TransferQueue: An asynchronous streaming data management module for efficient post-training

Paper | Zhihu

🎉 Overview

TransferQueue is a high-performance data storage and transfer module with panoramic data visibility and streaming scheduling capabilities, optimized for efficient dataflow in post-training workflows.

TransferQueue offers fine-grained, sample-level data management and load-balancing (on the way) capabilities, serving as a data gateway that decouples explicit data dependencies across computational tasks. This enables a divide-and-conquer approach, significantly simplifies the algorithm controller design.

🔄 Updates

• Dec 30, 2025: TransferQueue x verl integration is tested with the DAPO algorithm at scale (64 nodes, 1024 cards). It significantly optimizes host memory utilization and accelerates data transfers. Stay tuned for more details!
• Dec 20, 2025: 🔥 The official tutorial is released! Feel free to check it out.
• Nov 10, 2025: We disentangle the data retrieval logic from TransferQueueController PR#101. Now you can implement your own Sampler to control how to consume the data.
• Nov 5, 2025: We provide a KVStorageManager that simplifies the integration with KV-based storage backends PR#96. The first available KV-based backend is Yuanrong.
• Nov 4, 2025: Data partition capability is available in PR#98. Now you can define logical data partitions to manage your train/val/test datasets.
• Oct 25, 2025: We make storage backends pluggable in PR#66. You can try to integrate your own storage backend with TransferQueue now!
• Oct 21, 2025: Official integration into verl is ready verl/pulls/3649. Following PRs will optimize the single controller architecture by fully decoupling data & control flows.
• July 22, 2025: We present a series of Chinese blogs on Zhihu 1, 2.
• July 21, 2025: We started an RFC on verl community verl/RFC#2662.
• July 2, 2025: We publish the paper AsyncFlow.

🧩 Components

Control Plane: Panoramic Data Management

In the control plane, TransferQueueController tracks the production status and consumption status of each training sample as metadata. When all the required data fields are ready (i.e., written to the TransferQueueStorageManager), we know that this data sample can be consumed by downstream tasks.

For consumption status, we record the consumption records for each computational task (e.g., generate_sequences, compute_log_prob, etc.). Therefore, even when different computation tasks require the same data field, they can consume the data independently without interfering with each other.

To make the data retrieval process more customizable, we provide a Sampler class that allows users to define their own data retrieval and consumption logic. Refer to the Customize section for details.

In the future, we plan to support load-balancing and dynamic batching capabilities in the control plane. Additionally, we will support data management for disaggregated frameworks where each rank manages the data retrieval by itself, rather than coordinated by a single controller.

Data Plane: Distributed Data Storage

In the data plane, we provide a pluggable design that enables TransferQueue to integrate with different storage backends according to user requirements.

Specifically, we provide a TransferQueueStorageManager abstraction class that defines the core APIs as follows:

• async def put_data(self, data: TensorDict, metadata: BatchMeta) -> None
• async def get_data(self, metadata: BatchMeta) -> TensorDict
• async def clear_data(self, metadata: BatchMeta) -> None

This class encapsulates the core interaction logic within the TransferQueue system. You only need to write a simple subclass to integrate your own storage backend. Refer to the Customize section for details.

Currently, we support the following storage backends:

• SimpleStorageUnit: A basic CPU memory storage with minimal data format constraints and easy usability.
• Yuanrong (beta, #PR107, #PR96): An Ascend native data system that provides hierarchical storage interfaces including HBM/DRAM/SSD.
• Mooncake Store (alpha, #PR162): A high-performance, KV-based hierarchical storage that supports RDMA transport between GPU and DRAM.
• Ray Direct Transport (alpha, #PR167): Ray's new feature that allows Ray to store and pass objects directly between Ray actors.

Among them, SimpleStorageUnit serves as our default storage backend, coordinated by the AsyncSimpleStorageManager class. Each storage unit can be deployed on a separate node, allowing for distributed data management.

SimpleStorageUnit employs a 2D data structure as follows:

• Each row corresponds to a training sample, assigned a unique index within the corresponding global batch.
• Each column represents the input/output data fields for computational tasks.

This data structure design is motivated by the computational characteristics of the post-training process, where each training sample is generated in a relayed manner across task pipelines. It provides an accurate addressing capability, which allows fine-grained, concurrent data read/write operations in a streaming manner.

