Allow to disable bulkheads in a given thread

[kirs]

The default Semian's bulkhead implementation being not fiber safe makes it hard to experiment with concurrency in a larger app which relies on that implementation.

I'd like to introduce Semian.disable_bulkheads_for_thread which will let me experiment with async IO in an app that's using Semian's non-fiber-safe bulkheads.

Example:

Semian.disable_bulkheads_for_thread(Thread.current) do
  Async do
     # current thread has bulkheads disabled and can use ActiveRecord/Trilogy
  end
end

Without something like this, the 2nd fiber that tries to hit Activerecord fails with:

ActiveRecord::ConnectionAdapters::TrilogyAdapter::ResourceBusyError: [mysql_shard_0] timed out waiting for resource '1000mysql_shard_0'

[casperisfine]

Have you seen #560 ?

[kirs]

Yes. From what I understand, that patch is helpful if you implement a custom async-friendly Semian adapter like in @kovyrin's example.

That's the best path; however it's worth implementing a custom adapter only when you know that async is going to stay in the app for the long term.

Meanwhile, I see benefits of having a shortcut like disable_bulkheads_for_thread to try async in a monolith app that's using Semian's default adapter.

[kovyrin]

Do you expect threads other than current to be passed in here?

[kirs]

I generally don't. I've decided on that API to make it clear to the user that it disables it on a specific thread, not all threads.

I could also make it disable_bulkheads_for_current_thread but that seemed more verbose.

[kovyrin]

I like this 👍🏻

[casperisfine]

I don't know what to think about this. The point of bulkheads is to ensure all your capacity won't be stuck on a single slow resource.

So it seems to me what you want to do is to only acquire a single ticket for all your fibers, not just disable bulkheads.

But it's been a long time since I worked on all this, so I don't have any string opinion.

[kirs]

This is not a proper solution, it's more of an escape hatch to disable bulkheads to experiment with concurrency in an app that's using non-thread-safe bulkheads feature of Semian.

Bulkheads should be disabled (pass bulkhead: false) in a threaded environment (e.g. Puma or Sidekiq)

Once we run those experiments, we could work on proper thread and fiber-safe bulkheading.