Note that you can add metadata to the subscriptions that can later be used for filtering during dispatch.
Phoenix.PubSub.subscribe(YourApp.PubSub, "topic:user-id", metadata: [some: :extra_info])
which (if you are familiar with Registry) is similar to
Registry.subscribe(YourApp.PubSub, "topic:user-id", [some: :extra_info])
In general, the number of subscriptions doesn’t affect much, as the total throughput should stay about the same:
bench.exs
Mix.install([:benchee, :phoenix_pubsub])
{:ok, _pid} = Phoenix.PubSub.Supervisor.start_link(name: App.PubSub)
defmodule Sub do
use GenServer
def start_link(opts \\ []) do
GenServer.start_link(__MODULE__, opts)
end
@impl true
def init(opts) do
topic = Keyword.fetch!(opts, :topic)
Phoenix.PubSub.subscribe(App.PubSub, topic)
{:ok, nil}
end
@impl true
def handle_info(_message, state) do
{:noreply, state}
end
end
Enum.each(1..200_000, fn _ -> Sub.start_link(topic: "topic") end)
Enum.each(1..10, fn _ -> Sub.start_link(topic: "topic2") end)
map = %{"some" => "key", "then" => "some oter key"}
Benchee.run(
%{
"message" => fn topic -> Phoenix.PubSub.broadcast(App.PubSub, topic, "message") end,
"map" => fn topic -> Phoenix.PubSub.broadcast(App.PubSub, topic, map) end
},
inputs: %{"lots subs" => "topic", "few subs" => "topic2"}
)
> elixir bench.exs
Operating System: macOS
CPU Information: Apple M1
Number of Available Cores: 8
Available memory: 8 GB
Elixir 1.13.1
Erlang 24.2
Benchmark suite executing with the following configuration:
warmup: 2 s
time: 5 s
memory time: 0 ns
parallel: 1
inputs: few subs, lots subs
Estimated total run time: 28 s
Benchmarking map with input few subs...
Benchmarking map with input lots subs...
Benchmarking message with input few subs...
Benchmarking message with input lots subs...
##### With input few subs #####
Name ips average deviation median 99th %
map 147.35 K 6.79 μs ±1531.35% 4.99 μs 24.99 μs
message 95.00 K 10.53 μs ±9207.44% 2.99 μs 8.99 μs
Comparison:
map 147.35 K
message 95.00 K - 1.55x slower +3.74 μs
##### With input lots subs #####
Name ips average deviation median 99th %
map 3.12 320.16 ms ±9.22% 313.18 ms 387.97 ms
message 2.78 360.19 ms ±16.91% 352.94 ms 479.31 ms
Comparison:
map 3.12
message 2.78 - 1.13x slower +40.02 ms
