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Using ExAws to setup SQS, SNS, and consume messages

ExAws is a collection of Elixir libraries to interact with Amazon Web Services.

The ExAws documentation is in general very good, but I figured it would be nice to show example code of a concrete solution using a small subset of the functionality. Hopefully you can expand/modify the code to your own needs. I assume the reader is familiar with Elixir and has some basic AWS knowledge.

In this post I will describe how to accomplish the following with Elixir and ExAws:

  1. Create a SQS queue
  2. Create a subscription to an existing SNS topic
  3. Set SNS subscription attributes
  4. Consume messages from the SQS queue

The presented code will be functioning but of prototype nature. That means tests, error handling and refactoring opportunities are left to the reader as an exercise ;-)

If you want to jump straight to the code it is up on github.com/vorce/ex_aws_example.

Setup

First we need to add the ex_aws dependencies to our project:

In mix.exs:

defp deps do
  [
    {:ex_aws, "~> 2.1"},
    {:ex_aws_sqs, "~> 3.0"},
    {:ex_aws_sns, "~> 2.0"},
    {:sweet_xml, "~> 0.6"},
    {:hackney, "~> 1.15"},
    {:poison, "~> 4.0"}
  ]
end

and fetch them (mix deps.get).

Create a SQS queue

This is fairly straight forward with ExAws.SQS.create_queue/2. Let’s start with a fresh module to hold our functions that we will expand on as we go along.

defmodule ExAwsExample do
  @moduledoc "Collection of functions demonstrating how to use ExAws.SQS and ExAws.SNS"

  alias ExAws.SQS

  def create_queue(queue_name, opts \\ []) do
    queue_name
    |> SQS.create_queue(opts)
    |> ExAws.request()
  end
end

queue_name is just a string, opts is an optional keyword list containing any extra options you might need (outlined here) - leave it out completely or set it to the empty list ([]) if you don’t need any attributes.

Create a subscription to an existing SNS topic

To create a subscription we need to use ExAws.SNS.subscribe/3. Since you can subscribe different things to a SNS topic, it’s not as self explanatory how to use it. You can get the topic_arn from the AWS console (looks something like “arn:aws:sns:region:12345678:topicname”), this identifies the SNS topic you want to get messages from. It’s not immediately clear what values we can use for the protocol parameter, looking at the AWS console we can see the following options:

AWS SNS Subscription options

So far I know these values work: "email", "http", "https", and "sqs" - in this example we will use "sqs". The third parameter to the subscribe function is a string called endpoint, this is where the message from the topic will go. In our SQS case this will be the ARN of the SQS queue we want the messages from the SNS topic to end up in. How do we get the ARN of the queue then? Ideally it would be returned in the response from the create_queue call, but it’s not. We do get the queue URL back from that call though and we can transform it to the ARN.

With our new knowledge let’s add a new alias and two functions to our module.

alias ExAws.SNS

def create_sqs_subscription(topic_arn, queue_url) do
  queue_arn = queue_url_to_arn(queue_url)

  topic_arn
  |> SNS.subscribe("sqs", queue_arn)
  |> ExAws.request()
end

defp queue_url_to_arn(queue_url) do
  [_protocol, "", host, account_id, queue_name] = String.split(queue_url, "/")
  [service, region, _, _] = String.split(host, ".")

  "arn:aws:#{service}:#{region}:#{account_id}:#{queue_name}"
end

Let’s run our code. To do that we need to specify our AWS credentials. The easiest way is to set the two environment variables AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY (more info in ex_aws docs) for our iex session. We also want to explicitly set the region in config.exs: AWS_ACCESS_KEY_ID=... AWS_SECRET_ACCESS_KEY=... iex -S mix

Here’s how to use our two functions to create a queue, and then set up a subscription:

{:ok, %{body: %{queue_url: queue_url}}} = ExAwsExample.create_queue("my-great-queue")
{:ok, _} = ExAwsExample.create_sqs_subscription(my_topic_arn, queue_url) # assuming you know the arn of the SNS topic

After running this code you should see a queue in the SQS console called “my-great-queue”, and a subscription for it in the SNS console. At this point you might expect that you can publish messages to the topic and they end up in the queue - but not so fast.

Permissions

We need to make sure that we give permission to the topic to publish to our queue. You can do this in the AWS SQS console - in fact it will automatically add the correct permissions if you manually add a subscription to a SNS topic. You can of course also accomplish the same thing with code which is handy. We have to add a policy option in the create_queue call. A policy is just a json document, more details in the AWS docs. The policy below will grant the topic permission to send messages to our particular queue.

def queue_policy(region, account_id, queue_name, topic_arn) do
  ~s"""
  {
    "Version": "2012-10-17",
    "Statement": [
      {
        "Effect": "Allow",
        "Principal": {
          "AWS": "*"
        },
        "Action": [
          "sqs:SendMessage"
        ],
        "Resource": "arn:aws:sqs:#{region}:#{account_id}:#{queue_name}",
        "Condition": {
          "ArnEquals": {
            "aws:SourceArn": "#{topic_arn}"
          }
        }
      }
    ]
  }
  """
end

Let’s create a new queue with a policy:

my_policy = ExAwsExample.queue_policy("eu-west-1", "123456778900", "my-great-queue2", my_topic_arn)
{:ok, %{body: %{queue_url: queue_url}}} = ExAwsExample.create_queue("my-great-queue2", policy: my_policy)
{:ok, _} = ExAwsExample.create_sqs_subscription(my_topic_arn, queue_url)

You can now verify that messages end up in the queue by manually publishing something to your SNS topic in the AWS web console.