User Interface: Asynchronous & Synchronous Client

The interaction workflow of TransferQueue system is as follows:

1. A process sends a read request to the TransferQueueController.
2. TransferQueueController scans the production and consumption metadata for each sample (row), and dynamically assembles a micro-batch metadata according to the load-balancing policy. This mechanism enables sample-level data scheduling.
3. The process retrieves the actual data from distributed storage units using the metadata provided by the controller.

To simplify the usage of TransferQueue, we have encapsulated this process into AsyncTransferQueueClient and TransferQueueClient. These clients provide both asynchronous and synchronous interfaces for data transfer, allowing users to easily integrate TransferQueue into their framework.

In the future, we will provide a StreamingDataLoader interface for disaggregated frameworks as discussed in issue#85 and verl/RFC#2662. Leveraging this abstraction, each rank can automatically get its own data like DataLoader in PyTorch. The TransferQueue system will handle the underlying data scheduling and transfer logic caused by different parallelism strategies, significantly simplifying the design of disaggregated frameworks.

🔥 Showcases

General Usage

The primary interaction points are AsyncTransferQueueClient and TransferQueueClient, serving as the communication interface with the TransferQueue system.

Core interfaces:

• (async_)get_meta(data_fields: list[str], batch_size:int, partition_id: str, mode: str, task_name:str, sampling_config: Optional[dict[str, Any]]) -> BatchMeta
• (async_)get_data(metadata: BatchMeta) -> TensorDict
• (async_)put(data: TensorDict, metadata: Optional[BatchMeta], partition_id: Optional[str])
• (async_)clear_partition(partition_id: str) and (async_)clear_samples(metadata: BatchMeta)

Refer to our tutorial for detailed examples.

Collocated Example

verl

The primary motivation for integrating TransferQueue to verl now is to alleviate the data transfer bottleneck of the single controller RayPPOTrainer. Currently, all DataProto objects must be routed through RayPPOTrainer, resulting in a single point bottleneck of the whole post-training system.

Leveraging TransferQueue, we separate experience data transfer from metadata dispatch by

• Replacing DataProto with BatchMeta (metadata) and TensorDict (actual data) structures
• Preserving verl's original Dispatch/Collect logic via BatchMeta (maintaining single-controller debuggability)
• Accelerating data transfer by TransferQueue's distributed storage units

You may refer to the recipe, where we mimic the verl usage in both async & sync scenarios. Official integration to verl is also available now at verl/pulls/3649 (with subsequent PRs to further optimize the integration).

Disaggregated Example

Work in progress :)

🚀 Quick Start

Use Python package

pip install TransferQueue

Build wheel package from source code

Follow these steps to build and install:

1. Clone the source code from the GitHub repository

   git clone https://github.com/TransferQueue/TransferQueue/
   cd TransferQueue

2. Install dependencies

   pip install -r requirements.txt

3. Build and install

   python -m build --wheel
   pip install dist/*.whl

📊 Performance

Note: The above benchmark for TransferQueue is based on our naive SimpleStorageUnit backend. By introducing high-performance storage backends and optimizing serialization/deserialization, we expect to achieve even better performance. Warmly welcome contributions from the community!

For detailed performance benchmarks, please refer to this blog.

We also provide a stress test report that demonstrates 768 concurrent clients writing 1.4 TB of data into TransferQueue across 4 nodes. The system remains stable without any crashes or data loss, achieving 80% bandwidth.

🛠️ Customize TransferQueue

Define your own data retrieval logic

We provide a BaseSampler abstraction class, which defines the following interface:

@abstractmethod
def sample(
    self,
    ready_indexes: list[int],
    batch_size: int,
    *args: Any,
    **kwargs: Any,
) -> tuple[list[int], list[int]]:
    """Sample a batch of indices from the ready indices.

    Args:
        ready_indexes: List of global indices for which all required fields of the
        corresponding samples have been produced, and the samples are not labeled as
        consumed in the corresponding task.
        batch_size: Number of samples to select
        *args: Additional positional arguments for specific sampler implementations
        **kwargs: Additional keyword arguments for specific sampler implementations

    Returns:
        List of sampled global indices of length batch_size
        List of global indices of length batch_size that should be labeled as consumed
        (will never be retrieved in the future)

    Raises:
        ValueError: If batch_size is invalid or ready_indexes is insufficient
    """
    raise NotImplementedError("Subclasses must implement sample")

In this design, we separate data retrieval and data consumption through the two return values, which enables us to easily control sample replacement. We have implemented two reference designs: SequentialSampler and GRPOGroupNSampler.

