Deploying Elixir apps with Ansible

By Jake Morrison in DevOps on Sun 16 June 2019

We run high traffic apps on dedicated servers. Elixir runs great on bare metal, as it easily takes advantage of all the cores on the machine. We can get a machine with 24 CPU cores, 32 GB of RAM and 10TB of traffic for under $100 month. Try that in the cloud. You can cut your hosting bills by 90%.

Dedicated server price

When deploying to bare metal, we use Ansible, an easy-to-use standard tool for managing servers. It has reliable and well documented primitives to handle logging into servers, uploading files and executing commands: just what we need to deploy an Elixir app. We also use it when deploying to AWS, setting up AMIs for use in auto-scaling groups. This guide shows how to set up and run a Phoenix app, talking to a Postgres database. It is based on this working template and the principles in "Best practices for deploying Elixir apps". It is recommended that you get the example up and running before you go on to prepare your own applications for deployment.

Install Ansible

On your local dev machine, install Ansible with the Python pip command:

pip install ansible

See the Ansible docs for other options.

Clone the example application

git clone https://github.com/cogini/mix-deploy-example.git

Set SSH alias and Ansible host list

Ansible uses ssh to talk to the server. On your local dev machine, add an ssh host alias in the ~/.ssh/config file so you can reference the server using its name.

# Set this to the actual IP of your server
Host web-server1
    HostName 123.45.67.89

You can use any name you like, but it needs to match the name in the Ansible "inventory", e.g. ansible/inventory/hosts. This file puts hosts into groups so we can manage them together, e.g.:

[web_servers]
web-server1
web-server2

[db_servers]
db-server1
db-server2

[web-servers] is a group of web servers. web-server1 is a hostname from the Host line in your .ssh/config file.

The configuration variables defined in inventory/group_vars/all apply to all hosts in your project. They are overridden by vars for more specific groups like inventory/group_vars/web_servers or for individual hosts, e.g. inventory/host_vars/web-server.

Configure the Elixir app

We use our elixir-release Ansible role to set up and deploy the Elixir app.

Under the ansible dir, edit inventory/group_vars/all/elixir-release.yml to match the Elixir app you are deploying:

# External name of the app, used to name directories and the systemd unit
elixir_release_app_name: mix-deploy-example

See the docs for the elixir-release Ansible role for more options.

Configure OS accounts

We use our users Ansible role to manage the accounts used to deploy and run the app, as well as control access for system administrators and developers.

For security, we use separate accounts to deploy the app and to run it. The deploy account owns the code and config files, and has rights to restart the app. We normally use a separate account called deploy. The app runs under a separate account with the minimum permissions it needs. We normally create a name matching the app, e.g. foo or use a generic name like app.

Configure users and associated ssh keys in inventory/group_vars/all/users.yml. The following defines a user jake, getting the ssh keys from their GitHub profile, and a user ci for a CI/CD server account, getting the ssh key from a file.

users_users:
  - user: jake
    name: "Jake Morrison"
    github: reachfh
  - user: ci
    name: "CI server"
    key: ci.pub

users_deploy_users defines users that are allowed to log into the deploy account on the server via ssh, e.g.:

users_deploy_users:
  - jake
  - jenkins

users_global_admin_users defines admin users, e.g. for your ops team. The following creates a separate account called bob on the server with sudo:

users_global_admin_users:
  - bob

See the docs for the users Ansible role for more options.

We split the deployment into two phases, setup and deploy. In the setup phase, we do the tasks that require elevated permissions, e.g. creating user accounts, creating app dirs, installing OS packages, and setting up the firewall.

In the deploy phase, we push the latest code to the server and restart it. The deploy doesn't require admin permissions, so it can run from a regular user, e.g. the build server.

Set up the web server

Once things are configured, run Ansible to do initial server setup, creating users and configuring the firewall.

From your local dev machine, run this playbook:

ansible-playbook -u root -v -l web_servers playbooks/setup-web.yml -D

The -u flag specifies the user for bootstrapping, after that you would normally use your own admin user. The user needs to be root or have sudo permissions. Depending on the hosting provider's provisioning process, that might be root or a default user like centos or ubuntu with your keypair installed. See playbooks/manage-users.yml for other connection options, e.g. specifying a root password manually.

The -v flag controls verbosity, you can add more v's to get more debug info. The -D flag shows diffs of the changes Ansible makes on the server. If you add --check to the Ansible command, it will show you the changes it is planning to do, but doesn't actually run them.

This playbook uses the iptables and iptables-http roles to set up the base firewall and port forwarding to redirect port 80/443 to the non-privilged port the app listens on.

Set up the app

Next, set up the server to run the app, creating directories and configuring the systemd unit. The mix_systemd Elixir library creates a systemd unit file for the app, and the mix_deploy library generates utility scripts.

When deploying to a single server, you can build on the server and run the scripts to set it up. When deploying to more servers, we use the elixir-release Ansible role. It creates directories and copies the generated files from the local project to the server.

From your local dev machine, run this playbook:

ansible-playbook -u $USER -v -l web_servers playbooks/deploy-app.yml --skip-tags deploy -D

This runs all the tasks in the role except those tagged with deploy, which are instead run from the CI/CD server.

Deploy the app config

Instead of baking secrets like db passwords into the release file, we create a config file and copy it to the app config dir under /etc.

Here we use the Ansible vault to manage app secrets.

First, configure the db server settings in inventory/group_vars/all/db.yml db_password in inventory/group_vars/all/secrets.yml and secret_key_base in inventory/group_vars/web_servers/secrets.yml.

From your local dev machine, run this playbook to generate a TOML config file with the secrets and push it to the web servers:

ansible-playbook -u $USER -v -l web_servers playbooks/config-web.yml -D

See templates/app.config.toml.j2.

Deploy the app to web server

Finally, we are ready to deploy the app. We build the release on a build server, then push it to prod servers and restart.

On a CI/CD server, run the following playbook:

ansible-playbook -u deploy -v -l web_servers playbooks/deploy-app.yml --tags deploy --extra-vars ansible_become=false -D

-u deploy specifies that the CI server should connect to the target server as the deploy user using the ssh key we set up before.

Other tasks

Ansible is useful for all sorts of admin tasks.

This playbook sets up the build server, installing ASDF version manager and checking out the app from git:

ansible-playbook -u root -v -l build_servers playbooks/setup-build.yml -D

See inventory/group_vars/build_servers/vars.yml, particularly app_repo for the URL of the git repo.

You can install Ansible on the build machine with:

ansible-playbook -u $USER -v -l web_servers playbooks/setup-ansible.yml -D

The following playbook sets up a Postgres database:

ansible-playbook -u $USER -v -l db_servers playbooks/setup-db.yml -D

Configuration is in inventory/group_vars/db_servers/postgresql.yml.