Ruby::Box Practical Guide: Use Cases and Integration Patterns (Part 2)
· 6 min read
In Part 1, we covered what Ruby::Box is and how it provides namespace isolation. Now let’s explore practical patterns for integrating it into real applications.
Use Case: Plugin Systems
Plugin systems benefit significantly from Ruby::Box. Each plugin runs in its own isolated environment, preventing plugins from interfering with each other or the host application.
class PluginManager
def initialize
@plugins = {}
end
def load_plugin(name, path)
box = Ruby::Box.new
box.require(path)
# Access the plugin class from within the box
plugin_class = box.eval('Plugin')
@plugins[name] = {
box: box,
instance: plugin_class.new
}
end
def run(name, method, *args)
plugin = @plugins[name]
plugin[:instance].public_send(method, *args)
end
def unload(name)
@plugins.delete(name)
# Box becomes eligible for garbage collection
end
end
# Usage
manager = PluginManager.new
manager.load_plugin(:markdown, './plugins/markdown_plugin')
manager.load_plugin(:syntax_highlight, './plugins/syntax_plugin')
# Each plugin has its own isolated environment
# If markdown_plugin patches String, syntax_plugin won't see it
manager.run(:markdown, :process, content)
This pattern ensures that a misbehaving plugin cannot corrupt the global namespace or break other plugins.
Use Case: Multi-Tenant Configuration
Applications serving multiple tenants often need per-tenant configurations. Ruby::Box provides clean isolation without complex scoping logic.
class TenantContext
def initialize(tenant_id, config_path)
@tenant_id = tenant_id
@box = Ruby::Box.new
@box.require(config_path)
end
def config
@box.eval('TenantConfig')
end
def execute(code)
@box.eval(code)
end
end
# Each tenant gets isolated configuration
tenant_a = TenantContext.new('acme', './tenants/acme/config')
tenant_b = TenantContext.new('globex', './tenants/globex/config')
tenant_a.config.theme # => "dark"
tenant_b.config.theme # => "light"
# Global variables are isolated too
tenant_a.execute('$rate_limit = 100')
tenant_b.execute('$rate_limit = 500')
tenant_a.execute('$rate_limit') # => 100
tenant_b.execute('$rate_limit') # => 500
Use Case: Running Multiple Gem Versions
During migrations, you might need to run two versions of the same gem simultaneously. Ruby::Box makes this possible without separate processes.
# Load v1 API client in one box
v1_box = Ruby::Box.new
v1_box.eval <<~RUBY
$LOAD_PATH.unshift('./vendor/api_client_v1/lib')
require 'api_client'
RUBY
# Load v2 API client in another box
v2_box = Ruby::Box.new
v2_box.eval <<~RUBY
$LOAD_PATH.unshift('./vendor/api_client_v2/lib')
require 'api_client'
RUBY
# Compare behavior during migration
def compare_responses(endpoint, params)
code = "ApiClient.get('#{endpoint}', #{params.inspect})"
v1_response = v1_box.eval(code)
v2_response = v2_box.eval(code)
if v1_response != v2_response
log_difference(endpoint, v1_response, v2_response)
end
v1_response # Return v1 for now, switch to v2 when ready
end
Use Case: Isolated Monkey Patches for Testing
Some tests require monkey patches that would pollute the global namespace. Ruby::Box keeps these contained.
# test_helper.rb
def create_time_frozen_box(frozen_time)
box = Ruby::Box.new
box.eval <<~RUBY
class Time
def self.now
Time.new(#{frozen_time.year}, #{frozen_time.month}, #{frozen_time.day})
end
end
RUBY
box
end
# In your test
def test_subscription_expiry
box = create_time_frozen_box(Time.new(2026, 1, 1))
# Load and test code within the frozen-time box
box.eval <<~RUBY
expiry_date = Time.new(2025, 12, 31)
subscription = Subscription.new(expires_at: expiry_date)
raise "Expected expired" unless subscription.expired?
