Add optional handle_enqueue/2 and handle_dequeue/2 callbacks #7

sneako · 2020-04-26T13:09:53Z

Closes #5

josevalim · 2020-04-26T13:40:54Z

Hi @sneako! I had an idea, let’s make handle_enqueue(command, pool_state). The item is opaque but you can wrap it, for example to attach some information to be used on dequeue. On dequeue we receive the potentially wrapped item and unwrap it.

This can allow you to do things like having queues based on how long the items are in the queue (sojourn time). We can even improve enqueue/dequeue later to allow items to be skipped if we think that we won’t be able to serve it on time. In fact, we would be able to implement the whole deadline on top of those callbacks.

…rgument

sneako · 2020-04-26T19:50:35Z

@josevalim that sounds really interesting! I just pushed an update, am I on the right track here? I think the tests could probably be improved, but I just want to make sure I'm following correctly.

lib/nimble_pool.ex

Co-Authored-By: José Valim <[email protected]>

…and remove deadline code

lib/nimble_pool.ex

…ue skip a checkout

josevalim · 2020-04-27T22:01:17Z

Hi @sneako, sorry for the back and forth but I think that, after reviewing your changes, we need to bring the deadline in and keep it built-in. Here are the reasons:

If we don't bring the deadline in, we need to send a cancel request from timeout, as you did. But if we do a cancel request from timeout and you implement deadline by hand in Finch, then we will have to implement this clause, as you can skip something and still receive its timeout message
If we skip a message on handle_enqueue/handle_dequeue, we will have to send a message back to the client saying the checkout was skipped. If we don't have a deadline, there is a chance we can send this message after the timeout, polluting the process inbox in case it doesn't terminate

So it seems to me having deadlines will provide a better experience altogether. Can you please bring it back? :) WDYT?

The good news is that we can probably implement it inside the handle_dequeue and handle_enqueue callbacks themselves.

josevalim · 2020-04-27T22:03:10Z

lib/nimble_pool.ex

+    with {:ok, command, state} <- handle_dequeue(command, state),
+         {{:value, worker_server_state}, resources} <- :queue.out(resources) do


Let's please break it in two cases. Otherwise, if someone returns {:empty, state} from dequeue, it will actually work!

Also, we need to first checkout the resource, then we need to dequeue. :) Otherwise we may dequeue and not use it, meaning it would be dequeued multiple times.

josevalim · 2020-04-27T22:04:41Z

lib/nimble_pool.ex


    receive do
+      {^ref, :skipped} ->
+        Process.demonitor(ref, [:flush])
+        exit!(:skipped, :checkout, [pool])


Not a big fan of using :skipped here because it doesn't say much. What if we allow {:skip, Exception.t, pool_state} to be returned from the callbacks? Then in here, we simply do raise exception?

sneako · 2020-04-27T22:32:12Z

No apology necessary! This is a huge learning experience for me, so I appreciate you bearing with me as I fumble through this! I agree, having the deadline built in is a much nicer experience

…eturn types on :skip

sneako · 2020-04-28T09:58:46Z

Still have to update the docs, and could probably improve the tests a bit, but I think this is ready for another look :)

josevalim · 2020-04-28T12:19:29Z

lib/nimble_pool.ex

+
+        case apply_worker_callback(worker, :handle_checkout, args) do
+          {:ok, worker_client_state, worker_server_state} ->
+            case handle_dequeue(command, deadline, ref, mon_ref, state) do


Hrm, we need to call handle_dequeue before handle_checkout, otherwise handle_checkout will receive the wrapped handle_dequeue item. However, if we call it before, then handle_checkout can fail, which means we need to enqueue it again. Damn it. :D

So in a way, it feels like handle_checkout and handle_dequeue are the same callback. Maybe what we need to do is:

Let's remove handle_dequeue for now and merge this PR. In particular, maybe_checkout can look exactly how it was before

Let's pass pool_state to handle_checkout and handle_checkin and require it to be returned in their return types

Support {:skip, exception, pool_state} in handle_checkout

WDTY?

