Async Falcon Rails

Overview

Transform a Rails application to use the Falcon web server with async job processing, async Action Cable, and Redis-compatible database (Valkey for production). This skill handles the complete migration from Puma to Falcon, configures async job adapters, sets up Redis/Valkey for Action Cable and job queues, and configures Kamal deployment for production.

When to Use

Use this skill when the user:

• Wants to add async/Falcon stack to an existing Rails project
• Needs to migrate from Puma to Falcon web server
• Requests async job processing setup with Redis
• Wants to configure async Action Cable
• Needs Kamal deployment configuration for the async stack

Prerequisites

Before applying this skill, verify:

1. The project is a Rails application (check for Gemfile, config/application.rb)
2. The project structure includes config/environments/ directory
3. Bundle is available (bundle command works)
4. If Kamal deployment is needed, check for config/deploy.yml

Workflow

Follow these steps in order to transform a Rails application to use the async/Falcon stack:

Step 1: Update Gemfile Dependencies

Replace Puma with Falcon and add async dependencies:

bundle remove puma
bundle add falcon
bundle add async-job-processor-redis
bundle add async-job-adapter-active_job
bundle add async-cable
bundle add redis

After running these commands, verify the Gemfile includes:

• gem "falcon"
• gem "async-job-processor-redis"
• gem "async-job-adapter-active_job"
• gem "async-cable"
• gem "redis" (provides Async::Redis::Endpoint for endpoint parsing)

Step 2: Update Dockerfile for SSL Dependencies

CRITICAL: The async/Falcon stack requires OpenSSL development libraries to build properly. Without this, Docker builds will fail.

Edit the Dockerfile and add libssl-dev to the system dependencies.

Find the line that installs build packages (usually around line 40):

RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y build-essential git libyaml-dev pkg-config && \
    rm -rf /var/lib/apt/lists /var/cache/apt/archives

Update it to include libssl-dev:

RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y build-essential git libyaml-dev libssl-dev pkg-config && \
    rm -rf /var/lib/apt/lists /var/cache/apt/archives

Why this is needed: The Falcon web server and async gems depend on native extensions that require OpenSSL headers to compile. Without libssl-dev, the Docker build will fail with compilation errors.

Step 3: Create Async Job Configuration

Create config/initializers/async_job.rb with the following content:

require "async/job"
require "async/job/processor/aggregate"
require "async/job/processor/redis"
require "async/job/processor/inline"
require "async/redis/endpoint"

Rails.application.configure do
  # Resolve Redis endpoint from REDIS_URL; fallback to localhost for dev/test.
  redis_endpoint = Async::Redis::Endpoint.parse(
    ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" }
  )

  config.async_job.define_queue "default" do
    enqueue Async::Job::Processor::Aggregate
    # Ensure the job runner connects to the accessory container (or localhost in dev).
    dequeue Async::Job::Processor::Redis, endpoint: redis_endpoint
  end

  config.async_job.define_queue "local" do
    dequeue Async::Job::Processor::Inline
  end
end

This configuration:

• Sets up a "default" queue using Redis for job processing
• Parses the REDIS_URL environment variable to create a proper Redis endpoint
• Passes the endpoint to the Redis processor for both development and production
• Sets up a "local" queue for inline processing
• Uses Aggregate processor for enqueuing and Redis for dequeuing

Step 4: Update Procfile for Development

Update Procfile.dev to include the async job processor:

Add this line to the existing Procfile.dev:

jobs: bundle exec async-job-adapter-active_job-server

The complete Procfile.dev should include at minimum:

web: bin/rails s
jobs: bundle exec async-job-adapter-active_job-server

If the project uses Vite or other frontend build tools, keep those lines as well.

Step 5: Configure Development Environment

Edit config/environments/development.rb to use async_job queue adapter.

Find the section about ActiveJob (usually near config.active_job.verbose_enqueue_logs) and add:

config.active_job.queue_adapter = :async_job

Step 6: Configure Production Environment

Edit config/environments/production.rb to configure both async_job and Redis cache:

Find and update or add these configurations:

# For cache store (usually around line 50)
config.cache_store = :redis_cache_store, { url: ENV["REDIS_URL"] }

# For queue adapter (usually around line 53)
config.active_job.queue_adapter = :async_job

Step 7: Configure Application for Async/Cable

Edit config/application.rb to add async/cable support:

1. At the top of the file, after require "rails/all", add:

require "async/cable"

2. Inside the class Application < Rails::Application block, after config.load_defaults, add:

# Configure async/fiber support
config.active_record.permanent_connection_checkout = :disallowed
config.active_support.isolation_level = :fiber

These settings enable fiber-based isolation for async operations.

Step 8: Generate Action Cable Base Channel

Run the Rails generator to create the base Action Cable structure:

bin/rails generate channel BaseChannel

This creates:

• app/channels/application_cable/channel.rb
• app/channels/application_cable/connection.rb
• app/channels/base_channel.rb
• test/channels/base_channel_test.rb

Step 9: Mount Action Cable in Routes

Edit config/routes.rb to mount the Action Cable server.

Add this line at the top of the Rails.application.routes.draw block:

mount ActionCable.server => '/cable'

Step 10: Configure Cable to Use Redis

Edit config/cable.yml to use Redis for all environments:

Update the configuration to:

development:
  adapter: redis
  url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
  channel_prefix: PROJECT_NAME_production

test:
  adapter: test

production:
  adapter: redis
  url: <%= ENV.fetch("REDIS_URL") { "redis://localhost:6379/1" } %>
  channel_prefix: PROJECT_NAME_production

Replace PROJECT_NAME with the actual Rails application name (found in config/application.rb as the module name).

