Hi All,
I’m using a library (rabbit_mq) which has some arguments that i’d like to pass in via the child spec if possible or some other way vs being hardcoded in the source file. Thanks for any and all help.
use RabbitMQ.Producer, exchange: “example_queue”, worker_count: 3
The library has the following:
defmacro __using__(opts) do
quote do
alias RabbitMQ.Producer
require Logger
@confirm_type unquote(Keyword.get(opts, :confirm_type, :async))
@exchange unquote(Keyword.fetch!(opts, :exchange))
@worker_count unquote(Keyword.get(opts, :worker_count, 3))
@this_module __MODULE__
@behaviour Producer
##############
# Public API #
##############
def child_spec(opts) do
if @worker_count > max_workers() do
raise """
Cannot start #{@worker_count} workers, maximum is #{max_workers()}.
You can configure this value as shown below;
config :rabbit_mq, max_channels_per_connection: 16
As a rule of thumb, most applications can use a single digit number of channels per connection.
For details, please consult the official RabbitMQ docs: https://www.rabbitmq.com/channels.html#channel-max.
"""
end
config = %{
confirm_type: @confirm_type,
exchange: @exchange,
nack_cb: &on_publisher_nack/1,
worker_count: @worker_count
}
....
It is not hardcoded. What you are seeing is a convenience code that gets evaluated with the values you are passing to the use statement. Your worker_count parameter will be honoured.
Yes, those module attributes will get injected in the using module. That’s a fairly normal pattern for libraries utilising metaprogramming and that’s why they usually recommend you put the use statement in a dedicated single-responsibility module. Ecto.Repo does this as well.
Do not get worried about implementation details. Does the library work for your scenario?
Yes, the library works great. First library i found that handles rabbitmq messages reliably – e.g. when the rabbitmq broker goes down, it maintains a list of messages it has received and will send them after reconnect.
I was looking into the implementation to see a way around passing in the params differently. My way of trying to understand the language/platform. This is my first elixir project.
Thank you for the response. I think i’ve poorly stated my goal. I understand i can change those parameters, such as worker_count, but that would still reside inside my .ex file that contains the use statement.
Since I want to access more than one queue, I would need to have two separate source files with the exchange param set individually (that’s what i meant by hardcoded). My goal would be to have one source file launched twice from two entries in the supervisor child spec passing in these parameters vs having two copies of the same source file where the only difference is the parameter for the use statement.
Hopefully that makes more sense. I appreciate the help.
Something like this? Separates configuration from generation:
defmodule Queue do
def configuration do
%{
My.Queue_1 => [exchange: "example_queue", worker_count: 3],
My.Queue_2 => [exchange: "other_queue", worker_count: 2]
}
end
end
defmodule Rabbit do
for {module, options} <- Queue.configuration() do
defmodule module do
use RabbitMQ.Producer, unquote(options)
end
end
end
defmodule Tmp2.QConfig do
def configuration do
%{
Tmp2.Foo.Queue1 => [exchange: "example_queue", worker_count: 3],
Tmp2.Foo.Queue2 => [exchange: "other_queue", worker_count: 2]
}
end
end
defmodule Tmp2.Foo do
for {module, options} <- Tmp2.QConfig.configuration() do
defmodule module do
#use RabbitMQ.Producer, exchange: "example_queue", worker_count: 3
#use RabbitMQ.Producer, [exchange: "example_queue", worker_count: 3]
use RabbitMQ.Producer, unquote(options)
@impl true
def handle_publisher_nack(unackd_messages) do
Logger.error("Failed to publish messages: #{inspect(unackd_messages)}")
end
end
end
end
Seems as if the library does not call Macro.expand/2 on its use arguments. Not actually sure how to proceed in that case. Perhaps the rest of the community will have some ideas (and I’ll keep thinking on it).
Ok, that fixed the errors, thank you. When i run it now, the for loop only seems to generate one module not the qty in the config (2 for now). Any ideas?
Seems like this is the issue. Thanks again for your help.
Interactive Elixir (1.10.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Tmp2.Foo.Queue
Queue1 Queue2
My code is:
defmodule Tmp2.QConfig do
def configuration do
%{
Tmp2.Foo.Queue1 => [exchange: "example_queue", worker_count: 3],
Tmp2.Foo.Queue2 => [exchange: "other_queue", worker_count: 2]
}
end
end
defmodule Tmp2.Foo do
for {module, options} <- Tmp2.QConfig.configuration() do
defmodule module do
use RabbitMQ.Producer, options
require Logger
@impl true
def handle_publisher_nack(unackd_messages) do
Logger.error("Failed to publish messages: #{inspect(unackd_messages)}")
end
end
end
end