Nice, I think this makes sense. I was already thinking that I would only be using handle enqueue to implement round robin in Finch, and probably not need to implement handle_dequeue. I'll remove handle_dequeue, and update the other callbacks as specified in 2 & 3

josevalim · 2020-04-28T12:20:04Z

lib/nimble_pool.ex

-        state = remove(reason, worker_server_state, state)
-        state = %{state | requests: requests, monitors: monitors, resources: resources}
-        {:noreply, state}
+        state = remove_request(state, ref, mon_ref)


Good call on remove_request/3!

josevalim · 2020-04-28T15:08:42Z

lib/nimble_pool.ex

-              {:ok, client_state, worker_state} | {:remove, user_reason}
+  @callback handle_checkout(maybe_wrapped_command :: term, from, worker_state, pool_state) ::
+              {:ok, client_state, worker_state, pool_state}
+              | {:remove, user_reason}


Looks great. Let's just request the pool_state to be returned on {:remove, user_reason} too and we can ship it. The same for remove on checkin!

Hmm it looks like I will either have to add pool_state to all of the callbacks that can return remove, or define a second apply_worker_callback that accepts the pool state and returns it when it catches. I guess the second option sounds better

…checkin

josevalim · 2020-04-28T16:38:46Z

💚 💙 💜 💛 ❤️

josevalim · 2020-04-28T18:41:25Z

Hi @sneako ! I failed to notice we were passing more state than we should in my review, so I pushed a fix to master!

sneako · 2020-04-28T18:44:44Z

Ah right that makes sense! We definitely don't want to let the user mess with the other fields. Thanks for the heads up!

Add optional handle_enqueue/1 and handle_dequeue/1 callbacks

3326d11

update handle_enqueue & handle_dequeue to take command as the first a…

1f99ad4

…rgument

sneako marked this pull request as draft April 26, 2020 19:50

sneako changed the title ~~Add optional handle_enqueue/1 and handle_dequeue/1 callbacks~~ Add optional handle_enqueue/2 and handle_dequeue/2 callbacks Apr 26, 2020

josevalim reviewed Apr 27, 2020

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sneako and others added 3 commits April 27, 2020 14:03

Update lib/nimble_pool.ex

ce7313d

Co-Authored-By: José Valim <[email protected]>

Update lib/nimble_pool.ex

a443b87

Co-Authored-By: José Valim <[email protected]>

WIP: allow handle_enqueue/2 and handle_dequeue/2 to skip the request …

7ac516f

…and remove deadline code

sneako force-pushed the enqueue-dequeue branch from d609f8b to 7ac516f Compare April 27, 2020 19:48

josevalim reviewed Apr 27, 2020

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lib/nimble_pool.ex Outdated Show resolved Hide resolved

sneako added 3 commits April 27, 2020 22:54

handle_dequeue returns the command as well

2a0cf7d

reply with a :skipped message to the client when handle_enqueue/deque…

898850b

…ue skip a checkout

start updating handle_enqueue/dequeue docs

4cb13af

sneako force-pushed the enqueue-dequeue branch from 2467d67 to 4cb13af Compare April 27, 2020 21:47

josevalim reviewed Apr 27, 2020

View reviewed changes

put back deadline, add exception to handle_enqueue/dequeue callback r…

a90cc3c

…eturn types on :skip

fix up docs a bit more

3c9bd16

josevalim reviewed Apr 28, 2020

View reviewed changes

sneako force-pushed the enqueue-dequeue branch from b6edf18 to be100ad Compare April 28, 2020 15:01

remove handle_dequeue, allow handle_checkout to return skip tuple

9efc041

sneako force-pushed the enqueue-dequeue branch from be100ad to 9efc041 Compare April 28, 2020 15:04

Update nimble_pool.ex

4c6cc46

sneako marked this pull request as ready for review April 28, 2020 15:08

josevalim reviewed Apr 28, 2020

View reviewed changes

add pool state to :remove return values for handle_checkout & handle_…

ef94fc4

…checkin

josevalim merged commit 5a3928a into dashbitco:master Apr 28, 2020

sneako deleted the enqueue-dequeue branch April 28, 2020 16:39

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Add optional handle_enqueue/2 and handle_dequeue/2 callbacks #7

Add optional handle_enqueue/2 and handle_dequeue/2 callbacks #7

sneako commented Apr 26, 2020

josevalim commented Apr 26, 2020

sneako commented Apr 26, 2020

josevalim commented Apr 27, 2020

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josevalim commented Apr 28, 2020

sneako commented Apr 28, 2020 •

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		with {:ok, command, state} <- handle_dequeue(command, state),
		{{:value, worker_server_state}, resources} <- :queue.out(resources) do

Add optional handle_enqueue/2 and handle_dequeue/2 callbacks #7

Add optional handle_enqueue/2 and handle_dequeue/2 callbacks #7

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