Proposal: Avoid re-encoding in handle_out when used as a filter

gfo...@gmail.com

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Mar 4, 2020, 2:30:48 AM3/4/20

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Hi all, we are running into a performance bottleneck when using "intercept" and "handle_out" in a Phoenix channel.

In our use case, we have subscribers to a topic, but some subscribers have a set of "blocked" and "blocked by" users in their socket.assigns. We use the "handle_out" mechanism to check these maps at intercept time to prevent the message from being delivered to a given blocked receiver. Given that the state involved is only available on the socket of the receiver, we need to use intercept/handle_out. However, we pay a large performance cost, since the outgoing messages are uniquely encoded by Phoenix, despite the fact we are passing them through as-is.

In our case, where handle_out is a pass/fail filter on the message, it seems like there is an optimization opportunity. It would be nice if a facility existed where a intercept could be registered, but the goal of the intercept wasn't to mutate the message, but instead block or pass through the message as-is. This would allow the existing optimization of non-intercepted events to pass through the encoded message, without having to re-encode it.

Ergonomically, this could be done either by keeping the handle_out semantics but providing a new return value which signalled to phoenix that the original encoded payload should be pushed through as-is. Or, a new facility analagous to "handle_out" could be added which is expected to return a value indicating if the message should be passed through (eg "filter_out".) The former seems more in the style of phoenix, but hard to say.

This seems like a really strong optimization, given that a common use-case for handle_out is to perform late filtering on messages based upon things like security, rate limiting, etc, without the need to actually re-encode the message.

Thanks for any interest!

José Valim

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Mar 4, 2020, 3:45:13 AM3/4/20

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Hi Greg!

Phoenix master (upcoming v1.5) will use PubSub v2.0 where we have made the dispatching mechanism more flexible. I wonder if you can use the new mechanism instead to provide the flexibility that you need.

In master, when you call Phoenix.Endpoint.broadcast:

https://github.com/phoenixframework/phoenix/blob/29b16e0c3a230fa06aa6386bda9d745d3db3330f/lib/phoenix/endpoint.ex#L385

It boils down to be a call to Phoenix.PubSub with a custom module:

https://github.com/phoenixframework/phoenix/blob/29b16e0c3a230fa06aa6386bda9d745d3db3330f/lib/phoenix/channel/server.ex#L137

That has a dispatching function:

https://github.com/phoenixframework/phoenix/blob/29b16e0c3a230fa06aa6386bda9d745d3db3330f/lib/phoenix/channel/server.ex#L85

The parameters available on dispatching are given on subscription:

https://github.com/phoenixframework/phoenix/blob/29b16e0c3a230fa06aa6386bda9d745d3db3330f/lib/phoenix/channel/server.ex#L400-L401

This means you could put in a relatively straight-forward fashion setup a separate topic with custom dispatching rules that would also look at conditionals.

Another option is for you to drop channels and work directly with the socket, as can be shown here: https://github.com/phoenixframework/phoenix/blob/29b16e0c3a230fa06aa6386bda9d745d3db3330f/lib/phoenix/socket/transport.ex#L50 - this is available since v1.4.

What do you think about these options?

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gfo...@gmail.com

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Mar 4, 2020, 4:10:14 AM3/4/20

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Thanks for the quick reply José! I will investigate further and let you know how it goes.

Chris McCord

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Mar 4, 2020, 8:34:48 AM3/4/20

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Handle_out has a known performance penalty, and this particular usecase is one I give as a reason to *not* use intercept because you can accomplish the same goals without it. Instead, I would pass the list of blocks/filters to the client and have the client do the filtering as they receive messages. You then allow broadcast to reach all subscribers, but skip updating the UI if the message should be dropped. Is this possible on your side?

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Greg Fodor

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Mar 4, 2020, 9:50:06 AM3/4/20

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Hey Chris, thanks for the info. For us unfortunately that is probably not possible - the messages involved are being blocked for two reasons:

- the user has been blocked by another user, and we want to ensure they no longer receive messages by the blocking user for privacy reasons. (We could drop as you mention on the client in the other direction so the blocking person does not see the blocked messages in the UI, but not the other way around: the blocked user should no longer have any visibility into the blocker, even w a modified client)

- the client is in a “pause” state where we want to minimize client overhead so do not want to deliver incoming messages to it (in our case, the client is in an iframe on a site and is waiting for the user to optionally activate it, but needs to be connected to the channel for other messages)

A little more context: we are using Phoenix as a message bus for a 3D game, so our message load is fairly high throughout (approx ten tps per user) and the data involved is highly sensitive, such as body language transmitted via VR tracking. So we do start to hit this bottleneck w approx 500 ccu or so.

One thing that I’d be interested in is if there is a way to selectively “upgrade” a channel to have the intercepts after initially being cheap to send. Most channels connections do not have either blocking state nor are paused, so if we could enter into the intercept state after a block event occurs or the channel is marked as paused by the client that would also solve our problem since that would constitute the vast minority of connections, and eliminate the extra encoding load under high concurrency.

You can try it at hubs.mozilla.com if you want! When you are in a room with someone, hold space and “hide” an avatar - messages between you will be bidirectionally blocked by the server to ensure the (presumed) abuser cannot see your actions in the room.

Hope this helps, and thanks!

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Greg Fodor

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Mar 4, 2020, 10:43:36 AM3/4/20

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Actually I may be able to solve the blocking case - I think I can set things up so I alter the event type in the case of a connection involved in a block so it goes down the intercept route, but not otherwise (the client is unchanged.) ie, I stop intercepting event “foo”, and start intercepting event “foo-filtered”, migrate the handle_out to “foo-filtered”, and then on an incoming message if the connection is blocked or has blocks then rewrite the event to “foo-filtered” (and push “foo” in handle_our so the client sees no difference.)

Still need to think about pausing - but that can be solved at a higher level I think - the fact that a topic can be paused or not is known at the topic level for us, and this is rare, so I can probably just have this use the intercept route if and only if the topic is in the special category of being pausable.

Is there a more general way to maybe have a client “opt out” of receiving certain categories of events that don’t involve using handle_out?

Greg Fodor

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Mar 4, 2020, 12:08:10 PM3/4/20

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This seems to do the trick - PR is here if you are curious!

https://github.com/mozilla/reticulum/pull/329

Greg Fodor

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Mar 4, 2020, 12:58:43 PM3/4/20

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looks like this got us a 2x boost before bottleneck moves to cowboy processes :)

on that note (perhaps worthy of another thread) -- is it surprising that we see message backup in the cowboy processes as the final bottleneck (CPU bound) of a high throughput, low subscriber count per topic (~50) phoenix channel? i found it a bit surprising we didn't see the bottleneck ultimately manifest in the JSON encoding/decoding process of moving messages around, but instead it seems like its in the cowboy processes, and my experiments point to TLS (which we have enabled) as the culprit.

i'm still learning how to profile things but I ran a test with TLS disabled and saw a big jump in peak capacity before backup -- any knobs worth turning, compile flags for openssl/otp, cowboy configs, etc? we are on cowboy 1.7 using websockets.

thx again!

Greg Fodor

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Mar 4, 2020, 1:07:25 PM3/4/20

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Sorry, cowboy 2.7!

José Valim

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Mar 4, 2020, 1:30:25 PM3/4/20

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Unfortunately I do not have a lot of experience on fine tuning SSL. If an option, consider moving SSL to a layer up (a proxy) and see how the whole system compares.

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