vshesh

"sample" an agent every second?

I have a situation where I have a pubsub, and a process receiving events from that pubsub.
I want the receiving process to update some state. That’s pretty easy with a genserver or an agent.
Then, I want another process to “sample” the process with state every second and do something.

What is the best way to accomplish this? Agent + Task?

Does that change if I want to modify the schedule at which the sampling is happening over time? (so the sampler can receive an event that changes the frequency of sampling?)

In Rx I would use a behavior and sample it with rx.sample - anything like that here welcome.

And what is the best way to expose this to the rest of my app? Should I make a mini-supervisor tree with both of these processes since they’re linked (if the receiving process goes down, no sense having the sampling process). How do I make it possible for the supervision tree to treat both of these processess together as a “single process” that can be shut down together?

13 comments

#agent

0 1792 13

2024-09-18 16:16:55 UTC

Marked As Solved

ityonemo

I have a feeling process.send_after has some corner cases where you don’t want to use it for periodic events, since it doesn’t automatically cancel a previous firing cycle. If you’re not careful with your code you can wind up with 2x firings, 3x firings, 4x firings and so forth.

You can store the reference to your timer when you use send_interval, and that will allow you to make adjustments later, using :timer.cancel and storing a new timer ref when you rebuild the interval.

Post #4

Also Liked

kokolegorille

Simply pass the rate frequency as part of the server state. So You can modify it.

There is also Process.send_after.

There is an example in this tetris in Erlang, where the game loop accelarate over time.

http://www1.erlang.org/examples/small_examples/tetris.erl

Post #3

kokolegorille

It is also the case for Process.send_after, it returns a ref. And it is possible to read the elapsed time with Process.read_timer(ref), or cancel timer. I also store the ref in the server state.

Both are doing the same, but not the same way. I don’t use :timer module because it can get overloaded.

From Common Caveats — Erlang System Documentation v29.0.2

Creating timers using erlang:send_after/3 and erlang:start_timer/3, is much more efficient than using the timers provided by the timer module in STDLIB.

Post #8

thoughtarray

Hey all and future search engine visitors (like me). I wanted to mention something I feel was missed here. In case it matters to your program, you can ensure that your initial :tick message is sent after GenServer’s event loop is started by using the handle_continue/2 callback.

@impl true
def init(state) do
  {:ok, state, {:continue, :init}}
end

@impl true
def handle_continue(:init, state) do
  send(self(), :tick)
  {:noreply, state}
end

Post #14

Last Post!

thoughtarray

@impl true
def init(state) do
  {:ok, state, {:continue, :init}}
end

@impl true
def handle_continue(:init, state) do
  send(self(), :tick)
  {:noreply, state}
end

Post #14