Introduction

"The Ghosts of Action View Cache," presented by Hartley McGuire at RailsConf 2025, explores the past, present, and future of Rails’ Action View and its underlying caching, template compilation, and code maintenance mechanisms. The talk uses the narrative device of being visited by "ghosts" to walk through real-world problems and innovations in Rails view rendering.

Main Topic

The core theme of this talk is how the foundational components of Action View—especially dependency tracking and render parsing—enable efficient view caching, memory usage optimization, and dead code identification in Rails applications.

Key Points

• Fragment Caching vs. Low-level Caching:

  • Fragment caching in Action View is introduced as a safer and more efficient alternative to low-level caching with Rails.cache, reducing errors (such as caching relations instead of records) and improving performance by caching rendered HTML.
  • Action View handles cache invalidation automatically using template digests, removing the burden from developers.

• Cache Invalidation Mechanism:

  • Explains how the ActionView::Digestor generates cache keys based on template content and dependencies, enabling automatic and reliable cache invalidation whenever templates change.
  • The ActionView::DependencyTracker (ERB Tracker) determines which templates are rendered within others, using regular expressions to build dependency trees.

• Optimizing Memory Usage with Precompilation:

  • Rails' standard template compilation can lead to redundant memory usage across forked worker processes.
  • Introduces the actionview-precompiler gem, which parses and compiles ERB templates at application boot using Ruby’s ripper or prism parser, reducing redundant memory use.
  • The move from regex-based render parsing to real Ruby parsers (Prism in Ruby 3.4) allows for more robust and compatible dependency tracking across Ruby implementations and template languages.

• Dead Code Identification using Mark-and-Sweep:

  • Dead view templates often accumulate in large or long-lived Rails apps, especially during migrations or UI changes.
  • Describes a CLI tool (inspired by garbage collection mark-and-sweep) that uses Action View’s dependency tracking to find and report unused (dead) templates, allowing for codebase cleanup and further memory savings.

• Forward-looking Improvements and Recommendations:

  • Advocates for upstreaming precompilation and dead code detection tools directly into Rails for wider accessibility and easier maintenance.
  • Suggests making the Prism render parser the default in Rails 8.1 for more accurate render parsing and integration with various template languages.
  • Emphasizes the utility and extensibility of the foundational Action View components and challenges developers to leverage them for solving future problems.

Examples and Anecdotes

• Uses a narrative of a solo developer experiencing and fixing production issues to illustrate the practical relevance of caching, memory, and code hygiene problems.
• Describes personal and team-level experience cleaning up dead templates during a UI library migration, motivating the need for better, automated tools.

Conclusions and Takeaways

• Fragment caching in Action View offers safer, performance-oriented caching and seamless cache invalidation.
• Precompiling templates at boot with tools like actionview-precompiler (and leveraging Ruby parsers) significantly reduces memory usage.
• Automated identification and removal of dead view templates helps maintain cleaner codebases and improves performance.
• The composability of Action View’s core components presents opportunities for future innovations and should be more tightly integrated and configurable in Rails’ default toolset.