[TE] Refactor TransferEngine into Impl + forwarding wrapper

[alogfans]

Description

Separate interface and implementation of Mooncake TE, therefore TENT has better backward compatibility.

Type of Change

• Types
  • Bug fix
  • New feature
    • Transfer Engine
    • Mooncake Store
    • Mooncake EP
    • Integration
    • P2P Store
    • Python Wheel
  • Breaking change
  • CI/CD
  • Documentation update
  • Other

How Has This Been Tested?

Checklist

• I have performed a self-review of my own code.
• I have updated the documentation.
• I have added tests to prove my changes are effective.

[gemini-code-assist]

Summary of Changes

Hello @alogfans, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the TransferEngine component by separating its interface from its implementation. This architectural change, utilizing the Pimpl idiom, moves all internal state and complex logic into a new TransferEngineImpl class. The TransferEngine now acts as a lightweight wrapper, forwarding all method calls to its private implementation. This design improves modularity, reduces compilation dependencies, and is intended to provide better backward compatibility for external components like TENT.

Highlights

• Pimpl Idiom Adoption: The TransferEngine class has been refactored to utilize the Pimpl (Pointer to Implementation) idiom.
• Separation of Concerns: The public interface of TransferEngine is now distinct from its internal implementation details, which reside in a new TransferEngineImpl class.
• Improved Backward Compatibility: This architectural change aims to enhance backward compatibility for TENT by isolating changes to the implementation, reducing the impact on external components.

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[gemini-code-assist]

Code Review

This pull request refactors the TransferEngine class by separating its interface from its implementation using the Pimpl (pointer to implementation) idiom. The implementation details are moved into a new TransferEngineImpl class. This is a great architectural improvement that will help with backward compatibility and reduce compilation dependencies for clients of TransferEngine.

The changes are well-executed. I have a few suggestions to improve const-correctness, header hygiene, and performance. Most notably, the const qualifier was dropped from getLocalTopology, which is a breaking API change that should be reverted. I've also pointed out some unnecessary headers and a place where an extra copy can be avoided.

I am having trouble creating individual review comments. Click here to see my feedback.

mooncake-transfer-engine/include/transfer_engine.h (115)

The const qualifier was removed from this method's signature. The original method was const, and the implementation it forwards to in TransferEngineImpl is also const. Removing const is a breaking change for clients that might be calling this on a const TransferEngine object. Please restore the const qualifier to maintain API compatibility and const-correctness. You will also need to add const to the method's implementation in transfer_engine.cpp.

    std::shared_ptr<Topology> getLocalTopology() const;

mooncake-transfer-engine/include/transfer_engine_impl.h (18-21)

These headers appear to be unnecessary in this header file and were likely copied over during refactoring. Please remove them to keep includes minimal. If any of these are needed by transfer_engine_impl.cpp, they should be included there directly, preferably using their C++ standard library versions (e.g., <cstring>, <climits>, <cerrno>).

mooncake-transfer-engine/include/transfer_engine_impl.h (150-151)

The remote_agent parameter is taken by value. While the wrapper in TransferEngine uses std::move, this still results in an extra move construction for the std::string. To improve efficiency, consider taking remote_agent by const std::string&. This avoids the extra move while still allowing efficient handling of rvalues from the caller. Remember to update the implementation in transfer_engine_impl.cpp as well.

    int sendNotifyByName(const std::string& remote_agent,
                         TransferMetadata::NotifyDesc notify_msg);

mooncake-transfer-engine/src/transfer_engine_impl.cpp (20)

The code uses errno, which is defined in <cerrno>. Please explicitly include this header for correctness and portability, as its inclusion via other headers is not guaranteed.

#include <cerrno>
#include <cstring>