fork() support in Python!

[Yonatan Zunger]

Wow -- I just saw the notes for 16264, and that 1.15 now supports fork() in Python. This is huge and great news!

I just want to make sure I understand how this change works, and in particular what the consequences of the shutdown of the core-level gRPC resources in the child's post-fork handler means. The use case which is (IMO) most important is where you create some kind of Python object which has a gRPC stub inside it (e.g., a client object meant to talk to servers), then fork() (often through use of the core Python multiprocessing library), and use that object from within the child process as well. (This is important because the multiprocessing library is the only built-in parallelization mechanism that doesn't suffer from serialization due to the GIL) The overhead cost, IIUC, would be essentially a restart of the core resources, which is roughly equivalent to the cost of closing and reopening the channel, but not of having to reboot all the wrapping objects, like bigtable clients or whatever. (Which are notoriously slow to start up because they also want to look up all sorts of metadata from the server when they boot)

I could imagine several gotchas here: for example, that the cancellation of in-flight RPC's by the child during the reboot would also affect RPC's in flight due to other threads, meaning that the client object has to be entirely idle during the fork process.

Am I understanding the new change correctly? What are the intended use cases that it's meant to unlock?

Yonatan

[Srini Polavarapu]

Hi Yonatan,

Your understanding is correct. The design takes care of not affecting RPCs in parent process due to FD shutdown in the child's post-fork handlers. Child process will recreate its own connection when a Python object (with a gRPC stub inside) inherited from the parent needs to be used. This should handle the case you described in your post. That was one of the use cases in our mind when deciding how to solve the fork issue. Please give it a try and let us know if you see any issues.

-Srini