Step 11: Configure Kamal Deployment (If Applicable)

If the project uses Kamal for deployment (check for config/deploy.yml), update the deployment configuration:

11.1: Add Job Server

In the servers: section, add or uncomment the job server configuration:

servers:
  web:
    - 192.168.0.1
  job:
    hosts:
      - 192.168.0.1
    cmd: bundle exec async-job-adapter-active_job-server
    options:
      init: true

Key configuration:

• init: true: Skips health checks for the job server (avoids 30-second deployment wait)
• Job servers don't expose HTTP endpoints, so health checks would timeout unnecessarily

11.2: Configure Redis/Valkey Accessory

In the accessories: section (create if it doesn't exist), add Redis/Valkey configuration:

accessories:
  redis:
    image: valkey/valkey:9
    host: 192.168.0.1
    port: "127.0.0.1:6379:6379"
    directories:
      - redis_data:/data

Key considerations:

• Important: Use Valkey instead of Redis due to Redis licensing changes. Valkey is a Redis-compatible fork maintained by the Linux Foundation
• Use Valkey 9 or latest stable version
• Port binding "127.0.0.1:6379:6379" prevents public exposure (localhost only)
• Volume name redis_data prevents conflicts with other services (e.g., PostgreSQL)

11.3: Add REDIS_URL Environment Variable

In the env: section under clear:, add:

env:
  clear:
    REDIS_URL: redis://PROJECT_NAME-redis:6379/1

Replace PROJECT_NAME with the service name from the top of deploy.yml. The format follows Kamal's Docker naming convention: {service_name}-{accessory_name}.

11.4: Configure Multi-Architecture Builds

Update the builder: section to support multiple architectures:

builder:
  arch:
    - amd64
    - arm64

This enables building Docker images for:

• amd64: Intel/AMD processors (Windows, Linux, older Macs)
• arm64: Apple Silicon (M1/M2/M3 Macs), ARM-based Linux servers

Verification Steps

After completing the workflow, verify the setup:

1. Gemfile: Check that Puma is removed and all async gems are added
2. Dockerfile: Verify libssl-dev is included in the apt-get install line
3. Initializers: Verify config/initializers/async_job.rb exists and includes endpoint configuration (endpoint: redis_endpoint)
4. Environments: Confirm async_job queue adapter in development.rb and production.rb
5. Application: Verify async/cable require and fiber isolation config in application.rb
6. Cable: Check that Action Cable is mounted in routes.rb
7. Cable Config: Confirm cable.yml uses Redis for development and production
8. Kamal (if applicable): Verify job server with init: true, Redis accessory, REDIS_URL, and multi-arch build config in deploy.yml

Important Notes

Redis/Valkey Dependency

This stack requires Redis-compatible database to be running:

• Development: Start Redis locally with redis-server or use Docker
• Production: Valkey (Redis-compatible) is deployed as a Kamal accessory (configured in deploy.yml)
• Note: We use Valkey instead of Redis in production due to Redis licensing changes. Valkey is a fully Redis-compatible fork maintained by the Linux Foundation

Environment Variables

The REDIS_URL environment variable must be set:

• Development: Defaults to redis://localhost:6379/1 (configured in cable.yml)
• Production: Set via Kamal deploy.yml or environment configuration

Kamal Naming Conventions

When using Kamal:

• Redis accessory will be named: {service_name}-redis
• Use this name in REDIS_URL: redis://{service_name}-redis:6379/1
• Volume names should be descriptive: redis_data, postgres_data, etc.

Port Binding Security

The Redis port binding "127.0.0.1:6379:6379" ensures:

• Redis is accessible to containers on the same Docker network
• Redis is NOT exposed to the public internet
• Only localhost connections are allowed on the host

Troubleshooting

Docker Build Errors

If Docker build fails with compilation errors about OpenSSL or missing headers:

• Symptom: Build fails during gem installation with errors like "openssl/ssl.h: No such file or directory"
• Cause: Missing libssl-dev system dependency in Dockerfile
• Solution: Add libssl-dev to the apt-get install line in Dockerfile (see Step 2)
• Verification: Check that the Dockerfile includes libssl-dev in the package list alongside build-essential, git, libyaml-dev, and pkg-config

This is a critical dependency for Falcon and async gems - without it, the Docker image cannot be built.

Bundle Errors

If bundle add commands fail:

• Check that Gemfile is not locked with version conflicts
• Try bundle update to resolve dependency issues
• Verify network connectivity to rubygems.org

Redis Connection Errors

If the application cannot connect to Redis:

• Development: Ensure Redis is running locally (redis-cli ping should return PONG)
• Production: Check that REDIS_URL environment variable is set correctly
• Verify cable.yml configuration matches the REDIS_URL format

Kamal Deployment Issues

If Kamal deployment fails:

• Verify all placeholders (PROJECT_NAME, IP addresses) are replaced with actual values
• Check that Redis accessory is running: kamal accessory details redis
• Ensure REDIS_URL matches the Kamal service naming convention

Deployment Timeouts

If Kamal deployment hangs for 30 seconds when deploying the job server:

• Symptom: Deployment waits and then shows "Container not ready" for job server
• Cause: Job server doesn't expose HTTP endpoints, so health checks timeout
• Solution: Add options: with init: true to the job server configuration in deploy.yml (see Step 11.1)
• Why it works: init: true tells Kamal to skip health checks for this service

References

For detailed configuration templates and examples, see:

• references/configuration-templates.md - Complete file templates and patterns