The Possibility and Difficulties of Erlang/Elixir Hot Code Swapping in a Dockerized Environment

Hot Code Swapping with Erlang/Elixir in Docker Containers: Modular Solutions

Backend solution using Erlang/Elixir with a hidden node for distributed updates

% Define the Erlang distributed system setup
-module(hot_code_swap).
-export([start_hidden_node/0, distribute_update/1]).

% Start a hidden node for code updates
start_hidden_node() ->
    NodeName = "hidden_node@127.0.0.1",
    Cookie = mycookie,
    {ok, _} = net_kernel:start([{hidden, NodeName}, Cookie]),
    io:format("Hidden node started successfully~n").

% Distribute new code to other nodes
distribute_update(CodePath) ->
    Nodes = nodes(),
    io:format("Distributing code update to nodes: ~p~n", [Nodes]),
    lists:foreach(fun(Node) ->
        rpc:call(Node, code, add_patha, [CodePath]),
        rpc:call(Node, code, load_file, [my_module])
    end, Nodes).

% Example usage
% hot_code_swap:start_hidden_node().
% hot_code_swap:distribute_update("/path/to/new/code").

Updating Elixir Code with a Hot-Swappable Docker-Based Setup

Backend solution using Elixir with code reloading and node management

defmodule HotCodeSwap do
  @moduledoc "Handles hot code swapping in a distributed environment."

  # Start a hidden node for managing updates
  def start_hidden_node do
    :net_kernel.start([:"hidden_node@127.0.0.1", :hidden])
    IO.puts("Hidden node started.")
  end

  # Function to push updates to other nodes
  def distribute_update(code_path) do
    nodes = Node.list()
    IO.puts("Updating nodes: #{inspect(nodes)}")

    Enum.each(nodes, fn node ->
      :rpc.call(node, Code, :append_path, [code_path])
      :rpc.call(node, Code, :load_file, ["my_module.ex"])
    end)
  end
end

# Example usage
HotCodeSwap.start_hidden_node()
HotCodeSwap.distribute_update("/path/to/new/code")

Automating Docker Build and Restart for Hot Code Updates

Script for managing Docker containers with minimal downtime

#!/bin/bash
# Script to automate Docker-based hot code swapping

APP_NAME="my_elixir_app"
NEW_TAG="my_app:latest"
CONTAINER_NAME="elixir_app_container"

echo "Building new Docker image..."
docker build -t $NEW_TAG .

echo "Checking running container..."
RUNNING_CONTAINER=$(docker ps -q -f name=$CONTAINER_NAME)

if [ -n "$RUNNING_CONTAINER" ]; then
    echo "Stopping current container..."
    docker stop $CONTAINER_NAME
fi

echo "Starting updated container..."
docker run -d --name $CONTAINER_NAME $NEW_TAG
echo "Hot swap completed!"

Unit Tests for Distributed Erlang Hot Code Swap

Unit test suite written in Erlang to verify code distribution

-module(hot_code_swap_tests).
-include_lib("eunit/include/eunit.hrl").

start_hidden_node_test() ->
    ?assertMatch({ok, _}, net_kernel:start([{hidden, "test_node@127.0.0.1"}, test_cookie])).

distribute_update_test() ->
    CodePath = "/tmp/new_code",
    Nodes = [node1@127.0.0.1, node2@127.0.0.1],
    lists:foreach(fun(Node) ->
        ?assertEqual(ok, rpc:call(Node, code, add_patha, [CodePath]))
    end, Nodes).

Erlang/Elixir Hot Code Swap and Docker: FAQs

1. What is hot code swapping in Erlang/Elixir?
2. Hot code swapping allows developers to update a running application without stopping it, using commands like code:load_file.
3. Why does Docker conflict with hot code swapping?
4. Docker focuses on immutability, requiring updates to be deployed with a fresh container using commands like docker build and docker run.
5. What is the role of a hidden node in hot code swapping?
6. A hidden node, started with net_kernel:start, can distribute updates to other nodes without becoming publicly visible in the cluster.
7. Can hot code swapping work alongside Docker containers?
8. Yes, by using a control node to push updates dynamically or separating application updates from container management processes.
9. What are the limitations of hot code swapping?
10. While powerful, it requires careful planning to avoid version conflicts, and complex updates may still necessitate a full container restart.
11. How does Docker ensure reliability in updates?
12. Docker uses commands like docker stop and docker run -d to restart applications cleanly with minimal downtime.
13. What are the benefits of combining Docker and hot code swapping?
14. This combination ensures near-zero downtime for updates, ideal for critical systems like payment gateways or real-time communication apps.
15. How can you validate distributed code updates?
16. Use commands like rpc:call to verify updates across nodes and implement automated unit tests for safety.
17. What kind of projects benefit the most from hot code swapping?
18. Applications requiring high availability, like live streaming platforms, IoT systems, or multiplayer games, benefit significantly.
19. Can hybrid approaches work for managing updates?
20. Yes, by using Docker for base deployments and hot swapping for live updates, you can achieve both safety and flexibility.

Key Takeaways for Balancing Docker and Hot Code Swapping

Bringing hot code swapping to a Dockerized environment requires blending modern container practices with Erlang/Elixir’s dynamic code features. While it sounds complex, it’s achievable with careful planning and distributed update strategies.

Using hidden nodes to broadcast changes allows teams to maintain uptime for critical systems. For simpler workflows, combining container restarts with strategic hot swaps offers a reliable solution, minimizing disruptions. 🔧