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L24-cpp-extensible-api.md

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L24: C++ Extensible api

Abstract

This proposal will enable extending functionality of GRPC C++ API by removing the final keyword and making some class methods virtual. It will allow alternate implementations of the API by that subclass.

Background

Most GRPC C++ API classes are currently marked final. The rationale is that having virtual methods might have adverse memory size and performance impact. Additionally, adding final to public API classes later on would break existing implementations that use classes derived from public API. Hence it is better and safer to mark classes final unless there's a reason to make them extensible.

Related Proposals:

Proposal

  • Remove final keyword from all public API classes.
  • Move a subset of current private methods to protected. See the implementation PR for currently defined subset.
  • Add protected getters/setters for a subset of private member variables. See the implementation PR for currently defined getters/setters.
  • Mark core methods to virtual, so the functionality can be extended.
  • Each such change should go through thorough performance evaluation and should not be accepted if there is any observed performance degradation.
  • Do this work in stages. Begin with Server, ServerBuilder, and CompletionQueue classes, and extend to other classes as needed.

Rationale

The original rationale for keeping methods private and classes non-extensible has not held true. See grpc/grpc#14359 for details of performance eval in the extreme case. Meanwhile the final classes and non-virtual methods preclude any experimentation with implementation. This proposal will allow extending and experimenting with different server, client, and support structure implementations.

The performance impact of removing final from all public API classes was not measurable. Impact of adding virtual was noticable in case of Inproc transport microbenchmarks and only for 256K byte messages. However, this is a extreme case where virtual was added everywhere, which is not the intention behind this proposal. The more realistic case is seen in performance results of PR grpc/grpc#14517, which shows the impact of both adding virtual and removing final for a few important classes is negligible.

Implementation

The proposed implementation is in PR grpc/grpc#14517. It is limited in scope. It doesn't change every public class and method, but it gives enough extensibility to experiment with alternate implementations.

Open issues (if applicable)