Deep Dive into Ruby Internals: From Rails Caching to C-Level Object IDs 2024-10-27

Posted Oct 27, 2024

By Joey

4 min read

Deep Dive into Ruby Internals: From Rails Caching to C-Level Object IDs

When debugging Rails applications, sometimes we stumble upon fascinating aspects of Ruby’s internal workings. This article shares a journey that started with a simple Rails caching issue and led us deep into Ruby’s object model and memory management system.

The Starting Point: Rails Caching and Kaminari

It all began with what seemed like a straightforward Rails caching operation:

  
Rails.cache.write(Lesson.page(1))

This line threw an unexpected error. We were using Kaminari for pagination, and something wasn’t quite right. The error message was cryptic:

  
TypeError: can't dump anonymous module #<Module:0x000000012028d860>

Understanding Anonymous Modules

To understand what was happening, let’s first look at how anonymous modules are typically created in Ruby:

  
# Creating an anonymous module
dynamic_module = Module.new do
  def some_method
    "Hello from anonymous module"
  end
end

# Using the anonymous module
class MyClass
  include dynamic_module
end

Anonymous modules are commonly used for:

Creating dynamic mixins
Implementing plugin systems
Metaprogramming scenarios

However, they come with certain limitations, particularly when it comes to serialization.

Debugging with Ruby’s Built-in Debugger (rdbg)

To investigate further, we used Ruby’s built-in debugger:

  
require 'debug'

# Set a breakpoint to catch the TypeError
catch TypeError do
  Rails.cache.write(Lesson.page(1))
end

When the error occurred, we were inside the debugger and tried to inspect the module:

  
(rdbg) p self
#<Module:0x000000012028d860>

Attempting to find the source code or definition of this module led to an interesting discovery - there was none! The module was created at the C level, which explained why we couldn’t access its Ruby source code.

Marshal’s Restrictions

The error came from Marshal, Ruby’s built-in serialization system. Marshal has specific restrictions, one of which is the inability to dump anonymous modules. Here’s why:

  
# This works fine
Marshal.dump(String)

# This raises TypeError
anonymous_module = Module.new
Marshal.dump(anonymous_module)  # TypeError: can't dump anonymous module

This restriction exists because:

Anonymous modules lack persistent names
They might contain context-dependent behavior
Their state might be tied to the current runtime

Digging Deeper: Module Inspection

When debugging modules (named or anonymous), we can gather information using various Ruby methods:

  
module SomeModule
  def self.some_method; end
end

# Basic inspection
p SomeModule.name                    # "SomeModule"
p SomeModule.instance_methods(false) # [:some_method]
p SomeModule.ancestors              # [SomeModule, Object, Kernel, BasicObject]

# For anonymous modules
anonymous_mod = Module.new
p anonymous_mod.name                 # nil
p anonymous_mod.object_id            # Some number like 8980

The Connection: Ruby Object IDs and C-Level Memory Addresses

During our investigation, we discovered an interesting relationship between Ruby’s object IDs and C-level memory addresses:

  
# In debugger:
(rdbg) self
#<Module:0x000000012029fb50>
(rdbg) object_id
8980

The connection between these identifiers reveals how Ruby manages objects internally:

The hex address (0x000000012029fb50) is the actual memory location in C
The object_id (8980) is Ruby’s internal object identifier
There’s a mathematical relationship between them: object_id * 2 approximates the memory address

Here’s how we can work with these IDs:

  
# Getting object from Ruby object_id
obj = ObjectSpace._id2ref(8980)

# Getting hex representation of object_id
hex_addr = "0x%016x" % (obj.object_id * 2)

# For immediate values (small integers, symbols, etc.)
num = 42
p num.object_id      # Will be 85 (42 * 2 + 1)

Ruby’s Object Model Under the Hood

This investigation reveals several important aspects of Ruby’s object model:

Object Tagging System:
- Regular objects have even-numbered object_ids
- Immediate values (small integers, symbols) have special representations
- The lowest bits are used for type tagging
Memory Layout: ```plaintext Regular Objects: [ klass | flags | instance variables ] ^ | object_id * 2 points approximately here

Immediate Values: [ value | tag bits ]

3. Garbage Collection Considerations:
```ruby
require 'objspace'

# Track object allocation
ObjectSpace.trace_object_allocations do
  obj = Object.new
  p ObjectSpace.allocation_sourcefile(obj)
  p ObjectSpace.allocation_sourceline(obj)
end

Practical Implications

This deep dive has several practical implications for Ruby developers:

Caching Considerations:
- Be careful when caching objects that might contain anonymous modules
- Consider implementing custom serialization for such cases
Debugging Strategies:
- Use object_id and hex addresses to track objects across contexts
- Understand when you’re dealing with C-level vs. Ruby-level code
Memory Management:
- Awareness of how Ruby manages object identity
- Understanding of immediate values vs. regular objects

Conclusion

What started as a simple Rails caching issue led us through a fascinating journey into Ruby’s internals. We learned about:

Anonymous modules and their limitations
Ruby’s debugging capabilities
Marshal serialization restrictions
Object identity and memory management
The connection between Ruby’s object model and C-level implementation

This kind of deep understanding can be invaluable when debugging complex issues or optimizing Ruby applications. It reminds us that even seemingly simple operations can have complex underpinnings worth understanding.