Set SNS subscription attributes

A very useful attribute we can set when the subscription protocol is “sqs” is “RawMessageDelivery”, this attribute will skip the metadata/envelope around the actual contents of the message. We definitely want this on our subscription. To make it happen we need to use SNS.set_subscription_attributes/3.

In some cases you only want to forward a subset of the messages that flows to the SNS topic to the queue. To accomplish that we need to apply a filter policy to the subscription.

Ok, so we have two propertiers we need to set on the SNS subscription — time for another function:

def set_subscription_attributes(subscription_arn, opts \\ []) do
  filter = Keyword.get(opts, :filter, "{}") # default to an empy filter json document
  raw_message_delivery = Keyword.get(opts, :raw_message_delivery, "true")

  "FilterPolicy"
  |> SNS.set_subscription_attributes(filter, subscription_arn)
  |> ExAws.request()

  "RawMessageDelivery"
  |> SNS.set_subscription_attributes(raw_message_delivery, subscription_arn)
  |> ExAws.request()
end

The subscription_arn is conveniently returned in the response from the subscribe function. How the filter option look depend on the messages you expect, but here’s an example:

@filter_policy ~s"""
{
  "Source": ["MySystemX"],
  "PayloadType": ["user_event"],
  "PayloadVersion": [{"numeric": ["=", 1]}]
}
"""

So after successfully creating the SNS subscription, we can set the subscription attributes:

...
{:ok, %{body: %{subscription_arn: sub_arn}}} = ExAwsExample.create_sqs_subscription(my_topic_arn, queue_url)
ExAwsExample.set_subscription_attributes(sub_arn, opts)

Consume messages from the SQS queue

Now that we have building blocks for setting up our infrastructure we can build the consumer for the queue that will get and process the messages that comes in. We want the message consumer to continuously poll the SQS queue, so let’s create a worker and add it to our supervision tree.

Consumer GenServer

We need a new module (sqs_consumer.ex) that will contain our queue consumer functionality.

defmodule ExAwsExample.SQSConsumer do
  @moduledoc """
  Consumes messages from a SQS queue
  """
  alias ExAws.SQS

  require Logger

  use GenServer

  @account_id "041669274849"
  @queue_name "my-great-queue2"
  @queue_url "https://sqs.eu-west-1.amazonaws.com/#{@account_id}/#{@queue_name}"
  @topic_arn "arn:aws:sns:eu-west-1:#{@account_id}:test"
  @subscription_filter_policy "{}"

  def start_link(_) do
    GenServer.start_link(__MODULE__, :ok, name: __MODULE__)
  end

  @impl true
  def init(:ok) do
    Logger.debug("Setting up queue #{@queue_name} and subscription to topic #{@topic_arn}")

    policy = ExAwsExample.queue_policy("eu-west-1", @account_id, @queue_name, @topic_arn)

    {:ok, %{body: %{queue_url: queue_url}}} =
      ExAwsExample.create_queue(@queue_name, policy: policy)

    {:ok, %{body: %{subscription_arn: sub_arn}}} =
      ExAwsExample.create_sqs_subscription(@topic_arn, queue_url)

    subscription_opts = [filter: @subscription_filter_policy, raw_message_delivery: "true"]
    ExAwsExample.set_subscription_attributes(sub_arn, subscription_opts)

    schedule_check()

    {:ok, %{queue_name: @queue_name, last_message_time: nil}}
  end

  def schedule_check(check_interval \\ 1_000) do
    Process.send_after(self(), :get_messages, check_interval)
  end

  def handle_messages() do
    case get_messages(@queue_url, wait_time_seconds: 5, max_number_of_messages: 10) do
      {:ok, []} ->
        :ok

      {:ok, messages} ->
        Logger.info(
          "Received #{length(messages)} messages from queue #{@queue_name}, processing them..."
        )

        process_messages(messages)

      {:error, _} = unexpected ->
        Logger.error(
          "Could not get messages from queue #{@queue_name}, reason: #{inspect(unexpected)}"
        )
    end
  end

  defp get_messages(queue_url, opts) do
    result =
      queue_url
      |> SQS.receive_message(opts)
      |> ExAws.request()

    with {:ok, %{body: %{messages: messages}}} <- result, do: {:ok, messages}
  end

  def process_messages(messages) do
    Enum.each(messages, fn message ->
      Logger.info("Handling message: #{inspect(message)}")
      # do interesting stuff here
    end)

    messages
  end

  @impl GenServer
  def handle_info(:get_messages, state) do
    handle_messages()
    schedule_check()

    {:noreply, state}
  end
end

This is a lot at once, but what it is doing is connecting to the queue and asking for messages. To start the consumer we need to add it to the application supervisor.

We are missing one thing though. After we are done with the message we should remove it so that no other consumer will also process it. We need another function in the ExAwsExample module:

def delete_message(queue, receipt) do
  queue
  |> SQS.delete_message(receipt)
  |> ExAws.request()
end

The receipt is an identifier for the message, contained in the message itself. Let’s call delete_message/2 after process_messages/1 in SQSConsumer:

messages
|> Enum.each(fn %{receipt_handle: receipt_handle} ->
  ExAwsExample.delete_message(@queue_url, receipt_handle)
end)

Note: You really want to only delete the message if you know the processing of it went well. If something goes wrong consider using deadletter queues.

Putting it together

We now have most of the pieces we need to have a working application.

Full code and instructions on how to run it here.

Happy Elixir AWS hacking!


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