Recent Posts

Call Elixir from Erlang

Posted on 07 Jul 2017

I've been reading the Designing for Scalability with Erlang/OTP book and because of that I've been getting interested in writing some erlang code. I got curious if I could call my elixir code from erlang.

This is very simple. Since module and function names are just atoms in erlang you can call them like this:

'Elixir.IO':'puts'("Hello, world!").

Elixir puts all of it's modules "under" the Elixir atom. So to call an Elixir function named Api.User.changeset() you would use 'Elixir.Api.User':'changeset'().

Include Phoenix Channels in a Rails App

Posted on 01 Mar 2017

My side project Worfcam has a home page that lists the most recent photo taken from a device. It used to poll every 3 seconds for updates to the photos. I now use Phoenix channels to push new photos to the home page. This is how I did that. I got part of the idea from this blog post at akitonrails.com

Security note: it should be pointed out that this has very lax security as it trusts the rails process and elixir process are on the same machine and firewalled away from the world outside of connecting through nginx.

Rails Configuration

First include phoenix.js in your javascript. I managed to get it included by vendoring a hand compiled version straight from the phoenix github. After this is in place, you write your javascript the same way as you would a normal Phoenix app.

To connect the proper socket I load it from the environment. I add a new method to my AppContainer and use it in the javascript when connecting to the Phoenix socket. For local development this is ws://localhost:5300/socket. On production it is wss://example.com/socket.

class AppContainer
  # ...
  let(:channels_url) do
    ENV["CHANNELS_URL"]
  end
end

Pushing Events

The phoenix app will have the following route:

  scope "/", Channels do
    resources "/events", EventController, only: [:create]
  end

The controller will simply broadcast whatever gets sent in. This is why it trusts the network is locked down. A simple way to add authentication to this could be basic auth or digest auth. Or even OAuth 2.0 server to server workflow.

defmodule Channels.EventController do
  use Channels.Web, :controller

  def create(conn, %{"topic" => topic, "event" => event, "body" => body}) do
    Channels.Endpoint.broadcast(topic, event, body)
    conn |> send_resp(204, "")
  end
end

I have the Ecto Repo pointing to the same database and the models have a schema that matches what rails uses. My channels enforce that only the correct users can connect to channels. Channels will be straight forward normal channels.

To push events over from Rails I have a sidekiq job that sends the data required by the create action. Once in place events flow from Rails to Phoenix to the web browser.

class EventPushWorker
  include Sidekiq::Worker

  def perform(topic, event, body)
    return if Rails.env.test?
    response = Faraday.post(AppContainer.channels_event_url) do |req|
      req.headers["Content-Type"] = "application/json"
      req.body = {
        topic: topic,
        event: event,
        body: body,
      }.to_json
    end
  end
end

nginx Configuration

The only tricky part from here is getting Rails to talk to Phoenix in production. Local development works because you can talk directly to the server. For production I have nginx set up a location before the default location that passes to Rails.

upstram worfcam {
  server localhost:5000;
}

upstream channels {
    server localhost:5300;
}

location /socket {
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection $connection_upgrade;
    proxy_set_header Origin $http_origin;
    proxy_pass http://channels;
}

location / {
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto https;
    proxy_set_header Host $http_host;
    proxy_redirect off;
    proxy_pass http://worfcam;
}

Once this is in place, your Rails app doesn't even need to know that it's talking to Phoenix from the front end.

Foreman Systemd Export

Posted on 12 Feb 2017

I switched my side project to a single Linode instance this weekend in an effort to simplify its setup. In the process I decided to try foreman as a way of keeping the rails server up and running. I was very pleased with what came out of it.

Export scripts

To start you need to export systemd scripts into the /etc/systemd/system folder. Make sure to replace --user deploy with the user that your app should run as. This capistrano task will handle that.

namespace :app do
  desc "Reload systemd"
  task :systemd do
    on roles(:web) do
      within release_path do
        execute :sudo, :foreman, :export, :systemd, "/etc/systemd/system", "--user deploy"
        execute :sudo, :systemctl, "daemon-reload"
      end
    end
  end
end

The first deploy you won't need to daemon-reload, but whenever a systemd file changes the daemon needs to reload to know about the changes.

The export will create files that are named app-web@.service, where web is the name of the Procfile line. In systemd an @ service allows you to pass in parameters to the script. For this case, it will be the PORT environment variable.

Start/Stop/Restart

Once the scripts are in place. You need capistrano tasks that will. These tasks are all essentially the same and look like this:

namespace :app do
  desc "Start web server"
  task :start do
    on roles(:web) do |host|
      within release_path do
        execute :sudo, :systemctl, :start, "app-web@5000.service"
        execute :sudo, :systemctl, :start, "app-worker@5100.service"
      end
    end
  end
end

This will start up a web and worker instance with the default ports that foreman uses. As you add in new Procfile lines, you will need to add a new line here.

