must-understand, with_default_isolation_level, Rails World CFP and more!

Hi, it’s Vipul. Let’s explore this week’s changes in the Rails codebase.

Last Week for Rails World 2025 Call for Papers
This is the last week for The CFP for Rails World 2025!

Add must-understand directive according to RFC 9111
The must-understand directive indicates that a cache must understand the semantics of the response status code, or discard the response. This directive is enforced to be used only with no-store to ensure proper cache behavior.

class ArticlesController < ApplicationController
  def show
    @article = Article.find(params[:id])
    if @article.special_format?
      must_understand
      render status: 203 # Non-Authoritative Information
    else
      fresh_when @article
    end
  end
end

Use UNLINK for RedisCacheStore in ActiveSupport
The RedisCacheStore now uses Redis’s UNLINK command instead of DEL for cache entry deletion, enabling asynchronous and non-blocking removal of keys, which enhances performance.

Add Cache#read_counter and Cache#write_counter
This introduces 2 new methods on Rails.cache: read_counter and write_counter, to read values that are being incremented/decremented.

Rails.cache.write_counter("foo", 1)
Rails.cache.read_counter("foo") # => 1
Rails.cache.increment("foo")
Rails.cache.read_counter("foo") # => 2

Change redirect status code in SessionsController#destroy template from 302 to 303
The new Authentication generator introduced in Rails 8.0, creates a SessionsController that returns a 302 Redirect response in its destroy action. With this change it will now issue a 303(See Other) status code instead of 302(Found) when redirecting after logout, to comply with the 9110 spec

Include cookie name in length calculation
This change updates Rails to include the cookie name’s length when validating that a cookie stays within the 4KB limit, aligning its behavior with browser standards.

Introduce with_default_isolation_level in ActiveRecord
This change introduces the with_default_isolation_level method in ActiveRecord, allowing to set a default database isolation level for specific code blocks. This is particularly useful when migrating large applications to a new isolation level, as it enables enforcing the desired level in targeted areas, such as base controllers, facilitating smoother transitions and ensuring consistent transaction behavior.

class ApiV2Controller < ApplicationController
  around_action :set_isolation_level

  def set_isolation_level
    Product.with_default_isolation_level(:read_committed) do
      yield
    end
  end
end
# forces all controllers that subclass from ApiV2Controller to start getting new isolation level

With postgres adapter, prepend structure_load_flags instead of appending them
When using postgres adapter and the structure_load_flags options, the extra flags were appended instead of prepended to the default ones, causing the psql command to ignore some of the extra flags. Now, the default arguments args are appended to the extra_flags instead of the opposite.

You can view the whole list of changes here.
We had 15 contributors to the Rails codebase this past week!

Until next time!