The Sampler class or instance should be passed to the TransferQueueController during initialization. During each get_meta call, you can provide dynamic sampling parameters to the Sampler.

from transfer_queue import TransferQueueController, TransferQueueClient, GRPOGroupNSampler, process_zmq_server_info

# Option 1: Pass the sampler class to the TransferQueueController
controller = TransferQueueController.remote(GRPOGroupNSampler)

# Option 2: Pass the sampler instance to the TransferQueueController (if you need custom configuration)
your_own_sampler = YourOwnSampler(config)
controller = TransferQueueController.remote(your_own_sampler)

# Use the sampler
batch_meta = client.get_meta(
    data_fields=["input_ids", "attention_mask"],
    batch_size=8,
    partition_id="train_0",
    task_name="generate_sequences",
    sampling_config={"n_samples_per_prompt": 4}  # Put the required sampling parameters here
)

Refer to tutorial/04_custom_sampler.py for more details.

How to integrate a new storage backend

The data plane is organized as follows:

  transfer_queue/
  ├── storage/
  │   ├── __init__.py
  │   │── simple_backend.py             # Default distributed storage backend (SimpleStorageUnit) by TQ 
  │   ├── managers/                     # Managers are upper level interfaces that encapsulate the interaction logic with TQ system.
  │   │   ├── __init__.py
  │   │   ├──base.py                    # TransferQueueStorageManager, KVStorageManager
  │   │   ├──simple_backend_manager.py  # AsyncSimpleStorageManager
  │   │   ├──yuanrong_manager.py        # YuanrongStorageManager
  │   │   ├──mooncake_manager.py        # MooncakeStorageManager
  │   │   └──factory.py                 # TransferQueueStorageManagerFactory
  │   └── clients/                      # Clients are lower level interfaces that directly manipulate the target storage backend.
  │   │   ├── __init__.py
  │   │   ├── base.py                   # TransferQueueStorageKVClient
  │   │   ├── yuanrong_client.py        # YuanrongStorageClient
  │   │   ├── mooncake_client.py        # MooncakeStorageClient
  │   │   ├── ray_storage_client.py     # RayStorageClient
  │   │   └── factory.py                # TransferQueueStorageClientFactory

To integrate TransferQueue with a custom storage backend, start by implementing a subclass that inherits from TransferQueueStorageManager. This subclass acts as an adapter between the TransferQueue system and the target storage backend. For KV-based storage backends, you can simply inherit from KVStorageManager, which can serve as the general manager for all KV-based backends.

Distributed storage backends often come with their own native clients serving as the interface of the storage system. In such cases, a low-level adapter for this client can be written, following the examples provided in the storage/clients directory.

Factory classes are provided for both StorageManager and StorageClient to facilitate easy integration. Adding necessary descriptions of required parameters in the factory class helps enhance the overall user experience.

✏️ Contribution Guide

Contributions are warmly welcome!

New ideas, feature suggestions, and user experience feedback are all encouraged—feel free to submit issues or PRs. We will respond as soon as possible.

We recommend using pre-commit for better code format.

# install pre-commit
pip install pre-commit

# run the following command in your repo folder, then fix the check before committing your code
pre-commit install && pre-commit run --all-files --show-diff-on-failure --color=always

🛣️ Roadmap

• Support data rewrite for partial rollout & agentic post-training
• Provide a general storage abstraction layer TransferQueueStorageManager to manage distributed storage units, which simplifies Client design and makes it possible to introduce different storage backends (PR#66, issue#72)
• Implement AsyncSimpleStorageManager as the default storage backend based on the TransferQueueStorageManager abstraction
• Provide a KVStorageManager to cover all the KV based storage backends (PR#96)
• Support topic-based data partitioning to maintain train/val/test data simultaneously (PR#98)
• Release the first stable version through PyPI
• Support disaggregated framework (each rank retrieves its own data without going through a centralized node)
• Provide a StreamingDataLoader interface for disaggregated framework
• Support load-balancing and dynamic batching
• Support high-performance storage backends for RDMA transmission (e.g., Mooncake Store, Ray Direct Transport...)
• High-performance serialization and deserialization
• More documentation, examples and tutorials

📑 Citation

Please kindly cite our paper if you find this repo is useful:

@article{han2025asyncflow,
  title={AsyncFlow: An Asynchronous Streaming RL Framework for Efficient LLM Post-Training},
  author={Han, Zhenyu and You, Ansheng and Wang, Haibo and Luo, Kui and Yang, Guang and Shi, Wenqi and Chen, Menglong and Zhang, Sicheng and Lan, Zeshun and Deng, Chunshi and others},
  journal={arXiv preprint arXiv:2507.01663},
  year={2025}
}