Understanding and Debugging Anonymous Modules in Ruby: A Deep Dive with Kaminari

When working with Rails caching, you might encounter the cryptic error: TypeError: can't dump anonymous module. This article explores what anonymous modules are, why they can't be serialized, and how to debug these issues using Kaminari as a real-world example.

Understanding Anonymous Modules
Why Anonymous Modules Can't Be Serialized
The Kaminari Case Study
Debugging Techniques
Solutions and Best Practices

Understanding Anonymous Modules

Anonymous modules in Ruby are modules created without an explicit name. They're commonly created using Module.new or through dynamic meta-programming:

# Named module
module NamedModule
  def some_method
    "Hello"
  end
end

# Anonymous module
Module.new do
  def some_method
    "Hello"
  end
end

Anonymous modules are frequently used for:

Dynamic trait composition
Meta-programming features
Runtime behavior modification
Concern and mixin implementation

Why Anonymous Modules Can't Be Serialized

Ruby's Marshal, which Rails caching uses by default, can't serialize anonymous modules for several reasons:

No constant reference:

# This works
module Named
  def method; end
end
Marshal.dump(Named)

# This fails
anonymous = Module.new { def method; end }
Marshal.dump(anonymous) # TypeError: can't dump anonymous module

No guaranteed uniqueness:

# Each creates a new, unique module
3.times do
  Module.new { def method; end }
end

State restoration issues:

# How would this be restored?
class MyClass
  include Module.new { def dynamic; end }
end

The Kaminari Case Study

Kaminari creates anonymous modules during pagination setup. Here's a simplified version of what happens:

module Kaminari
  module PageScopeMethods
    # This creates anonymous modules dynamically
    Kaminari.config.instance_values.each do |key, value|
      # Dynamic module creation for configuration
      Module.new do
        define_method(key) { value }
      end
    end
  end
end

When you try to cache a paginated collection:

# This fails because the paginated collection includes anonymous modules
Rails.cache.write('posts', Post.page(1))

Debugging Techniques

1. Module Creation Tracking

module ModuleTracker
  def self.trace_module_creation
    TracePoint.new(:class) do |tp|
      if tp.self.is_a?(Module) && tp.self.name.nil?
        puts "Anonymous module created at:"
        puts "Location: #{tp.path}:#{tp.lineno}"
        puts "Backtrace:\n#{caller.join("\n")}"
      end
    end.enable
  end
end

# Usage
ModuleTracker.trace_module_creation
Post.page(1) # Will show module creation traces

2. Object Space Analysis

module AnonymousModuleFinder
  def self.find_anonymous_modules
    ObjectSpace.each_object(Module).select { |mod| mod.name.nil? }
  end
  
  def self.analyze_anonymous_module(mod)
    {
      object_id: mod.object_id,
      methods: mod.instance_methods(false),
      included_in: find_including_classes(mod),
      source_location: find_source_location(mod)
    }
  end
  
  private
  
  def self.find_including_classes(mod)
    ObjectSpace.each_object(Class).select do |klass|
      klass.included_modules.include?(mod)
    end
  end
  
  def self.find_source_location(mod)
    mod.instance_methods(false).map do |method|
      [method, mod.instance_method(method).source_location]
    end.to_h
  end
end

3. Cache Operation Monitoring

module CacheDebugger
  def write(name, value, options = nil)
    begin
      super
    rescue TypeError => e
      if e.message.include?('anonymous')
        debug_value(value)
        raise
      end
    end
  end
  
  private
  
  def debug_value(value)
    puts "Failed to cache: #{value.class}"
    if value.respond_to?(:included_modules)
      puts "Included modules:"
      value.included_modules.each do |mod|
        puts "- #{mod.name || '<anonymous>'}"
      end
    end
  end
end

Rails.cache.extend(CacheDebugger)

4. Method Resolution Tracing

module MethodResolutionTracer
  def method_missing(method, *args)
    if caller.any? { |line| line.include?('kaminari') }
      puts "Method missing: #{method}"
      puts "Called from: #{caller.first}"
    end
    super
  end
end

class ActiveRecord::Base
  prepend MethodResolutionTracer
end

5. Include Hook Monitoring

module IncludeMonitor
  def included(base)
    if self.name.nil?
      puts "Anonymous module included in #{base}"
      puts "Include location: #{caller.first}"
    end
    super
  end
end

Module.prepend(IncludeMonitor)

Solutions and Best Practices

Use Named Modules:

# Instead of
Module.new do
  def method; end
end

# Use
module NamedModule
  def method; end
end

Cache Serializable Data:

# Instead of caching the collection
Rails.cache.fetch('posts') do
  Post.page(1)
end

# Cache the data
Rails.cache.fetch('posts') do
  Post.page(1).map { |p| { id: p.id, title: p.title } }
end

Use Alternative Serialization:

config.cache_store = :memory_store, {
  serializer: JSON
}

Extract Dynamic Behavior:

# Instead of dynamic modules
class Post
  def self.paginate(page)
    # Direct implementation
  end
end

Conclusion

Anonymous modules are powerful but can cause serialization issues. When debugging these problems:

Track module creation
Monitor object space
Trace method resolution
Watch include hooks
Debug cache operations

The key is understanding where and why anonymous modules are created, and either:

Replace them with named modules
Avoid caching objects containing them
Use alternative serialization methods
Restructure the code to avoid dynamic module creation

Remember: Just because you can create anonymous modules doesn't mean you should, especially when caching is involved.

joeywang/Understanding and Debugging Anonymous Modules in Ruby: A Deep Dive with Kaminari.md