Proposal: GenServer.reply_after

[Khionu Sybiern]

I have a case where I use a GenServer to block other operations from happening in order to respect ratelimits of a third party. Right now, I spawn a Task that uses GenServer.reply after a sleep, then return :no_reply. Imo, it would be more ergonomic to have an equivalent to Process.send_after for GenServer.reply.

Example: https://gist.github.com/khionu/7da26e1c53108ee4b462ef570f19b61b

[José Valim]

Hi Khionu, can you please explain a bit more about the design? Why does it need to delay the response in such a way? Is it coming from the rate limiter process itself?

On Thu, Sep 2, 2021 at 21:02 Khionu Sybiern <ksyb...@octoml.ai> wrote:

I have a case where I use a GenServer to block other operations from happening in order to respect ratelimits of a third party. Right now, I spawn a Task that uses GenServer.reply after a sleep, then return :no_reply. Imo, it would be more ergonomic to have an equivalent to Process.send_after for GenServer.reply.

Example: https://gist.github.com/khionu/7da26e1c53108ee4b462ef570f19b61b

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[Khionu Sybiern]

The idea is that the rate limiter process itself doesn't need to be held up because of the caller's need to respect the rate limit. Because the delay is immediately calculable at the time, that can be immediately delegated from the rate limiter process, allowing it to move on to the next request.

[José Valim]

In this case, then I think I would have the GenServer return immediately and say: you can call the service at time X. The client gets the response, see how much time it is needing, and handles it accordingly. Either by sleeping or by sending a message to itself. WDYT?

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[Khionu Sybiern]

Maybe it could be moved to the client side of the API, but otherwise it would be a lot of boilerplate.

A point was raised on the Discord that this function could be a trap for people still getting used to idiomatic Elixir, and they proposed multiple other solutions, both in changing approach and how to do this without a Task or new GenServer function. I'll withdraw my proposal, and I'll summarize the discussion in the near future.

[José Valim]

Thanks. You could also send_after a message to yourself and then send it to the client. It is an extra step but it should be quite efficient too.

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