Distributed Elixir in Amazon ECS

Does the same applies for Elastic Beanstalk with applications deployed using Docker?

So after some investigation, it does look like the A records get removed immediately with the stop requests. It also appears that the application does not get a SIGTERM for quite a while after that – the container remains active all the way through to the end of the DEPROVISIONING state.

The limitation with the number of A records being limited to 8 stands, however. More than that and the cluster will experience lots of random disconnects and reconnects. It is possible that things may eventually stabilze (seems like it should in theory) – I am still looking at this.

Thanks for this write-up. However, note that Route53 multi-value DNS will only ever return 8 records maximum. This limits the number of nodes in a cluster using this strategy to 8. Any more than that, and the results returned by the dns query will vary each time, causing libcluster to think nodes are coming and going, and you will see lots of disconnects/reconnects.

There is also a possible issue with the task IPs getting removed from the private namespace zone only after a task has transitioned to the STOPPED state (which seems like a major design flaw), not giving a task being shut down a chance to drain any pending requests and account for the A record TTL. I have not fully investigated this, however.

I struggled with this for a week and eventually got it working. The information in this thread is enough to point in the right direction, but there is a big gap between the right direction and a successful deployment.

I’ve written up a fresh guide on deploying distributed Elixir to ECS Fargate tasks with Service Discovery and libcluster’s DNS Polling strategy. Hope it helps save someone some time!

I am using the bridge network mode, with the custom peerage provider for srv records.

 NAME                RESULT OF ATTEMPT
 api@172.31.13.73 true      
 api@172.31.6.233 false     

 LIVE NODES
 api@172.31.13.73 (self)

Not really sure why it won’t connect. Any help is appreciated.

We use a custom Peerage.Provider implementation like

defmodule AwsServiceDiscovery do
  @behaviour Peerage.Provider

  @impl true
  def poll do
    dns_name = Application.fetch_env!(:peerage, :dns_name)
    app_name = Application.fetch_env!(:peerage, :app_name)

    :inet_res.lookup(String.to_charlist(dns_name), :in, :srv)
    |> Enum.flat_map(fn {_priority, _weight, _port, srv_dns_name} ->
      :inet_res.lookup(srv_dns_name, :in, :a)
      |> Enum.map(fn ip ->
        :"#{app_name}@#{:inet.ntoa(ip) |> to_string}"
      end)
    end)
  end
end

Our tasks are using awsvpc network mode. It has some limits depending on EC2 instance type Elastic network interfaces - Amazon Elastic Compute Cloud . With Fargate you don’t need to think about this limitation

I use Fargate, so it uses the A record for service discovery.

Hey guys, thanks for all the info, it helped me a lot set it up myself.

You’re all running one node per instance, with a fixed port mapping from container to host, right?

AWS doesn’t allow A records in the service registry unless the networkMode of the container is awsvpc (where each container has its own elastic network interface and IP address), I suppose because there could be multiple instances per host, running on different ports. It has to be SRV, which is fine.

The DNSPoll strategy only polls A records by default, so I’m curious about whether you guys had to use a custom resolver like I did, or if there’s a more straightforward way?

Yep that’s exactly what we are using as well now. BTW the erlang communication port can be a single port.