Adding to a deploy

Finally you need to insert systemd into the deploy cycle. This is easy by adding the tasks in after the appropriate capistrano task deploy:publishing.

after 'deploy:publishing', 'app:systemd'
after 'app:systemd', 'app:restart'

Enhancements

Capistrano connects into the deploy user and that user has sudo rights. It would be more secure to only allow these specific commands to be sudoable. Add this to your sudoers file to enable specific commands only.

deploy ALL=(ALL) NOPASSWD: /usr/bin/systemctl restart app-web@5000.service
deploy ALL=(ALL) NOPASSWD: /usr/bin/systemctl restart app-worker@5100.service

The full config/deploy.rb file

namespace :app do
  desc "Start web server"
  task :start do
    on roles(:web) do |host|
      within release_path do
        execute :sudo, :systemctl, :start, "app-web@5000.service"
        execute :sudo, :systemctl, :start, "app-worker@5001.service"
        execute :sudo, :systemctl, :start, "app-clock@5002.service"
      end
    end
  end

  desc "Stop web server"
  task :stop do
    on roles(:web) do |host|
      within release_path do
        execute :sudo, :systemctl, :stop, "app-web@5000.service"
        execute :sudo, :systemctl, :stop, "app-worker@5001.service"
        execute :sudo, :systemctl, :stop, "app-clock@5002.service"
      end
    end
  end

  desc "Restart web server"
  task :restart do
    on roles(:web) do |host|
      within release_path do
        execute :sudo, :systemctl, :restart, "app-web@5000.service"
        execute :sudo, :systemctl, :restart, "app-worker@5001.service"
        execute :sudo, :systemctl, :restart, "app-clock@5002.service"
      end
    end
  end

  desc "Reload systemd"
  task :systemd do
    on roles(:web) do
      within release_path do
        execute :sudo, :foreman, :export, :systemd, "/etc/systemd/system", "--user deploy"
        execute :sudo, :systemctl, "daemon-reload"
      end
    end
  end
end

after 'deploy:publishing', 'app:systemd'
after 'app:systemd', 'app:restart'

Generating a Guardian Secret Key

Posted on 29 Dec 2016

I've been working on a Phoenix project at work and I added authentication for the API via Guardian. Guardian is easy to use except for setting up the secret key. It took a few tries to finally get a key that worked, and it wasn't easy to find out how.

Generate the key

To start, generate a key in iex -S mix.

jwk_384 = JOSE.JWK.generate_key({:ec, "P-384"})
JOSE.JWK.to_file("password", "file.jwk", jwk_384)

I placed this file in priv/repo/dev.jwk for development purposed. In staging/production I stick the file contents in an environment variable.

Configure Guardian

Once the key is generated, configure the application to load it.

config/config.ex
config :guardian, Guardian,
  allowed_algos: ["ES384"],
  issuer: "MytApp",
  ttl: { 30, :days },
  secret_key: fn ->
    secret_key = MyApp.config(Application.get_env(:my_app, :secret_key))
    secret_key_passphrase = MyApp.config(Application.get_env(:my_app, :secret_key_passphrase))

    {_, jwk} = secret_key_passphrase |> JOSE.JWK.from_binary(secret_key)
    jwk
  end,
  serializer: MyApp.GuardianSerializer
lib/my_app.ex
defmodule MyApp do
  def config({:system, env}), do: System.get_env(env)
  def config(value), do: value
end

I use this set of functions to allow loading config via environment variables.

Configure Environments

config/dev.exs
config :my_app, :secret_key_passphrase, "password"
config :my_app, :secret_key, File.read!("priv/repo/dev.jwk")

Configure the dev environment.

config/prod.exs
config :venu, :secret_key_passphrase, {:system, "SECRET_KEY_PASSPHRASE"}
config :venu, :secret_key, {:system, "SECRET_KEY"}

Conclusion

I hope this helps better explain how to set up Guardian for your Phoenix project.

Elasticsearch Cluster Snapshot & Restore

Posted on 01 Nov 2016

We recently needed to do a cross cluster snapshot and restore for elasticsearch. We were hosted on Elastic Cloud and found out the hard way that the tooling in place there does not work. We did find out that Elasticsearch has a backup repository system in place that works very well. We would have saved ourselves some time if we had just started with this.

Create a user in AWS IAM

First off create a user in AWS IAM that has access to S3. You can attach the full access or limit down to a single bucket as shown in the Elastic Cloud documentation.

{
  "Statement": [
    {
      "Action": [
        "s3:*"
      ],
      "Effect": "Allow",
      "Resource": [
        "arn:aws:s3:::bucket-name",
        "arn:aws:s3:::bucket-name/*"
      ]
    }
  ]
}

Create the Repository

Create the repository in each cluster. This is also taken from that guide. You also need the repository-s3 plugin installed in each cluster for this to work.

sudo bin/elasticsearch-plugin install repository-s3

curl -X PUT localhost:9200/_snapshot/bucket-name -d '{
  "type": "s3",
  "settings": {
    "bucket": "bucket-name",
    "region": "us-east1",
    "access_key": "AKIAYOURKEYHERE",
    "secret_key": "secret-key",
    "compress": true
  }
}'

Snapshot

On the old cluster, create a snapshot. You can check on the status as it processes.

curl -X PUT localhost:9200/_snapshot/bucket-name/snapshot-backup-name
curl -X GET localhost:9200/_snapshot/bucket-name/snapshot-backup-name/_status

There are a lot of options you can provide to the snapshot, including limiting to certain indices.

Restore

On the new cluster, restore the snapshot. You can easily view the status of the restore with regular elasticsearch monitoring tools. The index health with be shown as shards come online.

curl -X POST localhost:9200/_snapshot/bucket-name/snapshot-backup-name/_restore -d '{
  "indices": "one-index"
}'

Conclusion

Hopefully this is of use to others. Snapshotting and restoring manually is a very simple process and was much easier than trying to figure out a custom solution from your elasticsearch host.

Eric Oestrich
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