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Hey

I'm using gRPC with the async API. That requires constructing reactors based on classes like ClientBidiReactor or ServerBidiReactor

If I understand correctly, the gRPC works like this: It takes threads from some thread pool, and using these threads it executes certain methods of the reactors that are being used.

The problem

Now, the problem is when the reactors become stateful. I know that the methods of a single reactor will most probably be executed sequentially, but they may be run from different threads, is this correct? If so, then is it possible that we may encounter a problem described for instance here?

Long story short, if we have an unsynchronized state in such circumstances, is it possible that one thread will update the state, then a next method from the reactor will be executed from a different thread and it will see the not-updated value because the state's new value has not been flushed to the main memory yet?

Honestly, I'm a little confused about this. In the grpc examples here and here this doesn't seem to be addressed (the mutex is for a different purpose there and the values are not atomic).

I used/linked examples for the bidi reactors but this refers to all types of reactors.

Conclusion / questions

There are basically a couple of questions from me at this point:

  • Are the concerns valid here and do I properly understand everything or did I miss something? Does the problem exist?
  • Do we need to manually synchronize reactors' state or is it handled by the library somehow(I mean is flushing to the main memory handled)?
  • Are the library authors aware of this? Did they keep this in mind while they were coding examples I linked?

Thank you in advance for any help, all the best!

Konrad Rudolph
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Karol
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1 Answers1

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You're right that the examples don't showcase this very well, there's some room for improvement. The operation-completion reaction methods (OnReadInitialMetadataDone, OnReadDone, OnWriteDone, ...) can be called concurrently from different threads owned by the gRPC library, so if your code accesses any shared state, you'll want to coordinate that yourself (via synchronization, lock-free types, etc). In practice, I'm not sure how often it happens, or which callbacks are more likely to overlap.

The original callback API spec says a bit more about this, under a "Thread safety" clause: L67: C++ callback-based asynchronous API. The same is reiterated a few places in the callback implementation code itself - client_callback.h#L234-236 for example.

drfloob
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