RUBY
# Time.now is unchanged outside the box
Time.now # => Current actual time
end
Use Case: Shadow Testing
Run new code paths alongside production code to compare results without affecting users. This pattern is useful for validating refactors or new implementations.
class ShadowRunner
def initialize(production_box, shadow_box)
@production = production_box
@shadow = shadow_box
end
def run(method, *args)
code = "#{method}(#{args.map(&:inspect).join(', ')})"
# Production path returns the result
production_result = @production.eval(code)
# Shadow path runs asynchronously, logs differences
Thread.new do
shadow_result = @shadow.eval(code)
unless production_result == shadow_result
Logger.warn("Shadow mismatch for #{method}",
production: production_result,
shadow: shadow_result
)
end
end
production_result
end
end
Working Around Native Extension Issues
Native extensions may fail to install with RUBY_BOX=1 enabled. The solution is to separate installation from execution:
# Gemfile installation without Boxing
bundle install
# Application execution with Boxing
RUBY_BOX=1 bundle exec ruby app.rb
For CI/CD pipelines:
# .github/workflows/test.yml
jobs:
test:
steps:
- name: Install dependencies
run: bundle install
- name: Run tests with Ruby::Box
run: RUBY_BOX=1 bundle exec rspec
env:
RUBY_BOX: "1"
Working Around ActiveSupport Issues
Some ActiveSupport core extensions have compatibility issues. Load them in your main context before creating boxes:
# At application startup, before creating any boxes
require 'active_support/core_ext/string/inflections'
require 'active_support/core_ext/hash/keys'
# Now create boxes for isolated code
plugin_box = Ruby::Box.new
# Plugins can use the already-loaded extensions
Alternatively, selectively load only what you need inside boxes:
box = Ruby::Box.new
box.eval <<~RUBY
# Load specific extensions that are known to work
require 'active_support/core_ext/object/blank'
RUBY
Performance Considerations
Ruby::Box adds minimal overhead for most operations:
- Method dispatch: Slightly more indirection through separate method tables
- Object creation: Unaffected, objects pass freely between boxes
- Memory: Each box maintains its own class/module definitions
For performance-critical paths, cache class references:
class OptimizedPluginRunner
def initialize(box)
@box = box
# Cache the class reference once
@processor_class = box.eval('DataProcessor')
end
def process(data)
# Use cached reference instead of evaluating each time
@processor_class.new.process(data)
end
end
When to Use Ruby::Box
Good candidates:
- Plugin or extension systems where isolation is critical
- Multi-tenant applications with per-tenant customizations
- Testing scenarios requiring invasive monkey patches
- Gradual migration between gem versions
- Applications loading third-party code that might conflict
Poor candidates:
- Running untrusted or potentially malicious code (use OS-level sandboxing)
- Production systems until the feature stabilizes
- Applications heavily dependent on native extensions
- Simple applications without isolation requirements
Migration Strategy
If you’re considering Ruby::Box for an existing application:
Step 1: Test compatibility
# Run your test suite with Boxing enabled
RUBY_BOX=1 bundle exec rspec
Step 2: Identify issues
Look for failures related to:
- Shared global state across files
- Assumptions about class modifications being visible everywhere
- Native extension loading errors
Step 3: Refactor incrementally
Start with isolated subsystems that don’t share state with the rest of your application. Move more code into boxes as you gain confidence.
Step 4: Monitor in staging
Run your staging environment with RUBY_BOX=1 before considering production deployment.
What’s Next for Ruby::Box
The Ruby core team has discussed building a higher-level “packages” API on top of Ruby::Box. This would provide more ergonomic ways to manage gem isolation without manual box management. Track progress in Ruby Issue #21681.
Ruby::Box solves real problems around namespace pollution and gem conflicts. While still experimental, it’s worth exploring for applications where isolation matters. Start with non-critical paths, understand the limitations, and provide feedback to the Ruby core team as you experiment.
