_filter
 & channel_filter,
 return_fields=[
 "checkpoint_id",
 "type",
 "$.blob",
 "task_path",
 "task_id",
 "idx",
 ],
 num_results=1000,  # Increased limit for batch loading
 )

 batch_results = await self.checkpoint_writes_index.search(batch_query)

 # Group results by parent checkpoint ID
 writes_by_checkpoint: Dict[str, List[Any]] = {}
 for doc in batch_results.docs:
 parent_checkpoint_id = from_storage_safe_id(doc.checkpoint_id)
 if parent_checkpoint_id not in writes_by_checkpoint:
 writes_by_checkpoint[parent_checkpoint_id] = []
 writes_by_checkpoint[parent_checkpoint_id].append(doc)

 # Sort and format results for each parent checkpoint
 for parent_checkpoint_id in parent_checkpoint_ids:
 batch_key = (thread_id, checkpoint_ns, parent_checkpoint_id)
 writes = writes_by_checkpoint.get(parent_checkpoint_id, [])

 # Sort results by task_path, task_id, idx
 sorted_writes = sorted(
 writes,
 key=lambda x: (
 getattr(x, "task_path", ""),
 getattr(x, "task_id", ""),
 getattr(x, "idx", 0),
 ),
 )

 # Extract type and blob pairs
 # Handle both direct attribute access and JSON path access
 results_map[batch_key] = [
 (
 getattr(doc, "type", ""),
 getattr(doc, "$.blob", getattr(doc, "blob", b"")),
 )
 for doc in sorted_writes
 ]

Explanation of the Fix

  1. Filter Query: The code constructs a FilterQuery to retrieve documents from Redis. The return_fields parameter specifies the fields to be returned, including $.blob, which represents the JSON path to the blob data.
  2. Batch Results: The checkpoint_writes_index.search(batch_query) call fetches the documents from Redis based on the filter query.
  3. Grouping by Checkpoint ID: The retrieved documents are grouped by their parent checkpoint ID to facilitate processing.
  4. Sorting Results: Within each group, the documents are sorted based on task_path, task_id, and idx to maintain consistency.
  5. Extracting Type and Blob: The crucial part of the fix lies in how the type and blob data are extracted. The code uses getattr(doc, "$.blob", getattr(doc, "blob", b"")) to handle both cases:
    • It first attempts to retrieve the blob data using the JSON path $.blob.
    • If that fails (i.e., the $.blob attribute is not found), it falls back to directly accessing the blob attribute (doc.blob).
    • If neither is found, it defaults to an empty byte string (b"").

This approach ensures that the code can handle documents where the blob data is stored either as a direct attribute or under a JSON path, resolving the AttributeError.

Verifying the Solution

After applying the fix, it’s essential to verify that the issue is resolved and that no side effects are introduced. You can do this by running the same code that previously triggered the error. If the fix is successful, aget_state_history() should execute without raising the AttributeError, and you should be able to retrieve the state history correctly.

Contributing the Fix

If you've successfully implemented and verified the fix, consider contributing it back to the LangGraph project. This helps other users benefit from your solution and improves the overall stability of the library. You can contribute by submitting a pull request (PR) on the LangGraph repository. A pull request involves:

  1. Forking the Repository: Create a copy of the LangGraph repository in your GitHub account.
  2. Making the Changes: Implement the fix in your forked repository.
  3. Testing the Changes: Ensure that the fix works as expected and doesn’t introduce any new issues.
  4. Submitting the Pull Request: Propose your changes to the main LangGraph repository by creating a pull request.

The LangGraph maintainers will review your pull request, provide feedback, and, if everything looks good, merge your changes into the main codebase.

Conclusion

The AttributeError: 'Document' object has no attribute 'blob' when calling aget_state_history() with AsyncRedisSaver can be a roadblock, but understanding the root cause and applying the appropriate fix can quickly resolve it. By ensuring that the code handles both direct attribute access and JSON path access for retrieving the blob data, you can ensure the smooth operation of your LangGraph applications using Redis as a checkpointer backend. Remember to verify your solution and consider contributing it back to the project to help the community.

For more information on LangGraph and Redis integration, you can refer to the official documentation and community resources. You might also find helpful information on general debugging techniques and contributing to open-source projects. For more in-depth information about Redis and its capabilities, check out the official Redis documentation. This resource provides comprehensive details about Redis features, commands, and best practices.