RabbitMQ tutorial - Routing

Routing

(using the amqp Elixir library)

info

Prerequisites

This tutorial assumes RabbitMQ is installed and running on localhost on the standard port (5672). In case you use a different host, port or credentials, connections settings would require adjusting.

Where to get help

If you're having trouble going through this tutorial you can contact us through GitHub Discussions or RabbitMQ community Discord.

In the previous tutorial we built a simple logging system. We were able to broadcast log messages to many receivers.

In this tutorial we're going to add a feature to it - we're going to make it possible to subscribe only to a subset of the messages. For example, we will be able to direct only critical error messages to the log file (to save disk space), while still being able to print all of the log messages on the console.

Bindings

In previous examples we were already creating bindings. You may recall code like:

AMQP.Queue.bind(channel, queue_name, exchange_name)

A binding is a relationship between an exchange and a queue. This can be simply read as: the queue is interested in messages from this exchange.

Bindings can take an extra routing_key parameter. To avoid the confusion with a basic_publish parameter we're going to call it a binding key. This is how we could create a binding with a key:

AMQP.Queue.bind(channel, queue_name, exchange_name, routing_key: "black")

The meaning of a binding key depends on the exchange type. The fanout exchanges, which we used previously, simply ignored its value.

Direct exchange

Our logging system from the previous tutorial broadcasts all messages to all consumers. We want to extend that to allow filtering messages based on their severity. For example we may want the script which is writing log messages to the disk to only receive critical errors, and not waste disk space on warning or info log messages.

We were using a fanout exchange, which doesn't give us too much flexibility - it's only capable of mindless broadcasting.

We will use a direct exchange instead. The routing algorithm behind a direct exchange is simple - a message goes to the queues whose binding key exactly matches the routing key of the message.

To illustrate that, consider the following setup:

In this setup, we can see the direct exchange X with two queues bound to it. The first queue is bound with binding key orange, and the second has two bindings, one with binding key black and the other one with green.

In such a setup a message published to the exchange with a routing key orange will be routed to queue Q1. Messages with a routing key of black or green will go to Q2. All other messages will be discarded.

Multiple bindings

It is perfectly legal to bind multiple queues with the same binding key. In our example we could add a binding between X and Q1 with binding key black. In that case, the direct exchange will behave like fanout and will broadcast the message to all the matching queues. A message with routing key black will be delivered to both Q1 and Q2.

Emitting logs

We'll use this model for our logging system. Instead of fanout we'll send messages to a direct exchange. We will supply the log severity as a routing key. That way the receiving script will be able to select the severity it wants to receive. Let's focus on emitting logs first.

Like always we need to create an exchange first:

AMQP.Exchange.declare(channel, "direct_logs", :direct)

And we're ready to send a message:

AMQP.Basic.publish(channel, "direct_logs", severity, message)

To simplify things we will assume that 'severity' can be one of 'info', 'warning', 'error'.

Subscribing

Receiving messages will work just like in the previous tutorial, with one exception - we're going to create a new binding for each severity we're interested in.

{:ok, %{queue: queue_name}} = AMQP.Queue.declare(channel, "", exclusive: true)

for {severity, true} <- severities do
  binding_key = severity |> to_string
  AMQP.Queue.bind(channel, queue_name, "direct_logs", routing_key: binding_key)
end

Putting it all together

The code for emit_log_direct.exs:

{:ok, connection} = AMQP.Connection.open
{:ok, channel} = AMQP.Channel.open(connection)

{severities, raw_message, _} =
  System.argv
  |> OptionParser.parse(strict: [info:    :boolean,
                                 warning: :boolean,
                                 error:   :boolean])
  |> case do
    {[], msg, _} -> {[info: true], msg, []}
    other -> other
  end

message =
  case raw_message do
    []    -> "Hello World!"
    words -> Enum.join(words, " ")
  end

AMQP.Exchange.declare(channel, "direct_logs", :direct)

for {severity, true} <- severities do
  severity = severity |> to_string
  AMQP.Basic.publish(channel, "direct_logs", severity, message)
  IO.puts " [x] Sent '[#{severity}] #{message}'"
end

AMQP.Connection.close(connection)

The code for receive_logs_direct.exs:

defmodule ReceiveLogsDirect do
  def wait_for_messages(channel) do
    receive do
      {:basic_deliver, payload, meta} ->
        IO.puts " [x] Received [#{meta.routing_key}] #{payload}"

        wait_for_messages(channel)
    end
  end
end

{:ok, connection} = AMQP.Connection.open
{:ok, channel} = AMQP.Channel.open(connection)

{severities, _, _} =
  System.argv
  |> OptionParser.parse(strict: [info:    :boolean,
                                 warning: :boolean,
                                 error:   :boolean])

AMQP.Exchange.declare(channel, "direct_logs", :direct)

{:ok, %{queue: queue_name}} = AMQP.Queue.declare(channel, "", exclusive: true)

for {severity, true} <- severities do
  binding_key = severity |> to_string
  AMQP.Queue.bind(channel, queue_name, "direct_logs", routing_key: binding_key)
end

AMQP.Basic.consume(channel, queue_name, nil, no_ack: true)

IO.puts " [*] Waiting for messages. To exit press CTRL+C, CTRL+C"

ReceiveLogsDirect.wait_for_messages(channel)

If you want to save only 'warning' and 'error' (and not 'info') log messages to a file, just open a console and type:

# => mix run receive_logs_direct.exs --warning --error > logs_from_rabbit.log

If you'd like to see all the log messages on your screen, open a new terminal and do:

mix run receive_logs_direct.exs --info --warning --error
# => [*] Waiting for logs. To exit press CTRL+C, CTRL+C

And, for example, to emit an error log message just type:

mix run emit_log_direct.exs --error "Run. Run. Or it will explode."
# => [x] Sent '[error] Run. Run. Or it will explode.'

(Full source code for emit_log_direct.exs and receive_logs_direct.exs)

Move on to tutorial 5 to find out how to listen for messages based on a pattern.