elixir_nsq is a client library for NSQ. Use it in your Elixir or Erlang applications to handle messages asynchronously. This library seeks to be complete, well-tested, and easy to use.

This library used go-nsq and pynsq for reference, but is structured to better fit common Elixir workflows.

To use this, you will need to have NSQ.

{:ok, producer} = NSQ.Producer.Supervisor.start_link("my-topic", %NSQ.Config{
  nsqds: ["127.0.0.1:4150", "127.0.0.1:4151"]
})

# publish to the default topic "my-topic"
NSQ.Producer.pub(producer, "a message")
NSQ.Producer.mpub(producer, ["one", "two", "three"])

# specify a topic
NSQ.Producer.pub(producer, "different-topic", "another message")
NSQ.Producer.mpub(producer, "different-topic", ["four", "five", "six"])

NSQ.Producer.close(producer)

defp deps do
  [{:elixir_nsq, "~> 1.0.3"}]
end

defp applications do
  [:logger, :elixir_nsq]
end

The handler should return :ok to finish normally, :req to requeue the message, or {:req, delay} to specify your own requeue delay. If your message handler throws an exception, it will automatically be requeued and delayed with a timeout based on attempts.

{:ok, consumer} = NSQ.Consumer.Supervisor.start_link("my-topic", "my-channel", %NSQ.Config{
  nsqlookupds: ["127.0.0.1:4160", "127.0.0.1:4161"],
  message_handler: fn(body, msg) ->
    IO.puts "id: #{msg.id}"
    IO.puts "attempts: #{msg.attempts}"
    IO.puts "timestamp: #{msg.timestamp}"
    :ok
  end
})

The message handler can also be a module that implements handle_message/2:

defmodule MyMsgHandler do
  def handle_message(body, msg) do
    IO.puts "Handled in a module! #{msg.id}"
    :ok
  end
end

{:ok, consumer} = NSQ.Consumer.Supervisor.start_link("my-topic", "my-channel", %NSQ.Config{
  nsqlookupds: ["127.0.0.1:4160", "127.0.0.1:4161"],
  message_handler: MyMsgHandler
})

If your message is especially long-running and you know it's not dead, you can touch it so that NSQ doesn't automatically fail and requeue it.

def MyMsgHandler do
  def handle_message(body, msg) do
    Task.start_link fn ->
      :timer.sleep(30_000)
      NSQ.Message.touch(msg)
    end
    long_running_operation()
    :ok
  end
end

If you're not using nsqlookupd, you can specify nsqds directly:

{:ok, consumer} = NSQ.Consumer.Supervisor.start_link("my-topic", "my-channel", %NSQ.Config{
  nsqds: ["127.0.0.1:4150", "127.0.0.1:4151"],
  message_handler: fn(body, msg) ->
    :ok
  end
})

Check https://github.com/wistia/elixir_nsq/blob/master/lib/nsq/config.ex for supported config values.

NSQ.Consumer and NSQ.Producer provide the function event_manager/1 so that you can receive events from the NSQ client. You can keep your own stats/logs and perform actions based on that info.

defmodule EventForwarder do
  use GenEvent

  def handle_event(event, parent) do
    send parent, event
    {:ok, parent}
  end
end

def setup_consumer do
  {:ok, consumer} = NSQ.Consumer.Supervisor.start_link("my-topic", "my-channel", %NSQ.Config{
    nsqds: ["127.0.0.1:4150", "127.0.0.1:4151"],
    message_handler: fn(body, msg) ->
      :ok
    end
  })

  # subscribe to events from the event manager
  NSQ.Consumer.event_manager(consumer)
  |> GenEvent.add_handler(EventForwarder, self)
end

Potential event formats are:

• {:message, NSQ.Message.t}
• {:message_finished, NSQ.Message.t}
• {:message_requeued, NSQ.Message.t}
• :resume
• :continue
• :backoff
• :heartbeat
• {:response, binary}
• {:error, String.t, binary}

For your convenience, this is the overall process structure of elixir_nsq. In practice, the Connection.Supervisors and Task.Supervisors don't do much automatic restarting because NSQ itself is built to handle that. But they are useful for propagating exit commands and keeping track of all running processes.

Consumer Supervisor
  Consumer
    ConnInfo Agent
    Connection.Supervisor
      Connection
        Message.Supervisor
          Message
          Message
      Connection
        Message.Supervisor
          Message
          Message
  Connection discovery loop
  RDY redistribution loop

Producer Supervisor
  Producer
    ConnInfo Agent
    Connection.Supervisor
      Connection
      Connection

The included tests require two nsqds and two nsqlookupds. A Procfile for use with foreman is included to start these up. If you don't have foreman, you'll need to find a way to run those commands if you want to run the tests.

foreman start
mix test

Note that some tests intentionally cause processes to exit, so you might see some error logging as part of the tests. As long as they're still passing, that is considered normal behavior.

• Snappy cannot be supported because existing NIFs cannot correctly decompress the nsqd stream. I believe they need support for skipping the checksum.